April 2018 Climate Statistics

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April was characterized by changing weather patterns. Migrating high pressure ridges and low pressure troughs at upper levels brought alternating periods of above and below normal temperatures, but overall there was a gradual warming trend, and the monthly average was slightly above normal.

Most precipitation fell during the first 16 days of the month, when half the days had measurable rain. The final 14 days were dry, except for a couple of trace events. As a whole, April was slightly drier than normal.

The first half of the month also saw three strong wind events. Boise fared somewhat better than the surrounding region, where there was local damage, as winds in the Treasure Valley weren`t quite as strong.

On the 2nd a rapidly moving upper level trough from the Gulf of Alaska crossed our area. It was accompanied by a strong jet stream, and a thunderstorm which dropped small hail on some parts of the Treasure Valley.  Some of the jet energy reached the surface as the cold front came through, producing a gust of 49 mph at the airport.

On the 7th another cold front crossed the Boise area, propelled rapidly inland by an initially strong upper level trough approaching the Washington coast.  The front triggered thunderstorms, one of which generated a gust of 55 mph from the northwest at the airport.  Over a quarter inch of rain was measured with this storm. The trough itself faded out as it progressed inland.

On the 12th a deeper, colder trough moved over our area. Two tenths of an inch of snow fell at the airport, but it melted quickly.

On the 16th an even deeper trough pushed inland, accompanied by a quarter inch of rain and a trace of snow.  That system was followed by a more settled, warmer period, with temperatures near or above normal from the 19th through the 28th. On the morning of the 18th another weather system was poised just off the northwest coast, but instead of heading for Boise, it went south to California on the 19th then east over the Colorado plateau on the 20th.

Our area was dominated by high pressure from the 20th through the 27th.  On the 27th the temperature maxed out at 90°F, two degrees shy of the record.

The early taste of summer was ended by a cold front on the 28th, followed by a slow moving upper level trough on the 29th.  The combination of cold air aloft and surface heating in the trough created instability which triggered convective showers but only traces of rain at the Boise Airport. The trough remained on the 30th and was expected to linger a few more days.

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Interested in measuring precipitation? Join the CoCoRaHS observing network.

Join CoCoRaHS Today!cocorahs

CoCoRaHS is a practical, enjoyable and useful activity. If you have an interest in weather and would like to help your local community, as well as scientists and others interested in precipitation, then CoCoRaHS is for you. It only takes a few minutes a day and gives you the chance to participate in real hands-on science. You’ll be amazed at what you learn as you become more aware of the variable weather that impacts you, your neighbors, your state and our entire country.

Data on the web

Volunteers submit their observations using the CoCoRaHS website or apps. Observations are immediately available to the public via maps and data analysis tools, and to data users via the CoCoRaHS Web API. Data users such as scientists, resource manages, decision makers and others have come to rely on the high density, high quality measurements provided by CoCoRaHS observers.

CoCoRaHS is Educational

CoCoRaHS offers learning opportunities too. In addition to training materials, newsletters and the ‘Message of the Day’, members also enjoy opportunities to attend Webinars featuring experts in weather, climatology and other pertinent disciplines. CoCoRaHS offers classroom resources for K-12 teachers. Students get to collect and submit real scientific data – all while meeting State and National Standards in science, math, geography and more!

What is CoCoRaHS?

The Community Collaborative Rain, Hail and Snow Network, is a non-profit, community based, network of volunteers who measure and report rain, hail and snow in their backyards.

A brief History

CoCoRaHS came about as a result of a devastating flash flood that hit Fort Collins, Colorado in July 1997. A very localized storm dumped over a foot of rain in several hours while other portions of the city had only modest rainfall. The ensuing flood caught many by surprise, caused $200 million in damages, and resulted in five deaths. CoCoRaHS was born in 1998 with the intent of doing a better job of mapping and reporting intense storms. CoCoRaHS became a nationwide volunteer network in 2010 and is now international with observers helping provide critical precipitation observations, benefiting their country’s needs.

Volunteers of all ages welcome!

Individuals and family volunteers of all ages and all walks of life are the foundation of the CoCoRaHS network, Anyone can help. It only takes a few minutes to check the rain gauge and report your observations.

Training: “the Key to our success”

It is important that all CoCoRaHS precipitation reports be accurate and consistant. Training is provided on how to install gauges, properly measure precipitation and transmit reports. CoCoRaHS precipitation reports are accurate and very useful.

Why is there so much interest in rain, hail and snow?

Precipitation is essential for life. It varies greatly with topography, storm type and season. It really is true that it may pour on one side of the street and be dry on the other. A portion of a field may be pounded by hail while others nearby receive no damage. Snowfall may pile up in one neighborhood and only dust another. Rain, hail and snow are fairly easy to measure, and the data collected are very important. Meteorologists, hydrologists, engineers, builders, farmers . . . you name it, everyone seems to care about rain, hail and snow. That’s why we ask, “How much fell in your backyard?” There are limited observations across southwest Idaho and southeast Oregon, compared to the rest of the country, so we would love to have your observations. To learn more about the CoCoRaHS program and to see where your fellow observers have recorded rain amounts, visit http://www.cocorahs.org/.

Invite your neighbors, relatives and friends by sending them this “Join” link: http://www.cocorahs.org/application.aspx

March 2018 Climate Statistics

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The changeable weather of March 2018 was typical of late winter and early spring. The progression of low pressure troughs and high pressure ridges brought alternating periods of warm and cold temperatures, yielding a close-to-normal average. There were no record highs or lows.

Episodes of active weather and moderate precipitation occurred as each trough and its accompanying cold front entered our area.

The 0.54 inch rainfall on the 14th tied the record for the date set in 1927. The 1.89 inches for the month was 0.50 inch above normal and ranks in the wettest 20 percent of Marchs at the airport, and the wettest 25 percent at all Boise locations.

Snowfall for the month was 2.6 inches, mostly falling on the morning of the 25th. The monthly normal is 1.3 inches.

The first four days of March were dominated by an upper level trough which brought cold temperatures but little precipitation. The 23°F on the 4th was the low for the month.

Temperatures rose above normal on the 8th as an upper level ridge began to build over the region.

The ridge reached its maximum amplitude on the 12th, allowing temperatures to rise into the low 60s. As the ridge began to exit, southerly flow ahead of the next upstream trough pushed the temperature to 69°F on the 13th, the high for the month.

The trough sent a strong cold front inland, and convective showers ahead of the front were preceded by northwest winds gusting over 30 mph. When the showers reached the Boise airport just after 4 am, moderate rain began and persisted for about an hour before tapering off and ending. The trough drifted slowly inland on the 16th and 17th, preventing temperatures from rising above the upper 40s on those days.

The last weather maker of the month was another strong upper level trough which stalled off the coast of British Columbia on the 22nd.  This trough extended far enough south to tap subtropical moisture.

Like its predecessor, moderate rain preceded the cold front on the afternoon of the 22nd. The front passed the airport at about 7:30 pm, accompanied by the first thunderstorm of the spring season.  The strongest convection and heaviest rain stayed west and north of Boise.

As the trough moved inland and weakened on the 25th, it produced the last measurable precipitation of the month, falling as 2.1 inches of snow. The cold north pacific air was responsible for a high of only 45°F that day, the chilliest maximum since the start of spring.

Following this system, west-northwest flow aloft returned temperatures to near normal from the 28th through the 31st.

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February 2018 Climate Stats

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February 2018 can be described as three seasons in one month.

It began with early spring. The 1st through the 9th had the same average temperature as the last half of a normal march. The 59°F on the 2nd tied the record for the date set in 1881. The 61°F on the 3rd eclipsed the old record of 60°F  in 1953.

Temperatures from the 10th  through the 18th were near normal for late winter.

It was back to mid-winter from the 19th through the 28th, which averaged colder than a normal December.

The average temperature for the entire month was deceptively close to normal.

February is usually the driest winter month at Boise. This February had only 56 percent of normal precipitation, despite heavier than average snowfall. Measurable precipitation fell on only 4 days.

Snowfall totaled 7.4 inches, over twice the normal 2.8 inches. There was at least patchy snow cover each day during the last week of the month. The 4.9 inch snowfall on the 22nd set a record for the date.  The old record was 4 inches in 1912. The greatest snow depth at the Boise Airport was 5 inches on the 22nd.

A warm upper level high pressure ridge off the coast was responsible for the spring-like weather from the 1st through the 9th. It also kept most of the precipitation associated with Pacific weather systems east of our area.

The ridge gradually shifted west, allowing northwest flow aloft to transport cooler but not unseasonably cold air into the Intermountain region from the 10th through the 18th.  One weather system embedded in the flow was strong enough to bring Boise a quarter inch of precipitation on the 14th, including half an inch of snow.

The ridge continued to drift west as a very cold upper level trough deepened over western Canada. Strong northerly flow on the west side of an upper level low over Hudson Bay pushed the trough south of the border. The cold front ahead of the trough passed Boise at about 9 pm on the 17th. Following the front, the wind increased from the northwest, and a peak gust of 43 mph was recorded at the airport just after midnight.

Temperatures initially were not unseasonably cold, but by the 19th modified arctic air had begun to filter into the Treasure Valley, while the true arctic front was stalled in the central Idaho mountains.

Although the coldest air stayed well to our north and east, clear skies, light winds, and very dry air allowed the temperature to drop into the single digits in the Boise area by sunrise on the 20th. The low of 9°F at the airport was the coldest since the 8 degrees on December 24, 2017.

The cold pattern persisted through the end of the month, with a deep upper level trough anchored over the western U.S. and western Canada. Temperatures remained unseasonably cold, and weather disturbances moving into the trough from Alaska brought periods of snow.

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Idaho Spring Flood and Water Resources Outlook

The potential for spring flooding due to snowmelt is slightly elevated for portions of eastern and north central Idaho. The spring flood risk for the rest of the state is average to below average. SWE-2-12

Good soil moisture recharge from autumn rains and well above average reservoir storage has resulted in a slightly elevated threat of spring flooding along the mainstem Snake River in eastern Idaho, and along smaller tributaries above Idaho Falls. Spring flood risk is also slightly elevated in the Clearwater Basin which is currently holding one of the greatest snowpack percentages in the state. Elsewhere, the absence of low elevation snow and areas of below average mid elevation snow suggest an average or below average spring flood threat.

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 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.

Precipitation and Temperature

As of early February, Water Year precipitation was near normal or above normal for the Panhandle, Spokane, Clearwater, and Salmon Basins, as well as the Upper Snake Basin near the Wyoming border. The Clearwater Basin had the greatest anomalies at 130 to 150 percent of  average. Aside from the Snake River headwaters region, Water Year  precipitation across southern Idaho stood at 60 to 80 percent of  average with south side Snake River Basins having the lowest  percentages.  Average temperatures for the Water Year have been above average for almost the entire state, especially across southern Idaho. 1

 

Snowpack

As of February 12, the highest snowpack percentages in the state were 114 and 116 percent of median in the Clearwater Basin and the Upper Snake above Palisades. Percentages were 91 to 107 percent of median for basins along the Montana border in eastern Idaho, and the Salmon, Spokane, and Panhandle Basins. Elsewhere in south central and southeast Idaho the snowpack was 57 to 81 percent of median, decreasing to 33 to 49 percent of median in the Owyhee and Bruneau Basins in southwest Idaho. Daily snowpack readings indicate record low levels for a handful of SNOTEL locations in southern Idaho.  Mountain snowpack in Idaho typically builds through March. Early April snow conditions will be pivotal to water supply conditions through the summer.  23

Reservoirs

Reservoir storage across Idaho is in good shape. Major reservoir systems across the northern half of the state were holding near average or above average storage as of February 1. Across the southern half of the state, with the exception of Brownlee at 85 percent and Mann Creek at 45 percent of average, most major projects had well above average storage which is great news considering the below average snowpack in many basins. 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 record setting precipitation and snowpack last year, abnormally dry conditions have returned to portions of west central and southern Idaho. Weather and precipitation for the remainder of winter and this spring will determine whether or not conditions improve or deteriorate for areas experiencing short term dryness. Good reservoir carryover will help ease drought concerns for those served by major storage projects.

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Long Range Outlook

The outlook through the end of February favors below normal temperatures and normal or below normal precipitation for the state. The outlook for March, April, and May favors below normal temperatures for the Panhandle, and above normal temperatures across far southern Idaho. The precipitation outlook for March, April, and May slightly favors wetter than normal conditions for the Panhandle, but does not shift the probabilities one way or another for the rest of the state.

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Water Supply Forecast

National Weather Service April through September water supply volume forecasts vary from 90 to 135 percent of average for most of the central Idaho mountains and north across the Clearwater and Panhandle regions. In southern Idaho, forecasts for the Big Lost Basin, the mainstem Snake River and tributaries above American Falls range from 80 to 125 percent of average, with the exception of Willow Creek near Ririe with a forecast of 38 percent of average. Forecasts for the rest of southern Idaho range from 39 to 73 percent of average with the lowest percentages in southwest Idaho. These forecasts may change considerably over the next couple of months since seasonal snow accumulation and rainfall typically occur during February, March, and April.

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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

U.S. Drought Monitor
www.droughtmonitor.unl.edu/

National Drought Mitigation Center
www.drought.unl.edu/

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/

January 2018 Climate Stats

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January 2018 was the fourth warmest on record at the Boise Airport.  For all Boise locations it tied January 1914 for sixth place. The monthly average of 37.8 degrees was 6.5 degrees above normal, and it even exceeded February’s normal of 36.5 degrees. Only the first four days of the month were below normal.

It is noteworthy that, prior to this year, the top five warmest Januarys at the airport occurred during El Niño winters, and three of those were strong El Niños.  January 2018 didn’t fit the pattern, as a weak La Niña was in effect.

There were two record highs, each occurring ahead of strong cold fronts. The 58°F on the 18th and the 59°F on the 30th broke the previous daily records of 57°F in 1998 and 56°F in 1992.

The unusually mild temperatures resulted from a combination of a progressive pattern and a persistent warm upper level high pressure ridge over the Western U.S.

Westerly flow aloft brought mild moist Pacific weather systems inland across the northern intermountain region, temporarily displacing the ridge, which always managed to rebuild. The active pattern hindered the formation of temperatures inversions and blocked invasions of cold air from western Canada.

Precipitation totaled 1.36 inches, close to the January normal of 1.39 inches. Most of it fell during two episodes.

On the 9th a cold front produced nearly half an inch of rain at the Boise Airport.  During the afternoon of the 19th another cold front generated nearly a quarter inch of rain. The upper level low pressure trough which followed the front brought an additional two tenths of an inch of rain.

On the 30th the last cold front of the month passed the airport at 1:40 pm MST, with a wind gust of 34 mph from the northwest, but no precipitation.

The only measurable snowfall was 0.1 inch on the 25th. The total for January 2017 was 21.5 inches. Normal is 5.1 inches.  There have been Januarys with less snow. Only traces fell in 1934, 1961, and 2003.

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McCall, Idaho recorded its 4th warmest January on record in 2018.  Daily average minimum temperatures in January 2018 averaged 21.4°F, 3rd warmest on record.

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Burns, Oregon recorded their 6th warmest January on record in 2018.

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Baker City, Oregon had their 3rd warmest January on record in 2018.  Average maximum temperatures reached 42.9°F, tied for the warmest on record in January since records began in 1928.

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Jerome, Idaho had their 3rd warmest January on record since 1916.  Two record highs were set in the month of January.  Average minimum temperatures tied for the second warmest on record in January at 28.5°F.

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Ontario, Oregon had their 4th warmest January on record.  A daily record high temperature of 54°F was recorded on January 12.

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With the upper level high pressure ridge over the Great Basin,the storm track was directed to the north across British Columbia, Washington, northern Idaho, and western Montana, leading to warmer than normal temperatures.

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December 2017 Climate Stats

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Temperature inversions plagued the Treasure Valley most of the month, resulting in below normal temperatures at Boise on all but eight days. Although it ranked among the coldest 25 percent of Decembers since airport records began in 1940, it averaged 5 degrees warmer than December 2016.

A persistent high pressure ridge kept the Boise area dry through much of the month. Precipitation was nearly half an inch below normal, and half the December 2016 total.

The month started out fairly mild under west-southwest flow aloft ahead of an upper level low pressure trough. The trough crossed the Boise area on the 3rd, bringing the largest one-day precipitation of the month, mainly in the form of rain.

As this system exited to the east, an upper level high pressure ridge built offshore and expanded inland.

By the 5th an inversion had become established in the Treasure Valley, and temperatures remained below freezing from the 7th through the 15th.

On the 16th an upper level trough destabilized the atmosphere enough to break the inversion, and temperatures continued to warm as the upper level flow shifted into the west and southwest in advance of tet another upper level trough. The 19th and 20th were the warmest days of the month, with highs of 52 and 51.

By the 21st the trough had moved east of our area. It was followed by moist northwest flow aloft and a cold front which brought snow totaling 6 inches on the 22nd and 23rd. The accumulation of 5 inches was the first measurable snow cover since February 4.

A combination of modified arctic air, overnight clearing, fresh snow cover, and calm winds caused the airport temperature to fall to 8 degrees by sunrise on the 24th, the coldest reading since the 7 degrees on January 18.

More snow was on the way. It started at about 4 pm Christmas eve as a warm front approached from the west. By Christmas morning 3 inches of new snow had fallen.

Warming continued at higher elevations, ensuring that cold air would remain trapped in the valley for a few more days.

On the 30th a cold front passed through the area. It was felt as a warm front in the valley, as colder air aloft behind the front broke the inversion, allowing the airport temperature to climb to 43 degrees that afternoon.

On the 31st strong high pressure provided mostly clear skies.  Although a shallow inversion had formed overnight, the sun warmed the surface enough during the day to mix the air, and temperatures around Boise warmed into the 35-40 degree range.

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Precipitation across much of southern Idaho and Oregon was well below normal as a persistent upper level high pressure led to an inversion with cool conditions in the valleys and warm conditions in the mountains.

November 2017 Climate Stats

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November`s average of 43.8 degrees is the tenth warmest at the Boise Airport. Aside from a cool spell from the 4th through the 8th, temperatures averaged above normal on most days. Only ten nights had lows were below freezing. The low of 47 on the 22nd set a record high minimum for that date, eclipsing the old record of 46 set in 1921.

Measurable rain fell on half the days. The monthly total of 1.77 inches is nearly half an inch above normal and ranks among the wettest 25 percent of Novembers at the airport.

Typical of fall, the weather pattern was active, with passing weather systems bringing showery periods and changeable temperatures.

On the 3rd, a low pressure trough from British Columbia brought Boise the coolest weather since last winter, and over half an inch of rain from the 3rd through the 6th. Below normal temperatures persisted through the 8th. The low of 25 on the 7th was the low for the month and the coldest reading since February 25.

Warmer more seasonable weather returned on the 9th. A weak high pressure ridge over the western U.S. and a persistent low pressure trough off the northwest coast kept us under southwest flow aloft, maintaining near normal or slightly above normal temperatures through the 20th.

Weather disturbances moving inland weakened as they traversed the ridge, but they retained enough moisture for moderate amounts of rain on the 10th, and again on the 14th and 15th.

On the 21st an unseasonably warm high pressure ridge built over the Desert Southwest and northwest Mexico, creating a source of warm air for the northern intermountain region. From the 20th through the

27th, Boise’s daily average temperatures ranged from 6 to 19 degrees above normal.

Moisture and weak weather systems moving through the north portion of the ridge generated nearly daily showers as far south as northern Nevada. Amounts were mostly light, but a stronger disturbance on the 24th produced a quarter inch of rain at the Boise airport.

The ridge weakened following a Pacific weather system which crossed the great basin the 27th, and temperatures at Boise returned to near normal on the 29th following a Pacific cold front.

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Idaho 2017 Water Year Summary

Overview

The 2017 Water Year will go down as one to remember due to record precipitation, record snowpack, and significant flooding that impacted the state. Extreme wet conditions resulted in a number of disaster declarations due to snow, flooding, and landslides, along with runoff volumes not seen for decades across portions of southern Idaho.

The Water Year started off with a bang as a series of Pacific storms brought record precipitation to the region in October 2016, recharging the soil moisture and setting the stage for abundant spring runoff. Winter was highlighted by very cold temperatures across the state, and extreme snowpack across much of southern Idaho.  As temperatures warmed during late winter and early spring, low and mid elevation snowmelt combined with rain, on top of saturated and frozen soils, led to widespread flooding across the Magic Valley, the Upper Snake Plain, and extensive flooding and landslides across northern Idaho.  This was followed by significant flooding on several main stem rivers and tributaries of central and southern Idaho later in the spring.  Water managers were challenged by the extreme runoff and many regulated and unregulated river systems experienced prolonged high flows and flooding. On a positive note, reservoir systems were filled to capacity resulting in plenty of water for the growing season and above average carry-over heading into 2018.

Despite the cold winter, compared to the 30 year normal, temperatures over the entire 2017 Water Year were above normal for most of Idaho with the greatest anomalies across the southern half of the state.  Water Year precipitation was 110 to 200 percent of normal for almost the entire state. Near the end of the snow accumulation season, snowpack was around 100 percent of normal in northern Idaho while many basins in central and southern Idaho were carrying 150 to 200 percent of normal snowpack. Despite all the water the state received, hot and dry summer weather allowed for a return of moderate drought to portions of central and northern Idaho with streamflows in some basins falling below normal by summers end.

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Temperature

Despite one of the coldest winters in recent decades, temperatures experienced over the entire 2017 Water Year ranked either above or much above normal.  After a relatively warm fall, well below normal temperatures persisted across low elevations for most of the winter months which allowed the low elevation snowpack to accumulate to unprecedented levels in some of the lower valleys. For some locations in northern Idaho it was the coldest winter in over 30 years. The extreme cold led to ice jams which resulted in flooding on some rivers and streams across the state.  For the entire water year, statewide temperature anomalies generally ranged from minus 1 to plus 3 degrees (F). Last year’s anomalies ranged from plus 2 to plus 4. Above normal temperatures during late winter and spring had a significant impact on the ripening of the snowpack which led to extreme melt rates and flooding in many areas. Had it not been for below normal temperatures experienced during the early winter months, anomalies for the year as a whole would be greater.

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Precipitation

Fall rains were well above average across most of the state, excluding the southwest corner. Record setting rainfall in October was key in recharging soil moisture and setting the stage for an excellent runoff season in the spring. An active winter ensued with percentages varying between 150 and 300 percent above normal across almost the entire state. This was largely due to numerous atmospheric river events that battered the Pacific Coastline, bringing ample moisture to the region. The first and second quarters of the 2017 Water Year therefore ended on a high note. Spring and summer precipitation percentages were largely lower than in the previous months, save portions of the Central Mountains, Eastern Magic Valley and the Upper Snake Plain, which still received above average precipitation. Summertime thunderstorm activity was somewhat limited for most of Idaho. South central and southeast Idaho was the exception, with bouts of monsoonal moisture bringing normal or above normal precipitation.

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Snowpack

The 2017 Water Year snowpack was quite healthy and hit record levels across portions of southern Idaho. Winter storms brought heavy snow to southern Idaho in December and January, not only to the mountains, but to the lower valleys as well. Southern Idaho snowpack ranged from around 100 to 160 percent of median by February, while northern Idaho snowpack was lagging at around 80 percent.  Snowpack in the Big Wood, Little Wood and Big Lost Basins reached 180 to 190 percent of median by early March; daily records were seen at many SNOTEL sites in the central Idaho mountains. By April 1 (when Idaho’s overall snowpack typically peaks), basin snowpack ranged from 95 to 170 percent of median statewide, with the highest percentages in southern Idaho. Runoff from low and mid elevation snowmelt in February and March caused widespread sheet flooding in the Magic Valley and across portions of the Upper Snake Plain. Rapid snowmelt, record rainfall, and saturated soils led to extensive flooding and landslides across northern Idaho in March and early April.  Record or near record high elevation snow and extreme melt rates led to prolonged flooding on many rivers and streams across the central mountains during the spring.

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Reservoirs

Wet fall weather and a large winter snowpack made for a challenging spring for reservoir operations.  Not only was the snowpack well above normal, but extreme snowmelt rates occurred leading to very large inflow peaks on some of the reservoir systems. Operators for large federal reservoirs and smaller privately owned reservoirs were taxed in keeping up inflows and flood control space. Some of the smaller privately owned reservoirs in southern Idaho were pushed to the brink on their reservoir capacity. Flood flows occurred below a number of reservoirs in southern Idaho, with 101 consecutive days of flood flows below Lucky Peak Dam on the Lower Boise River. Owyhee Reservoir was filled to capacity for the first time since 2011. By late spring, flood control operations ceased, reservoirs were full, and an abundant supply of water was available for the growing season along with above average carry-over in most systems for the start of the 2018 Water Year. Runoff volumes ranked near the top of historical records across most of southern Idaho, with record volumes at some locations fed by central mountain snowmelt.

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Streamflow

Record October precipitation pushed monthly streamflows above average across much of central and northern Idaho in the fall of 2016. Cold temperatures and low regulated flows, typical of winter, held streamflows at or below normal across most of Idaho in December and January.  A dramatic increase in streamflows occurred in February and March as warming temperatures initiated low and mid elevation snowmelt which was accompanied by periods of rain. High elevation snowmelt sustained the high streamflows through the spring and even through the summer across portions of central and southern Idaho.  Hot and dry weather allowed some basins to recede below normal during the summer months, particularly across northern Idaho.

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Drought

Wet weather during the fall and winter eliminated the abnormally dry conditions and pockets of moderate drought that were present at the beginning of the 2017 Water Year. However, above normal summer-time temperatures and below normal precipitation allowed areas of drought to redevelop across portions of central and northern Idaho by summers end.

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October 2017 Climate Stats

October 2017 ranks among the coldest 22 percent of Octobers at the Boise airport. Temperatures averaged 7.1 degrees below normal from the 1st through the 16th, and 1.3 degrees above normal for the rest of the month. Precipitation totaled near normal. The first freezing temperature of the season was the 31°F on the 9th, a day ahead of the average date.

There were no record highs, lows, or daily precipitation amounts.

During the first half of the month, cold upper level troughs were the dominant features. Brief periods of west or southwest flow aloft raised temperatures to near normal on the 6th, 7th, and 10th, but there were no above normal daily averages.

During the last half of the month, temperatures were near or above normal most days, thanks to west or southwest flow aloft, and strong upper level high pressure ridges on the 23rd and 24th, and again on the 27th and 28th.

This warmer pattern was briefly interrupted by a north Pacific cold front which crossed the Boise area around 4:30 am on the 20th. The frontal passage was accompanied by northwest winds gusting up to 42mph at the airport, causing visibility restrictions in blowing dust. Rain settled the dust a couple of hours later as a cold upper level trough pushed inland behind the front. Precipitation on the 20th and 21st totaled 0.37 inch at the airport. One location closer to the foothills reported around half an inch of rain.
On the 29th, the upper level ridge, which had kept Boise warmer than normal on the 27th and 28th, shifted west. This put our area under northwest flow aloft, allowing cooler air from British Columbia to spread south across the northern intermountain region.