2022 Idaho Spring Flood and Water Resources Outlook

Spring Flood PotentialSEWApr622

The risk for spring flooding due to snowmelt is low across Idaho. Snowpack in the lower valleys has melted out and the remaining mid and high elevation snowpack is well below normal for this time of year with the exception of northern Idaho where snowpack percentages are near normal. Snowpack in most basins peaked early and is on pace to melt out several weeks earlier than normal. 

The primary factors in the development of spring flooding are the occurrence of persistent above normal temperatures, and rain on snow precipitation events. Even if mainstem rivers do not reach flood stage, smaller creeks and streams can still overflow their banks. Under the right scenario, spring flooding is possible even for areas that have low snowpack. 

 

Water Supply 

NWSFApr622ational Weather Service water supply forecasts for April through September 2022 call for near normal runoff volumes for northern Idaho watersheds. Meanwhile, forecasts vary considerably across central and southern Idaho and are generally well below normal. Forecasts for the Snake River Headwaters near the Wyoming border and the Bear River in far southeast Idaho are generally 50 to 70 percent of normal. Volume forecasts for the Lost River Basins and Salmon River Basin are 60 to 80 percent of normal while forecasts for the Wood River Basins are 35 to 55 percent of normal. Forecasts for the Middle Snake River and the    Boise, Payette, and Weiser Basins range from 50 to 75 percent of normal. Across south central and southwest Idaho near the Nevada border runoff volumes are only expected to be 40 to 45 percent of normal. 

 

Temperature and Precipitation

In October, southern Idaho and the Panhandle Region had average to well above average precipitation while the Clearwater and Salmon River Basins in central Idaho were below average.  November precipitation was average to above average across the north while southern Idaho received well below average precipitation. Autumn was relatively warm statewide, especially in November when temperatures were 3 to 6 degrees (F) above average. December moisture varied considerably across Idaho but the majority of the state received average or above average precipitation. December temperatures were on the cool side across northern Idaho and a little above average across southern Idaho. 

AnoApr622

January precipitation was below average across most of the state while temperatures were near average across northern Idaho and below average across the south. Northern regions of Idaho were a mix of below average and above average precipitation in February while record dryness occurred across central and southern Idaho. February was a cold month with below average temperatures throughout the state.  March precipitation was below average with the exception of some of the Idaho Panhandle where precipitation was near average. March temperatures were average to below average across northern and eastern Idaho and a little above average elsewhere across the state.  The three month period from January through March, 2022 was one of the driest on record for many locations across central and southern Idaho. 

Snowpack

Mountain snowpack was well above normal across most of the state in early January. However, snowpack percentages trended down afterward and by the end of February had decreased by 50 to 60 percentage points across much of central and southern Idaho. As of April 6, mountain snowpack was well below normal with the exception of northern Idaho where snowpack percentages are near normal. April 6 snowpack percentages were the highest in the Spokane and Clearwater Basins at 99 and 91 percent of median, while the lowest was in the Owyhee Basin at only 48 percent of median. Idaho snowpack as a whole typically builds through March and peaks in early April.

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Reservoirs

April 1 reservoir storage in northern Idaho, far southeast Idaho, on the Henrys Fork arm of the Upper Snake, the Payette Basin, and on the main stem Snake River was generally around normal. Elsewhere across central and southern Idaho, reservoir system storage ranged from 39 to 88 percent of normal. Reservoirs on the Snake River above Heise were at a combined 57 percent of normal. Magic Reservoir in south central Idaho was only at 39 percent of normal while Owyhee Reservoir was only 63 percent of normal. Weather patterns and irrigation demand will drive reservoir operations over the next several months. Wet spring weather or extended periods of above normal temperatures resulting in rapid snowmelt and large reservoir inflows could result in significant fluctuations in reservoir discharge and downstream river levels.

Drought

Drought continues to plague much of Idaho. Very dry conditions across much of central and southern Idaho from January through March have led to intensification of drought in some areas. Temperature and precipitation patterns for the remainder of spring and this summer will determine whether or not drought conditions improve or deteriorate. The U.S. Seasonal Drought Outlook indicates that drought is likely to persist across central and southern Idaho at least through the end of June. 

DRApr622

Long Range Outlook

The outlook for April through June favors below normal temperatures across the Idaho Panhandle and above normal temperatures for most of southern Idaho, and equal chances of either below, above, or normal temperatures elsewhere. The precipitation outlook favors below normal precipitation across central and southern Idaho, and equal chances for either below, above, or normal precipitation across the Idaho Panhandle.

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Online Resources

Water Supply Volume Forecasts…
National Weather Service-Northwest River Forecast Center www.nwrfc.noaa.gov/ws/

National Weather Service-Colorado Basin River Forecast Center
www.cbrfc.noaa.gov/

USDA-Natural Resources Conservation Service
www.nrcs.usda.gov/wps/portal/nrcs/main/id/snow/

Snowpack Information…
National Weather Service-Northwest River Forecast Center
www.nwrfc.noaa.gov/snow/

National Weather Service-National Operational Hydrologic Remote Sensing Center
www.nohrsc.noaa.gov/

USDA-Natural Resources Conservation Service
www.nrcs.usda.gov/wps/portal/nrcs/main/id/snow/

Reservoir Storage…
Bureau of Reclamation Reservoir Storage
www.usbr.gov/pn/hydromet/select.html

USDA-Natural Resources Conservation Service
www.wcc.nrcs.usda.gov/basin.html

Drought Information…
U.S. Drought Portal
www.drought.gov

Peak Flow Forecasts…
Northwest River Forecast Center
www.nwrfc.noaa.gov/peak/

Colorado Basin River Forecast Center
www.cbrfc.noaa.gov/rmap/peak/peaklist.php

Temperature and Precipitation Outlook…
Climate Prediction Center
www.cpc.ncep.noaa.gov/

 

Winter 2021-2022 in Review

Although December was warmer or much warmer than normal area wide, temperature inversions which prevailed in January and February caused the winter as a whole to average colder than normal in the Baker Valley and much of the Snake River Valley. Higher elevations, which were above the inversion, averaged warmer than normal for the season. 

This winter was drier than normal primarily due to one of the driest Februarys on record, although some areas were actually wetter than normal in December and January. 

3 month temp anomaly 3 month precip anomaly

December

The first three and a half weeks of December were relatively mild. Then real winter arrived on the 27th, lowering the average temperature for the month only slightly. December as a whole still averaged warmer or much warmer than normal nearly everywhere.

Precipitation was below normal in some areas and above normal in others. Some valley locations received more snow than usual. The month’s total of 15 inches at Boise was nearly three times normal, making it the snowiest December since 2008.

Weak high pressure aloft kept temperatures mild through the 5th. On the 1st,  a new record high of 62 was set at Burns. The 53 at McCall tied their record for the date.  On the 3rd, the high of 58 at Twin Falls set a new record for the date. 

On the 5th, a strong high pressure ridge building northward off the coast put our area under northwest flow aloft, allowing a cold front to push south from B.C. On the 6th a disturbance following the cold front brought a mix of rain and snow. 

Highs rose 10-15 degrees between the 6th and the 8th under a weak high pressure ridge. Then a fast moving Pacific cold front cooled highs by 10-15 degrees on the 9th and 10th

Temperatures began to recover on the 11th under westerly flow aloft through a transitory high pressure ridge.  As the ridge shifted east on the 12th, the flow backed into the southwest, and highs rose above normal from the 12th through the 14th ahead of another Pacific cold front.

Following that front, a pair of low pressure troughs brought snow to most of our area.  Boise received 2.1 inches, a record for the date.

A weak high pressure ridge contributed to above normal temperature on the 20th.

Showers began on the 22nd ahead of a deep low pressure trough. The unsettled weather continued through the 27th.  Precipitation totals were quite variable:

Boise .64
Jerome .59
McCall .95
Mountain Home .09
Ontario .14
Baker .05
Burns .37 (inches). 

Meanwhile, back on the 17th, a high pressure ridge was building just south of the Aleutians. By the 24th, it had grown into a massive feature between Alaska and Hawaii. It would contribute to much colder weather for the end of the month.

By the 28th, the trough had moved east of the Rockies, leaving our area under north-northwest flow aloft between the ridge and the trough. On the 29th and 30th,  disturbances embedded in this flow brought several inches of snow to the lower valleys and heavier accumulations in the mountains. 

On the 31st, Arctic air which had been residing in B.C. and adjacent areas of northern Washington began to filter into southeast Oregon and southwest Idaho, bringing clearing skies and falling temperatures. Although temperatures that day averaged only around 10 degrees below normal, temperatures on New Year’s Day averaged around 20 degrees below normal. 

December 2021 temp anomaly December 2021 precip anomaly

January

It was the coldest January since 2017 at many valley locations, although temperatures in the mountains averaged above normal because of a stubborn temperature inversion.  

Precipitation showed just the opposite pattern, with above normal totals in the lower valleys and below normal amounts in the mountains. 

On the 1st,  Arctic air from British Columbia provided the winter’s coldest weather. With clear skies, light winds, and snow cover, conditions were ideal for radiative cooling.

 

At Boise, the low at the “official” location at the airport was -3. It was the first subzero reading since January 2017. It wasn`t quite that cold elsewhere around Boise. The “warmest” reported low was 8 above zero in town in an area near the Boise River.

Lows at other locations include:

Baker City -20
Burns -16
Challis -12
Idaho Falls -13
Jerome -3
McCall -15
Mountain Home 2
Ontario -3
Rexburg -9
Salmon -12
Stanley -29.

On the 3rd, an upper level trough was approaching the Washington coast. Ahead of the trough strong mid-Level flow from the southwest mixed down to the surface, replacing the Arctic air with milder Pacific air. 

The trough weakened as it crossed the Northwest on the 4th, but strong flow from the Pacific continued to transport relatively warm air inland.  

The 7th was the “warmest” day of the month at some locations, but  there was a wind chill, so temperatures in the 40s felt more like the 20s. Here are some high temperatures plus peak winds and gusts (mph) that day. 

Boise 49 W33G50
Baker City 47 NW29G48
Burns 44 W35G47
Jerome WSW22G29
McCall 38 SSW26G34
Mountain Home E18
Ontario 49 SW16G26

By the 8th only traces of snow remained in the lower valleys.

On the 9th an upper level high pressure ridge found a home near the west coast. It would be the dominant feature for the rest of the month.

As expected at this time of year, a temperature inversion developed under the ridge. Pacific systems weakened as they passed around and through the ridge, so there was no precipitation until the 20th.

On the 20th a stronger Pacific system crossed our area from the northwest,  causing the ridge to shift offshore. Light amounts of snow fell, but it disappeared in the lower valleys the next day as temperatures rose above freezing.

The ridge was alive and well as it expanded back inland on the 21st and 22nd, and the inversion remained. Low stratus and night and morning fog plagued the valleys. 

With surface high pressure centered over southwest Idaho and southeast Oregon, the pressure gradient was too weak to generate enough wind to sweep the stratus out. Finally on the 27th, high pressure strengthened over eastern Idaho, resulting in easterly winds strong enough to clear the valleys.

The following days were sunny, but nights were clear and cold without the protective blanket of low clouds. 

January 2022 temp anomalyJanuary 2022 precip anomaly

February

It was the coldest February since 1993, and also one of the driest on record. At Boise, it was the second driest February in 145 years. And it was the driest ever at the airport, where records go back to 1940. 

A persistent upper level high pressure ridge, centered to our west through most of the month, was the main cause of both anomalies. 

The temperature inversion which formed in January persisted through February 8th. Although highs managed to rise above freezing on most days, the cold nights were mainly responsible for keeping daily averages below normal. 

On the 4th the low of 10 on Twin Falls tied the record for the date. 

On the 9th the ridge shifted farther inland. Clear skies, along with increasing sun angle and longer days, generated enough mixing to bring some of the warmer air aloft down to the surface. 

At Burns the highs of 57 on the 9th and 62 on the 10th set a new record for those dates.

Temperatures averaged right around normal through the 20th.

On the 19th an upper level trough moved down the coast from the Gulf of Alaska as an upper level ridge amplified south of the Aleutians. The trough arrived over the Pacific Northwest on the 20th. As it amplified southward over the western U.S. on the 21st, Arctic air east of the Rockies had already penetrated as far south as Nebraska. Farther west, a lobe of the same Arctic air was entering Washington from British Columbia. On the 22nd Arctic air from both sources converged over southwest Idaho. The resulting precipitation was hit and miss. Some valley locations received from 1 to 4 inches of snow.

The coldest temperatures of the month were recorded between the 21st and the 27th. 

On the 23rd the low of 0 at Burns set a new record for the date. The 10 at Twin Falls tied the record set in 2018. 

On the 25th the low of 7 at Twin Falls set a new record for the date,  and the low of 9 at Baker tied the record set in 1933. 

On the 26th the low of 0 at Twin Falls set a new record for the date. 

Temperatures gradually moderated as an upper level high pressure ridge moved inland. On the 28th highs were above normal. 

February 2022 precip anomaly February 2022 temp anomaly

Fall 2021 Season in Review

Overall, fall was warmer and wetter than normal. Temperatures were generally above normal in September and October, and above or much above normal in November. September precipitation ranged from above to below normal. In October the entire area was wetter or much wetter than normal. The trend reversed in November, with below normal precipitation nearly everywhere.

September

September 2021 precipitation

September 2021 temperature

Temperatures were above normal over more than half of the area. Precipitation presented a patchwork pattern, reflecting the predominantly convective nature of the heavier precipitation events.

A weak upper level low pressure trough hanging over the northwest U.S. kept temperatures 5-10 degrees below normal through the 3rd.

A warming trend commenced on the 4th as an upper level high pressure ridge built over the Intermountain Region. Temperatures topped out at 10-15 degrees above normal on the 9th. Wildfire smoke limited heating to some extent, but temperatures still managed to reach the 93 at Baker City, Burns, and Twin Falls, 94 at Ontario, 95 at Jerome, and 98 at Boise. Except for Ontario, these were all records for the date.

The high the following day was 20 degrees cooler as a Pacific cold front and upper level trough crossed our area. Many locations experienced wind gusts in the 40-50 mph range as the front passed.

Following the trough, temperatures remained cool on the 11th. Slightly warmer westerly flow aloft from the 12th through the 15th provided highs in the lower 80s at lower elevations…near normal for mid September.

Another Pacific cold front passed on the evening of the 15th, lowering highs on the 16th by around 10 degrees.

Southwest flow aloft ahead of a much stronger Pacific cold front added 10-15 degrees to highs on the 17th and 18th. As the front passed, gusts of 40 to 55 mph were measured at many locations. Precipitation with the front was generally light, but a bit more fell on the 19th under the upper level trough which followed the cold front. Highs on the 19th and 20th were mostly in the 50s and 60s, the coolest of the season so far.

A high pressure ridge brought a warming trend on the 21st and 22nd. Warming was briefly interrupted on the 23rd as a weak trough crossed our area. The ridge strengthened behind the trough, and highs rose into the 80s at lower elevations from the 25th through the 27th.

During the afternoon of the 27th, desert winds from the south brought dry air with humidity dropping into the single digits at several locations.

On the 27th gusts of 45-50 mph were reported at many locations following a strong cold front. On the 28th the winterlike trough following  the front lowered snow levels to around 5000 feet. At Bogus Basin, nearly 3 inches had accumulated at the snow stake just before 11 am MDT.

Temperatures in the north Pacific air that afternoon were as much as 30 degrees cooler than the previous day`s highs.

By sunrise on the 29th, cool air and clear skies allowed the temperature to drop into the mid and upper 30s in the Treasure and Magic Valleys. Since winds were light, that was cold enough for patchy frost. McCall froze with 29 degrees, but the coldest readings were in eastern Oregon, with 23 at Baker City and 20 at Burns.

The month ended under a high pressure ridge which brought sunny skies and near normal temperatures.

October

October 2021 prepitation October 2021 temperature

October was noteworthy for heavy rainfall events. Precipitation was over 150 percent of normal across wide areas of southeast Oregon and southwest Idaho. Significant areas had over 300 percent of normal.

During the first five days of the month, temperatures rose to summerlike values under a high pressure ridge. On the 4th, southwest flow aloft ahead of a low pressure trough raised the temperatures into the 80s at many valley locations.

After reaching the coast, the trough deepened far enough south on the 6th and 7th to pick up tropical moisture. As the system moved inland on the 8th, precipitation was hit and miss. Ontario reported no rain, only .04 inch was measured at Baker City, and Burns got only a trace. But Boise picked up .46 inch, Jerome .65 inch, McCall .63 inch, and Twin Falls received a total of .71 inch on the  7th and 8th.

Cool showery weather persisted through the 10th.

A colder trough moved inland on the 11th. Temperatures averaged 10-15 degrees below normal through the 14th. There was only light of precipitation.

During the next couple of days, a ridge brought a warming trend. As it shifted east, southwest flow ahead of the next Pacific cold front raised the temperature from the upper 60s to the mid 75s on the 17th.  Cool air behind the front lowered highs on the 18th by 10-15 degrees.

By the 19th a weak ridge had again built over the Intermountain Region.

Southwest flow aloft intensified as the next trough approached the coast. Like the system earlier in the month, it had entrained tropical moisture. As it moved inland over the northwest states on the 22nd, rainfall totals ranged from a quarter inch to over half an inch in the valleys. McCall got .88 inch.

Another even deeper trough from the Aleutians intensified as it approached the coast on the 24th, resulting in even stronger southwest flow from the tropics to the Pacific Northwest and Northern Rockies by way of California.

This cool-season phenomenon is known as an “atmospheric river” or AR. It is a narrow region of poleward transport of moisture, where strong flow taps very moist air from the tropics and carries it to higher latitudes. An intense low pressure system developing on the west side of the AR, plus the associated cold front and warm front, lend lift to the extremely moist and potentially unstable air. So the northern Sierra Nevada, Cascade, and Owyhee ranges, which usually create “rain shadows”, have limited effect on the amount of rain falling downwind.

Heavy precipitation was widespread on the 24th and 25th. By far the heaviest amounts fell in the Boise mountains, where totals exceeding 2 inches were common. The winning totals were 3.05 inches 4 miles southwest of Featherville, and 4.83 inches 1 mile south-southeast of Featherville.

The 24th was not only wet, but very windy. Gusts over 40 mph were common. Several locations experienced gusts over 50 mph. A gust of 65 mph was measured at a location 6 miles north of Wagontire in Harney County, Oregon. In Washington County, Idaho, a gust of 64 mph was measured 15 miles west-northwest of Cambridge.

The ridge warmed temperatures to 10-15 degrees above normal on the 28th and 29th. Then another Pacific cold front dropped temperatures to near normal on the the 30th.

November

November 2021 precipitation November 2021 temperature

November was unseasonably warm and dry.  The month began with above normal temperatures under an upper level high pressure ridge. Disturbances moving into the ridge brought only light precipitation.

Southwest flow aloft brought more light showers on the 5th and 6th, and temperatures cooled to near normal on the 7th following another weak Pacific cold front.

On the 9th moderate amounts of precipitation accompanied a Pacific cold front and upper level low pressure trough. Totals from a quarter to a half inch were common, the largest one day precipitation totals of the month at many locations.

On the 11th a high pressure ridge initiated a warming trend, and temperatures averaged 10-15 degrees above normal through the 15th. Disturbances moving around the northern periphery of the ridge brought light showers from the 11th through the 14th.

During the night of the 16th, a strong but dry cold front from the Gulf of Alaska was followed by the coldest air so far this season, with lows below freezing at nearly all locations. Both Burns and McCall registered lows of 15 degrees.

On  the 19th a Pacific cold front triggered scattered thunderstorms. In the Boise area, small hail was reported at a few locations, and there were a couple of reports of wind gusts over 50 mph.

A high pressure ridge built in from the southwest on the 20th and kept our area dry through the 22nd.

On the 23rd another pacific cold front brought showers and cooler temperatures.

A high pressure ridge centered off the California coast expanded north on the 24th. It kept the area dry through the end of the month, but it created a temperature inversion, resulting in air stagnation in the lower valleys. Despite the inversion, afternoon heating under mostly sunny skies pushed temperatures above normal during the final week of the month.

https://twitter.com/NWSBoise/status/1465115860431867906

Season in Review – Summer 2021

Summer was, in a word, hot…the hottest on record at several locations. Many monthly and daily temperature records were broken or tied. It was also drier than normal overall, with the notable exception of the Treasure Valley.

June-August 2021 3-Month Averages vs  Previous Warmest              
               Summer 2021         Previous warmest
Boise              78.0                   76.6/2015
Baker City         67.2                   69.0/1961  
Burns              69.6                   69.2/1961
Jerome             74.2                   74.2/1961
McCall             65.6                   65.3/1961
Ontario            79.4                   78.5/1961
Rome               71.0                   72.4/2007
Twin Falls         73.9                   75.1/2013
June-August 2021 Monthly Averages vs Previous Warmest
             June               July               August
Boise        75.9  70.5/2007    83.8  83.1/2007    74.4  78.7/2001
Baker City   65.4  66.0/1961    71.3  72.3/1985    64.9  71.3/1961
Burns        66.9  66.9/2015    74.5  73.2/1960    67.3  72.3/1967
Jerome       73.5  71.8/1974    79.0  77.9/1985    70.1  77.3/1967
McCall       62.9  63.3/2015    71.1  68.7/2017    62.8  67.9/1961
Ontario      75.9  80.0/1971    85.1  82.3/2007    75.9  80.0/1971
Rome         68.9  72.4/2015    75.2  78.1/2007    68.8  74.2/1967
Twin Falls   72.2  73.5/2015    78.8  79.6/2007    70.6  77.1/2013
2021 Monthly Highs vs Previous Monthly Highs
             June               July              August
Boise        105  110/2015      107  111/1960     105  110/2018
Baker City   103  102/1961      101  105/2020      98  109/2018
Burns        103  102/2015      102  107/2002     101  103/2018
Jerome       100  110/1940      100  108/1973      99  107/1940
McCall        96   97/2015       95  102/1928      93  104/1928
Ontario      107  109/2015      107  113/1967     106  113/1961
Rome         101  107/2015      105  110/2002     103  107/2018
Twin Falls   100  103/2013      100  107/2003      96  102/2013

June

June temp anomalyJune precip anomaly

A persistent and very warm upper level high pressure ridge resulted in a number of record highs. It was the warmest June on record at Boise, Burns (tied), and Jerome.  The high of 103 at Boise on the 3rd was not only a new record for the date, it was also the highest for so early in the season.

A dry pacific cold front crossed the Boise area on the 4th, and temperatures were back to near normal from the 6th through the 9th.

A cold upper level low pressure trough from the Aleutians arrived at the northwest coast on the 7th. After deepening southward, it moved inland over the Pacific Northwest and northern California on the 9th and crossed the Intermountain Region on the 10th. The 0.71 inch of rain at Boise that day was only 0.04 inch short of Boise’s normal precipitation for the entire month of June.

Temperatures rose from the 11th through the 13th under southwest flow aloft ahead of an offshore trough.

As the trough moved inland on the 14th and 15th, the coolest air was diverted north of our area by an expanding upper level high pressure ridge over the four corners. So the trough brought only slight cooling, lowering temperatures to near normal on the 16th.

As the ridge amplified, temperatures rose again.

A low pressure trough formed off the California coast on the 20th. A disturbance generated by the trough lifted north, heading for southeast Oregon and southwest Idaho. It spawned strong, but mostly dry, thunderstorms in southeast Oregon on the 22nd. Cloud cover and cool air in the trough kept temperatures in the 60s and lower 70s in Harney County, while Boise`s high soared to 98 degrees. The increased pressure gradient caused by the temperature difference enhanced outflow from the storms as they drifted north, resulting in strong west winds in the Treasure Valley. A gust of 59 mph was measured at the Boise Airport, but there were reports of even stronger gusts elsewhere in the valley.

On the 23rd, an upper level ridge began to build off the coast. By the 26th it had moved inland over B.C. and the northwest U.S., unusually far north for such a strong and very warm ridge. Temperatures responded with a number of record highs from the 28th through the 30th.

During the hottest part of the day, humidities were low, falling into the teens and single digits.

Moisture circulating around the ridge resulted in numerous showers and thunderstorms around the region on the 30th, but many locations reported only sparse precipitation.

jun2021recs

July

July temp anomalyJuly precip anomaly

July 2021 was the warmest on record at several locations, including Boise, Burns, Jerome, McCall, and Ontario.  It could have been even warmer. Persistent smoke from wildfires restricted sunlight from reaching the surface, so highs were slightly lower than they would have been under clear skies.

At Boise, several other temperature records were broken, tied, or approached.

Consecutive days
     highs >= 90...44(6/17 - 7/31) ranks 2nd (50 in 1875 ranks 1st)
           >= 100...9(6/28 - 7/6) ties 2015, 2006, and 2003 
     lows  >= 60...43(6/19 - 7/31) ranks 1st (37 in 2007 ranks 2nd)
           >= 70...11(6/27 - 7/7) ranks 1st (5 in 2015 ranks 2nd)
Number of days
     highs >= 90...31...ties July 2017 (30-year average is 22)
           >= 100...12...ranks 4th (15 in 2003 ranks 1st)
     lows  >= 60...31...ties July 2007 (30-year average is 20)
           >= 70...16...ranks 1st (9 in 2007 ranks 2nd)
                                 (30-year average is 3)

It was a dry month overall, but a storm on the 31st brought significant rain to many Idaho locations.

An upper level warm high pressure ridge was responsible for the heat. Weak cold fronts associated with Pacific weather systems crossing western Canada occasionally pushed far enough south to lower temperatures by a few degrees. But other than breezy northwest winds, they had little noticeable effect.

By the 20th the ridge had shifted east and was centered over Colorado. Southwest flow between the ridge and an upper level trough centered over the B.C. coast transported monsoon moisture north of the Nevada border. The resulting thunderstorms brought generally light precipitation, along with gusty winds.

On the morning of the 22nd, a cold front passed our area as the upper level flow began to shift into the west, carrying the monsoon moisture east. Cooler drier air following the front resulted in the below normal average daily temperatures from the 22nd through the 24th.

On the 28th a brief influx of monsoon moisture generated early morning showers and thunderstorms, but precipitation was sparse. Skies cleared, and it was another hot afternoon and evening. Drier southerly flow brought even higher temperatures on the 29th and 30th.

By the 26th the center of the upper level ridge had migrated to the central Great Plains. Easterly flow south of the ridge picked up very moist air from the Gulf of Mexico and carried it west to New Mexico and Arizona. Southerly flow on the west side of the ridge then transported it north to the northern Intermountain Region.

On the 31st a low pressure system formed over Nevada. As it moved north, it interacted with the Gulf moisture, setting off numerous showers and thunderstorms with locally heavy rain. The Boise and Ontario areas, the Magic Valley, southern Twin Falls County, and the central Idaho mountains got the brunt of the rain.

 

jul2021recs

August

August temp anomalyAugust precip anomaly

 

In contrast to June and July, August was a relatively wet month with near normal temperatures, although it certainly had its share of record daily high temperatures. But smoky skies prevented highs from reaching their full potential. Temperatures were mostly above normal for the first half of the month and mostly below normal for the last half.

An upper level high pressure ridge kept temperatures hot from the 2nd through the 5th.

On the 6th a cold front and weak upper level trough crossed our area. Temperatures warmed briefly on the 7th ahead of a stronger trough from the Gulf of Alaska. That trough, and the northwest flow that followed, cleared the smoke and kept temperatures below normal through the 10th.

The heat returned on the 11th as an upper level ridge centered off the coast built inland. The heat persisted through the 16th. By the 14th the smoke had overspread our area again.

A major pattern change on the 17th ended the heat. An upper level trough deepened south from Canada over the Pacific Northwest and the Intermountain Region as an upper level ridge between Alaska and Hawaii strengthened and expanded north. The resulting northwest flow aloft put us in the path of more troughs coming out of Alaska. One of these systems brought light but measurable rain on the 20th and 21st, plus another brief respite from the smoke.

 

aug2021recs

2021 Spring in Review

Temperatures did not depart significantly from normal. Precipitation was below normal region-wide, with a large portion of our area receiving less than 50 percent of normal spring precipitation.

March

Typical of early spring, March was a month of changeable weather and wide temperature variations.  Temperatures averaged near normal. It was a dry month, with much of the region receiving fifty percent or less of average March precipitation.  On the 5th, southeast winds ahead of a slow-moving cold front raised temperatures to record highs at some locations. The high of 69 at Ontario broke the old record of 66 set in 2012.  The front weakened as it moved inland, bringing only light precipitation.  A second cold front crossed the area on the evening of the 7th ahead of a cold low pressure trough from the Gulf of Alaska. Again only light precipitation fell as the cooling trend continued.  Skies cleared as the cold air moved inland on the 10th, resulting in some of the coldest lows of the month:

Baker City 20
Boise 25
Burns 22
McCall 5
Mt. Home 19
Ontario 22
Rome 16
Twin Falls 19 (11th)

An upper-level high pressure ridge building over the Pacific Northwest brought a warming trend from the 11th through the 14th.

A weak cold front crossed the area on the 15th. After brief cooling on the 16th, high pressure provided gradual warming, resulting in the warmest high temperatures for the season so far.  On the 19th, moderate amounts of rain accompanied a cold front. A record daily rainfall of 0.27 inch was set at Twin Falls.  An upper-level low pressure trough with much cooler air followed the front, but an even colder system was on the way.  On the 22nd, a strong cold front and upper-level trough from the Gulf of Alaska brought more precipitation.  A record daily rainfall of 0.46 inch was set at Boise, including half an inch of snow during the evening. Most of the precipitation was in the form of thunderstorm rain which preceded the snow.  Many locations experienced some the coldest highs of the month on the 22nd and 23rd:

Baker City 49 (23rd )
Boise 46
Burns 49
Jerome 46
McCall 35
Mt. Home 49
Ontario 54
Rome 49 (23rd )
Twin Falls 44 (23rd )

Another cold upper-level trough from the Gulf of Alaska continued the cold wet weather on the 24th and 25th.  An upper-level ridge brought much warmer weather. On the 28th, a record high of 70 was set at Twin Falls, and a record high of 75 was tied at Baker City. But very windy and much colder weather would soon arrive from the Gulf of Alaska.  During the evening of the 28th, a very strong cold front crossed the area.  Northwest winds increased rapidly behind the front, driven by a steep surface pressure gradient and enhanced by a powerful jet stream on the southern periphery of the upper-level trough which followed the front.

A gust of 61 mph was measured at the Boise airport shortly after midnight on the 29th.  No major damage was reported, but a number of shallow-rooted trees were toppled, and there were numerous broken branches. Downed lines left around 23,000 Boise customers without power, and a semi trailer was blown over on I-84 near Gowen Road, blocking most of the west-bound lane for several hours.  Afternoon highs on the 20th were 10-15 degrees below normal, and breezy northwest winds made it feel even colder.  A warming trend commenced during the afternoon of the 31st as a high pressure ridge moved in from the west.

precipitation percent of normal Marchtemperature departure from normal March

 

 April

April 2021 is memorable for dry windy weather and temperature extremes, although the average temperature for the month was close to normal.  It was an unusually dry April, with about half of the area receiving less than fifty percent of normal precipitation.  A high pressure ridge kept temperatures warm from the 1st through the 4th.  Record highs were set at Twin Falls on the 1st, 2nd , and 3rd with 73, 72, and 75.  On the 3rd Ontario tied a record high of 80, originally set in 1990.  Temperatures were below normal on the 5th and 6th following a cold front on the 4th. Another cold front from the Gulf of Alaska initiated a cool spell which persisted through the 14th. A record low of 17 was set at Baker City on the 7th.  Record lows were set on the 9th at Burns, Ontario, and Baker City, with 14, 22, and 12.  A record low of 15 was set at Burns on the 11th. Record lows were set at Twin Falls and Baker City on the 12th, with 23 and 16.  Early on the morning of the 19th, a dry cold front from western canada crossed the area. After the front passed, northwest winds increased through the day, and gusts around 50 kts were observed at a few locations.  A high pressure ridge near the coast maintained dry weather from the 20th through the 23rd.  The ridge moved inland and weakened on the 24th, allowing moist and unstable air to spread inland ahead of a low pressure trough.

Locally moderate amounts of rain fell with showers and a few thunderstorms on the 24th and 25th:

Baker City .19 .25
Boise .53
Burns .07 .27
McCall .25 .37

Then on the 26th, a disturbance from the Gulf of Alaska moved down the coast and into the trough, generating more rain.

On the 27th, a strong high pressure ridge building inland from the northwest coast kicked the trough south to the Mexican border.  The ridge brought a warming trend which culminated on the 30th.  A record high of 83 was set at Burns on the 29th.  A record high of 86 was set at Twin Falls on the 30th.

precipitation percent of normal April temperature departure from normal April

 

 

May

May continued the dry pattern of April. Precipitation was below normal across most of the area, but some locations in Malheur, Canyon, and northwest Owyhee Counties reported near or above normal precipitation.  This was probably the result of isolated convective showers.  Much of the area had no measurable rain from the 2nd through the 20th.  Cool spells alternated with warm spells, resulting in an average temperature for the month that was very close to normal for most of the area.  There were no record highs or lows.  Early on the morning of the 1st, a Pacific cold front crossed southeast Oregon and southwest Idaho. That afternoon the moist and unstable air which followed the front spawned thunderstorms with gusty outflow winds. Gusts around 60 mph were observed in Ada and Elmore Counties. Damage was minimal, but 5500 customers in north Meridian were without power for awhile.  At Boise, a gust of 62 mph at 3:15 pm MDT, was the strongest gust for May on record going back to 1980.

A high pressure ridge brought a warming trend which started on the 2nd and culminated on the 6th with highs of 89 at Mountain Home, 90 at Boise, and 91 at Ontario.  Highs on the 7th were around 20 degrees lower following a strong Pacific cold front. Highs on the 8th were 10 additional degrees cooler under the cold low pressure trough which followed the front.  Temperatures slowly recovered starting on the 9th, maxing out on the 17th under a high pressure ridge.  Another Pacific cold front crossed the area on the 18th ahead of a cold wet low pressure trough from the Aleutians. By the 20th the trough had settled over the Intermountain Region.  The system produced the first significant rain of the month at Boise and Burns, where nearly half an inch fell on the 22nd. Mountain Home got a third of an inch, but only two tenths of an inch was measured at Ontario. A record rainfall of .32 inch was set at Twin Falls on the 21st, breaking the old record of .23 inch set in 2016.

The trough finally exited into Canada on the 24th, but close on its heels was another trough which brought more rain along with scattered thunderstorms on the 25th and 26th. Around a quarter inch of rain fell at Boise, Burns, and Ontario. Baker City got over a third of an inch, and nearly three-quarters of an inch fell at McCall.  A weak high pressure ridge brought brief warming on the 27th.  During the early morning hours on the 28th, the cold front preceding the last trough of the series crossed our area. No measurable rain accompanied the trough at lower elevations east of the Cascades, but temperatures were 10-15 degrees cooler.  On the 29th a high pressure ridge started a warming trend as it began to build over the region. Temperatures were above normal for the last three days of the month.

precipitation percent of normal May temperature departure from normal May

Winter 2020-2021 in Review

December

Temperatures were above normal in the mountains and below normal at lower elevations due to temperature inversions.  The entire region was drier or much drier than normal. The driest areas were the central Snake River Valley, the Camas Prairie, and the Boise Mountains.   An upper-level high pressure ridge dominated through the 10th, keeping our region dry. Burns was an exception, where a weak upper level low pressure trough brought light snow on the 6th.  From the 11th through the 16th, a temperature inversion resulted in areas of night and morning fog. A trough from the Gulf of Alaska brought light snow on the 13th and 14th. It was too weak to mix out the inversion, as most of its energy crossed the Intermountain Region well south of our area. 

The inversion finally broke as a stronger trough crossed the area on the 17th, bringing 5-10 inches of snow to the mountains, but only light precipitation in the valleys. An exception was the .55 inch rainfall at Jerome, which established a new record for the date.  Mild westerly flow aloft brought a warming trend from the 18th through the 22nd.  High temperature records were tied or broken at several locations on the 21st and 22nd

City New Record Old Record Year
Baker City 56 55 1972
Burns 53 51 1972
Jerome 58 58 1969
McCall 43 43 2019
Twin Falls (21st) 59 53 2014
Twin Falls (22nd) 50 50 2005

A strong Pacific cold front crossed the area on the 22nd, followed by a 10-20 degree temperature drop. Strong northwest winds gusting to 40-50 mph followed the front. A gust of 57 mph was measured north of Andrews in Harney County.  The trough which followed the front produced light precipitation at lower elevations. Moderate amounts of snow fell in the mountains, including 5 inches at Brundage.  

https://twitter.com/NWSBoise/status/1341508962311688197/

Following this system, a strong ridge kept skies mostly clear through Christmas eve.

A weak trough brought mainly light precipitation on the 25th and 26th. Boise was an exception, where a third of an inch of precipitation fell on the 26th, which included half an inch of snow. A few inches of snow fell in the mountains. 

Another temperature inversion formed on the 26th, and locally dense fog returned to the valleys.  With cold air trapped in the valleys, temperatures failed to reach the freezing mark at many of the usually warmer locations in the 28th and 29th

https://twitter.com/NWSBoise/status/1343759866368675842?s=20

The final trough of the month weakened as it moved inland on the 31st, but it was able to drop several inches of snow on the mountains, including 8 inches at McCall and 6 inches at Tamarack.

DecemberDecemberp

January

Temperatures were above normal across the region, and much above normal in the Snake River Valley and parts of southeast Oregon.  Most of southern Idaho and parts of Baker and Harney Counties in Oregon were drier than normal, while the Treasure Valley and west central Idaho received above normal precipitation.  

https://twitter.com/NWSBoise/status/1345922598413062145

Strong westerly flow across the Pacific was charged with abundant moisture as it swept inland with a warm front on the 3rd, resulting in heavy precipitation. Totals from a half inch to an inch of water were common, falling as rain at lower elevations and snow in the mountains. Bogus Basin accumulated 7 inches.  Strong west winds, mostly in the 40 to 50 mph range, followed a cold front on the 4th.  A gust of 63 mph was measured at Wagontire in Harney County.  On the 5th another warm front brought heavy snow to the mountains. Banner Summit got 14 inches. Bogus Basin and Tamarack received 10 inches. Atlanta also got 10 inches. Bogus Basin got 7 additional inches on the 7th as a trough crossed the area. Little if any precipitation fell at lower elevations from the 5th through the 7thOn the 12th a weak but moist trough embedded in the westerly flow brought more snow to the mountains, while relatively light precipitation fell at lower elevations. At Council 7 inches was measured, and 6 inches fell at Brundage and McCall.  A cold front crossed the area on the 13th, followed by wind gusts of 30-45 mph.

High pressure kept the area mostly dry from the 14th through the 20th.  From the 21st through the 26th, weak troughs moving down the coast from the Gulf of Alaska brought only light precipitation to our valleys and a few inches of snow to the higher elevations.  A record low temperature of 10 degrees was set at Twin Falls on the 25th, breaking the old record of 12 set in 2008.  On the 27th a stronger trough centered over southeast Alaska deepened southward just offshore. Southerly flow aloft on the east flank of the trough was felt at the surface as gusty south to southeast winds, generally in the 40 to 50 mph range. A gust of 68 mph was measured at Trail Gulch, 14 miles east-southeast of Hollister.   Also on the 27th, moderate amounts of snow fell in eastern Oregon and west-central Idaho, due mainly to lifting of the air by the mountains. At McCall 9 inches was measured, and 7 inches fell at Midvale. At Huntington in Baker County 5 inches was reported. 

Januaryp

January

February

February was a month of very active weather. Temperatures were near or above normal at most locations. February is normally warmer than January, but this year it was actually colder than January at Boise, McCall, Mountain Home, and Ontario. It’s not the first time this has happened, but at Boise it was the fourth year in a row.

On average, February is the driest winter month, but this year it was the wettest in the Treasure Valley and eastern Oregon. However, the Boise Mountains, Camas Prairie, and Magic Valley were drier than normal.  The month began with above-normal temperatures under southwest flow aloft ahead of an offshore trough. As the trough moved inland, a cold front crossed our area during the afternoon and evening of the 2nd.  This marked the beginning of a pattern change which would lead to long overdue winter weather for the valleys. Northwest flow aloft developed on the 4th and strengthened during the following days.  On the 5th, a fast-moving trough from the Gulf of Alaska brought a few inches of snow to the mountains, but its main impact was strong west to northwest wind. Gusts of 45-55 mph were common, but by far the strongest wind measured was 92 mph at Soldier Mountain.  Meanwhile on the 3rd, arctic air had plunged south across the Canadian border east of the Rockies. The mountains, and strong northwest winds aloft, kept it out of Idaho. A second invasion of even colder arctic air entered Montana on the 5th. By the 6th, its western margin had stalled along the Montana border.  Meanwhile, the “milder” western portion of the same arctic airmass was drifting south through the mountains of British Columbia. It entered Washington on the 8th and northeast Oregon on the 9th.  The front marking its leading edge reached eastern Oregon and southern Idaho on the 11th.  Moist air streaming inland ahead of an approaching trough was lifted over the cold air north of the front, resulting in snow which became heavier as the trough drew closer on the 12th and crossed our area on the 13th.  Snowfalls of 3 to 6 inches were common in the valleys on the 12th.  Up to 6 more inches fell on the 13th, with heavier accumulations in the mountains. By the time the snow ended, 7 to 10 inches had accumulated in the Treasure Valley. McCall, which nearly always gets way more snow than Boise, also reported a storm total of 10 inches. Totals of 13 inches were measured at both Bogus Basin and Magic Mountain ski areas, while Twin Falls reported only 2 inches.  The 4.4 inches which fell at the Boise airport on the 12th broke the old record for the date of 1.3 inches set in 1966, and the 5.5 inches on the 13th broke the old record of 4.3 inches set in 1995.  

On the 15th another trough from the Pacific brought more snow, with 1 to 4 inches in the valleys and 5 to 10 inches in the mountains. Wind gusts of 40 to 55 mph were observed in Harney County and the Magic Valley.  From the 16th through the 22nd, Pacific systems brought more snow to the mountains but generally light precipitation in the valleys. With afternoon temperatures above freezing, there was little if any snow accumulation in the lower valleys.  On the 23rd, a cold front was followed by a wind event which mainly affected southeast Oregon and areas east of Boise. There were many reports of gusts in the 45 to 55 mph range, but a gust of 67 mph was measured at Twin Falls.  On the 26th, another strong cold front swept across the region. One of the stronger gusts was 59 mph from the west-northwest at the Boise airport. A brief snow squall followed the front, but less than half an inch fell at the airport.  

On the 27th a trough from the Gulf of Alaska brought heavy snow to the mountains. Some of the heavier totals were 12 inches at Mores Creek Summit, 15 inches at Banner Summit, 20 inches at Tamarack, and 22 inches at Brundage.

2021 Idaho Spring Flood and Water Resources Outlook

Spring Flood Potential

Screen Shot 2021-03-23 at 10.28.21 AMThe risk for spring flooding due to snowmelt is near average across most of Idaho. The exceptions are the Big Lost Basin and Little Wood Basin, and the Medicine Lodge, Beaver, and Camas Basin in central and east-central Idaho where the risk is slightly below average due to well below normal snowpack and low soil moisture. Above normal
temperatures in December and January helped limit the snowpack in the low elevations which significantly reduces the snowmelt flood risk during late winter and early spring.

The primary factors in the development of spring flooding are the occurrence of persistent above normal temperatures, and rain on snow precipitation events. Even if mainstem rivers do not reach flood stage, smaller creeks and streams can still overflow their banks. Under the right scenario, spring flooding is possible even for areas that have low snowpack. Additionally, wildfire burn scars can have a significant impact on local flood potential during spring snowmelt.Screen Shot 2021-03-23 at 10.28.15 AM

Water Supply

National Weather Service water supply volume forecasts for the spring and summer of 2021 are near normal for northern Idaho watersheds. Meanwhile, forecasts vary considerably across central and southern Idaho with subpar snowpack and longer term hydrologic drought leading to well below normal forecasts in some areas. Forecasts for the Snake River headwaters region along the Wyoming border and the West Central Mountains are generally 75 to 85 percent of normal. Volume forecasts for the rest of the Central Mountains and southern Idaho watersheds are well below normal ranging from 25 percent to 70 percent of normal. The lowest forecasts are for the Big Wood and Little Wood Basins, and the Big Lost River Basin. Low snowpack and poor streamflows in these areas could lead to water supply concerns, especially for those relying on natural flows. These forecasts may change considerably over the next couple of months since seasonal snow accumulation and rainfall typically occur during March and April.

Temperature and Precipitation

The 2021 Water Year started out well across northern Idaho with above normal precipitation in October, while rather dry conditions prevailed across southern Idaho. Most of the state received near normal or above normal precipitation in November with the exception being near the Canadian border and portions of central Idaho where only 50 to 70 percent of normal occurred. December was a very dry month for the state as a whole. Precipitation in January was improved for some areas but dry conditions continued for the majority of Idaho. A favorable storm track in February brought normal or well above normal precipitation to the majority of the state except for the Northern Panhandle and the Central Mountains east of the Sawtooths which received less than normal precipitation. Temperatures were around normal in October and November with warmer than normal conditions prevailing in December and January. February temperatures were near normal across far southern Idaho and below normal across the remainder of the state.

Screen Shot 2021-03-23 at 10.28.08 AM

Snowpack

As of March 3, mountain snowpack was near normal for most of Idaho. The Clearwater Basin led the way with 113 percent of normal while the Little Wood and Big Lost River Basins were lagging at 66 and 69 percent of normal. Idaho snowpack as a whole typically builds through March and peaks in early April.Screen Shot 2021-03-23 at 10.28.00 AM

Reservoirs

Reservoir storage across Idaho was in good shape as of early March with major reservoir systems holding near average water with the exception of the Wood and Lost River Basins. Across southern Idaho, basin-wide total storage in the Bear River Basin was near 136 percent of average while Southside Snake Basins were near 81 percent of average. Storage in West Central Basins was near 103 percent of average while the Wood and Lost Basins were only 59 percent of average. The Upper Snake River Basin was at 120 percent of average, the Clearwater Basin at 103 percent of average and Panhandle Region was 106 percent of average. Weather patterns, irrigation demand, and flood control needs will drive reservoir operations over the next several months. Wet spring weather or extended periods of above normal temperatures resulting in rapid snowmelt and large reservoir inflows could result in significant fluctuations in reservoir discharge and downstream river levels.

Drought

Drought conditions continue to plague portions of southern Idaho. The Wood River Basins and Big Lost River Basin of south central Idaho are the focus for drought due to subpar snowpack and precipitation dating back to the winter of 2019-2020. Meanwhile, long-term dryness along the Utah and Nevada border is resulting in drought conditions as well. Weather and precipitation for the remainder of winter and this spring will determine whether or not drought conditions improve or deteriorate.

Screen Shot 2021-03-23 at 10.27.52 AM

Long Range Outlook

The outlook for March through May favors below normal temperatures across the Idaho Panhandle and above normal temperatures along the Utah and Nevada border. Elsewhere across the state, the chances for either below, above, or normal temperatures are equal. The precipitation outlook slightly favors above normal precipitation across the Panhandle Region. Elsewhere the precipitation outlook is equal chances for either below, above, or normal precipitation.

Screen Shot 2021-03-23 at 10.27.43 AM

On-line Resources

Water Supply Volume Forecasts…
National Weather Service-Northwest River Forecast Center www.nwrfc.noaa.gov/ws/

National Weather Service-Colorado Basin River Forecast Center
www.cbrfc.noaa.gov/

USDA-Natural Resources Conservation Service
www.nrcs.usda.gov/wps/portal/nrcs/main/id/snow/

Snowpack Information…
National Weather Service-Northwest River Forecast Center
www.nwrfc.noaa.gov/snow/

National Weather Service-National Operational Hydrologic Remote Sensing Center
www.nohrsc.noaa.gov/

USDA-Natural Resources Conservation Service
www.nrcs.usda.gov/wps/portal/nrcs/main/id/snow/

Reservoir Storage…
Bureau of Reclamation Reservoir Storage
www.usbr.gov/pn/hydromet/select.html

USDA-Natural Resources Conservation Service
www.wcc.nrcs.usda.gov/basin.html

Drought Information…
U.S. Drought Portal
www.drought.gov

Peak Flow Forecasts…
Northwest River Forecast Center
www.nwrfc.noaa.gov/peak/

Colorado Basin River Forecast Center
www.cbrfc.noaa.gov/rmap/peak/peaklist.php

Temperature and Precipitation Outlook…
Climate Prediction Center
www.cpc.ncep.noaa.gov/

Idaho 2020 Spring Flood and Water Resources Outlook

Spring Flood Potential

swe2020The risk for spring flooding due to snowmelt is near normal across most of Idaho. The exception is the Big Lost Basin and Wood River Basins where the risk is below normal due to well below normal snowpack and low soil moisture.  Relatively warm winter temperatures and a rather dry February have limited the snowpack in the low elevations which significantly reduces the snowmelt flood risk during late winter and early spring.

The primary factors in the development of spring flooding are the occurrence of persistent above normal temperatures, and rain on snow precipitation events. Even if mainstem rivers do not reach flood stage, smaller creeks and streams can still overflow their banks. Under the right scenario, spring flooding is possible even for areas that have low snowpack. Additionally, wildfire burn scars can have a significant impact on local flood potential during spring snowmelt.

Water Supply

The water supply runoff volume forecasts for the spring and summer of 2020 are near average for northern Idaho watersheds. Meanwhile, forecasts vary considerably across central and southern Idaho.  Forecasts for the Snake River headwaters region and along the Utah and Nevada border generally indicate near normal runoff volumes. Forecasts for the Middle Snake River and Central Mountain watersheds indicate below or well below normal runoff volumes. The lowest runoff volume forecasts are for the Big Wood and Little Wood basins, ranging from 33 to 55 percent of normal. Low streamflows across portions of the Central Mountains could lead to water supply concerns for those relying on natural flows. These forecasts may change considerably since seasonal snow accumulation and rainfall typically occur during March and April.ws2020

Temperature and Precipitation

A wet September closed out the 2019 Water Year, but very dry conditions dominated through the first quarter of the 2020 Water Year. Precipitation for October through December was only 40 to 70 percent of average for most of the state with the driest conditions across the Central Mountains and southern Idaho. The January storm track brought above normal precipitation to most of Idaho with the exception of the Wood River Basins and Big Lost Basin where precipitation was below normal. In February, the Clearwater and Lemhi Basins, the Salmon River Basin, and areas along the Wyoming border had near normal or above normal precipitation while the rest of the state was below normal. Water Year to date precipitation in general is normal to below normal for northern Idaho and areas along the Wyoming, Utah, and Nevada borders. For the Snake Plain and Central Mountains, Water Year precipitation is below or well below normal. After a cold October where mean temperatures were 5 to 10 degrees below normal for most of the state, above normal temperatures have dominated for the 2020 Water Year.

t2020p

Snowpack

As of March 5, mountain snowpack was near normal for most of Idaho.  The Boise River Basin was lagging at 84 percent of normal. The Big Wood and Little Wood River Basins and Big Lost River Basin had the lowest snowpack percentages ranging from 56 to 68 percent of normal.  Idaho snowpack as a whole typically builds through March and peaks in early April.

swe2020per

Reservoirs

Reservoir storage across Idaho was in good shape as of early March with major reservoir systems holding near average or well above average water. Across southern Idaho, reservoir storage in the Bear River Basin and Southside Snake River Basins was 144 to 153 percent of average. Storage in West Central Basins, Wood and Lost Basins, and the Upper Snake River Basin ranged from 110 to 168 percent of average. Meanwhile, reservoir storage in the Clearwater and Panhandle Regions was 96 to 104 percent of average. Weather patterns, irrigation demand, and flood control needs will drive reservoir operations over the next several months. Wet spring weather or extended periods of above normal temperatures resulting in rapid snowmelt could result in significant increases in reservoir outflows and river levels.

Drought

After a drought free start to the 2020 Water Year, dry fall conditions followed by low snowpack led to the development of drought conditions across portions of the Central Idaho Mountains. Weather and precipitation for the remainder of winter and this spring will determine whether or not drought conditions improve or deteriorate.

drought

Long Range Outlook

The outlook for March through May favors above normal temperatures across the south half of Idaho while chances for either below, above, or normal temperatures are equal across the north. The precipitation outlook slightly favors above normal precipitation along the Wyoming border, otherwise the probabilities are not shifted one way or another for the rest of the state.

MAM2020

MAM2020pOn-line Resources

Water Supply Volume Forecasts…
National Weather Service-Northwest River Forecast Center
www.nwrfc.noaa.gov/ws/

National Weather Service-Colorado Basin River Forecast Center
www.cbrfc.noaa.gov/

USDA-Natural Resources Conservation Service
www.nrcs.usda.gov/wps/portal/nrcs/main/id/snow/

Snowpack Information…
National Weather Service-Northwest River Forecast Center
www.nwrfc.noaa.gov/snow/

National Weather Service-National Operational Hydrologic Remote Sensing Center
www.nohrsc.noaa.gov/

USDA-Natural Resources Conservation Service
www.nrcs.usda.gov/wps/portal/nrcs/main/id/snow/

Reservoir Storage…
Bureau of Reclamation Reservoir Storage
www.usbr.gov/pn/hydromet/select.html

USDA-Natural Resources Conservation Service
www.wcc.nrcs.usda.gov/basin.html

Drought Information…
U.S. Drought Portal
www.drought.gov

Peak Flow Forecasts…
Northwest River Forecast Center
www.nwrfc.noaa.gov/peak/

Colorado Basin River Forecast Center
www.cbrfc.noaa.gov/rmap/peak/peaklist.php

Temperature and Precipitation Outlook…
Climate Prediction Center
www.cpc.ncep.noaa.gov/

2019 Idaho Water Year Summary

Overview:

The 2019 Water Year can be characterized as a year with a lot of variability, which is quite normal when talking about weather in Idaho.  Overall, it was a good year with respect to moisture as the majority of the state received normal or above normal precipitation. The exceptions were east-central Idaho and the panhandle region where water year precipitation was below normal. With respect to temperature, mean temperatures across the state were close to normal.

April 1 snowpack percentages were above normal for central and southern Idaho and a little below normal across northern Idaho. Northern Idaho and areas near the Nevada border received the better snow through the first half of the accumulation season while most of central and southeast Idaho lagged considerably.  In February, a series of cold and moisture storm systems brought abundant snow to the state which more than made up for the drier conditions earlier in the water year.

Springtime was rather typical with periods of warm and dry weather followed by cool and wet conditions.  Except for portions of east-central Idaho and the panhandle, spring precipitation was normal or above normal across the state. Above normal temperatures prevailed across northern Idaho during the spring while temperatures across central and southern Idaho were generally near normal or below normal. A healthy snowpack and favorable spring weather produced good spring runoff with most reservoirs topping off and adequate water supply for the state.

Summertime temperatures and precipitation were close to normal across Idaho. With the exception of the panhandle region, normal to above normal streamflows were sustained through the spring and summer months. Major irrigation projects serving Idaho ended the 2019 Water Year on a good note with normal or above normal carryover.

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Precipitation:

Well below normal precipitation occurred across most of the central mountains and far southwest Idaho during the 1st quarter (Oct.-Dec. 2018) of the 2019 Water Year. Meanwhile, the rest of the state varied from a little below to a little above normal. January and March were quite dry across the state as a whole, but sandwiched in between was an exceptionally wet February with precipitation 150 to 300 percent of normal for the majority of the state. April was another wet month, highlighted by widespread river and small stream flooding across much of west-central Idaho April 7-13 when 2 to 5 inches of rain fell on top of snowmelt. Above normal precipitation fell across southern Idaho in May while most of central and northern Idaho saw below normal precipitation. June was a rather dry month for the state followed by near normal precipitation for most of the state through the summer. The water ended on a high note with above normal precipitation in September, helping recharge soils after the moisture sapping summer months.

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Temperature:

Mean temperatures for the 2019 Water Year were near normal for most Idaho. During the 1st quarter (Oct.-Dec. 2018) of the water year mean temperatures were generally above normal for northern and central Idaho, while most of southern Idaho experienced normal to below normal temperatures. Typical monthly variability occurred during the January through June period. Cold temperatures in February and March were vital in building and maintaining a good snowpack while cool temperatures across southern Idaho in May and June resulted in a slower snowmelt and good streamflows heading into the warm and dry season.  Summertime temperatures as a whole were close to normal.Capture

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Snowpack:

Overall it was a good snow year for Idaho.  The storm track favored northern and far southwest Idaho the early part of the accumulation season with January 1 snowpack percentages ranging from 88 to 117 percent of normal. Elsewhere, snowpack lagged considerably by January 1, especially across the central mountains where percentages ranged from 44 to 73 percent of normal. Subpar snowpack was the rule through the month of January but a favorable storm track in February brought a series of cold and wet storm systems to the region. From February 1 to March 1, snowpack increased by 25 to 60 percentage points across most of central and southern Idaho while gains of 10 to 15 percent occurred across northern Idaho. By early April, when Idaho’s overall snowpack typically peaks, snowpack percentages from the Salmon River Basin south to the Nevada and Utah border ranged from 106 to 145 percent of normal while northern Idaho basins ranged from 85 to 95 percent of normal. This set the stage for an adequate water supply through the growing season.

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Streamflow:

A warm and very dry summer in 2018 left streamflows in Idaho below to much below normal in many river basins to start the 2019 Water Year, particularly across the panhandle, west-central mountains, and southern Idaho.  Streamflow percentiles varied considerably from basin through the core winter months. The snowmelt season and good spring precipitation brought robust streamflows to the state in April. Heavy rain combined with snowmelt caused main stem river flooding, flash flooding, and small stream flooding across much of central Idaho April 7-13 with record or near record crests observed on a number of waterways. Favorable spring weather resulted in a nearly ideal melt of the mountain snowpack and extended healthy streamflows through the summer for southern and central Idaho. Across the panhandle region, lower snowpack and drier spring weather resulted in below normal streamflows in many basins from late spring through summer.

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Reservoirs:

Reservoir storage carryover to start the 2019 Water Year was near normal. A healthy snowpack by early April and good spring runoff allowed major reservoir systems to either fill or come close to filling.  Flood control releases were necessary on some reservoir systems during spring runoff resulting in high flows on some rivers. As the irrigation season hit full stride reservoir systems began drawing down in mid to late June. By the end of the growing season reservoir storage was normal to above normal for almost all reservoirs across the state with good carryover for the 2020 Water Year.

CaptureCaptureDrought:

Moderate to severe drought plagued much of the panhandle region along with west central and southern Idaho through the 1st quarter (Oct.-Dec. 2018) of the 2019 Water Year. However, abundant snowfall and good spring precipitation erased drought conditions by early April. Moderate to severe drought returned to the panhandle region over the summer while central and southern Idaho remained drought free. Minimal drought across the state contributed to a rather quiet wildfire season.

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2019 Idaho Spring Flood and Water Resources Outlook

Spring Flood Potentialswe38

The potential for spring flooding due to snowmelt is slightly elevated for much of the central Idaho Mountains, particularly in the Big Lost and Wood River Basins, as well as the Upper Boise, Payette, and Weiser Basins. For the remainder of the state the spring flood potential due to snowmelt is average.

The primary factors in the development of spring flooding are the occurrence of persistent above normal temperatures, and rain on snow precipitation events. Even for areas that may have low snowpack, spring flooding is possible under the right scenario. Additionally, wildfire burn scars can have a significant impact on local flood potential during spring snowmelt.

Water Supplywatersupply

The water supply forecast for the spring and summer of 2019 is near average for the majority of watersheds across Idaho. Exceptions are some above average forecasts in the central mountains and Upper Snake Region, and some below average forecasts along the southern border and in the Idaho Panhandle.

Temperature and Precipitation

Wet weather at the beginning of the Water Year (Oct. 2018) was beneficial in recharging the soil moisture after a very dry and warm summer. However, fall precipitation as a whole was below normal across the state and subpar precipitation continued through most of January. February precipitation was exceptional across west-central and southern Idaho with much of the area receiving 200 to 300 percent of normal. February temperatures were well below normal across the state, especially in northern Idaho. Precipitation for the 2019 Water Year through February has been normal or above normal for most of central and southern Idaho, and a little below normal for most of the Panhandle Region. Mean temperatures for the 2019 Water Year have been near normal.

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Snowpack

As of March 8, snowpack from the Salmon River Basin to the southern border was above normal. The highest percentages were in the Big Lost Basin, Wood River Basins, Payette Basin, and Weiser Basin, ranging from 133 to 152 percent of median. Northern Idaho snowpack varied from 89 to 100 percent of median for the Northern Panhandle, Spokane, and Clearwater Basins. Idaho snowpack as a whole typically builds through March and peaks in early April.

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Reservoirs

Reservoir storage across Idaho is in good shape as of early March with major reservoir systems holding near average or above average water. Across southern Idaho, reservoir storage in the Bear River Basin was near 140 percent of average and Southside Snake River Basin storage was near 95 percent of average. The Upper Snake Basin and Wood River and Lost River Basins was near 125 percent of average. Storage in West Central Basins was near 98 percent of average, and the Clearwater and Panhandle Regions were near 97 and 104 percent of average. Weather patterns, irrigation demand, and flood control needs will drive reservoir operations over the next several months. Wet spring weather or extended periods of above normal temperatures resulting in rapid snowmelt could result in significant increases in reservoir outflows and river levels.

Drought

Moderate to severe drought plagued portions of Idaho through the fall and early winter, especially across southern Idaho and near the Utah and Nevada border. Drought conditions were eased by well above normal precipitation and a large increase in mountains snowpack during February. The latest U.S. Drought Monitor shows lingering drought across far southwest Idaho and along the Utah border. Weather and precipitation for the remainder of winter and this spring will determine whether or not drought conditions continue to improve or deteriorate.

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Long Range Outlook

The outlook for March through May favors above normal temperatures across Idaho. The precipitation outlook slightly favors wetter than normal conditions over far southeast Idaho, but does not shift the probabilities one way or another for the rest of the state.

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On-line Resources

Water Supply Volume Forecasts…
National Weather Service-Northwest River Forecast Center www.nwrfc.noaa.gov/ws/
National Weather Service-Colorado Basin River Forecast Center www.cbrfc.noaa.gov/
USDA-Natural Resources Conservation Service www.nrcs.usda.gov/wps/portal/nrcs/main/id/snow/

For a comprehensive report on each river basin in Idaho, see the Natural Resources Conservation Service report:   Idaho Water Supply Outlook Report March 1, 2019

Snowpack Information…

National Weather Service-Northwest River Forecast Center www.nwrfc.noaa.gov/snow/
National Weather Service-National Operational Hydrologic Remote Sensing Center www.nohrsc.noaa.gov/
USDA-Natural Resources Conservation Service www.nrcs.usda.gov/wps/portal/nrcs/main/id/snow/

Reservoir Storage…

Bureau of Reclamation Reservoir Storage  www.usbr.gov/pn/hydromet/select.html
USDA-Natural Resources Conservation Service https://www.wcc.nrcs.usda.gov/basin.html

Drought Information…

U.S. Drought Portal www.drought.gov

Peak Flow Forecasts…

Northwest River Forecast Center www.nwrfc.noaa.gov/peak/
Colorado Basin River Forecast Center www.cbrfc.noaa.gov/rmap/peak/peaklist.php

Temperature and Precipitation Outlook…

Climate Prediction Center  www.cpc.ncep.noaa.gov/