June 2018 Climate Statistics

June`s average temperature was one degree above normal, giving a ranking of 23rd warmest in the 79 years of Boise Airport records. Highs reached or exceeded 90 degrees on 6 days, which is exactly normal for June. There were no record highs or lows.

It was a month of changeable temperatures, owing to a procession of upper level troughs and ridges from the north Pacific. Highs ranged from 63°F on the 17th to 94°F on the 25th.

It was one of the drier Junes, with only a quarter inch of precipitation. Along with June 1957, it ranks 20th driest at the Boise Airport.

Measurable rain fell on only four days: the 7th, 9th, 17th, and 18th. With the exception of the 17th, rainfall totals were under a tenth of an inch.

On the 14th a trough from the Gulf of Alaska stalled over the northwest states. Cooler air aloft flowing into the trough from western Canada caused it to intensify, and it strengthened further as it entrained moisture from east of the Rockies. On the 17th, 0.13 inch of rain fell at the airport, but much heavier amounts were reported in northern and eastern Idaho and portions of the central Idaho mountains between the 16th and 18th.

Strong winds were observed on three occasions early in the month. At 7:30 pm on the 3rd, outflow from a shower ahead of a strong cold front produced a gust of 53 mph from the south, resulting in blowing dust reducing visibilities. At 6:25 pm on the 7th, outflow from a thunderstorm gusted to 41 mph from the southeast, accompanied by brief moderate rain. And at 12:50 pm on the 9th, a gust of 43 mph from the northwest followed another strong cold front.

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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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Idaho Spring Flood and Water Resources Outlook

M1The potential for spring flooding due to snowmelt is elevated for portions of eastern Idaho and northern Idaho. The spring flood risk for the rest of the state is average or below average. There remains an elevated risk of spring flooding across portions of the Upper Snake Basin due to above average mountain snowpack. This includes the mainstem Snake River above American Falls Reservoir and the Henrys Fork near Rexburg. An elevated flood risk also exists across the Panhandle Region where some of the highest snowpack percentages in the state reside. Elsewhere across Idaho, early May snowpack conditions suggest a low probability of spring flooding due to snowmelt. 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 where drought conditions exist, or that have low snowpack, spring flooding is possible under the right conditions. Additionally, burn scars can have a significant impact on local flood potential during spring snowmelt and rain events.M2

Water Supply

National Weather Service April through September water supply volume forecasts for northern and eastern Idaho, and the mainstem Snake River across southern Idaho range from 115 to 150 percent of average. Elsewhere, water supply forecasts are 85 to 110 percent of average for most of the Central Mountains and only 30 to 70 percent of average for south central and southwest Idaho. The lowest forecast percentages are in far southwest Idaho in the Bruneau and Owyhee River Basins at less than 40 percent of average.M3

Temperature and Precipitation

As of May 1, temperatures for the 2018 Water Year have been average or slightly below average across the northern half of the state while most of southern Idaho has experienced a little above average temperatures. Water Year precipitation was above average for the Panhandle, Spokane, and Clearwater Basins. The Clearwater Basin had the greatest anomalies in the state at 130 to 150 percent of average. The Salmon Basin and Upper Snake Basin near the Wyoming border were generally 100 to 130 percent of average. Elsewhere in southern and southwest Idaho the Water Year precipitation was mostly in the 70 to 90 percent of average range with pockets of around 60 percent.

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Snowpack

As of May 2, the highest snowpack percentages in the state ranged from 138 to 145 percent of median in the Clearwater, Spokane and Northern Panhandle Region. Not far behind were basins in eastern Idaho such as the Little Lost and Birch Basins, Henrys Fork, Teton, and Snake Basin above Palisades at 120 to 130 percent. The Payette, Boise, Salmon, Wood and Lost Basins ranged from 74 to 109 percent of median. Southside Snake River Basins along the Nevada border were a mixed bag ranging from a low of 17 percent of median in the Owyhee Basin to a high of 86 percent of median in the Raft River Basin. Northern Idaho basins and those near the continental divide reached their peak snowpack in mid to late April. The low elevation snow is gone and melting of the high elevations will increase over the next few weeks as temperatures warm.

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Reservoirs

Reservoir storage across Idaho is in good shape. As of May 1, storage in major reservoir systems throughout Idaho was 100 percent of average or greater, except where systems were heavily drafted to make space for anticipated snowmelt runoff. Weather patterns and irrigation demand will continue to drive operations through late spring as reservoirs are topped off. Wet spring weather or extended periods of warmth resulting in rapid snowmelt and large reservoir inflows could result in significant fluctuations in reservoir discharge and downstream river levels.

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Drought

Idaho is currently free from any official drought classification according to the U.S. Drought Monitor. However, below average precipitation for the Water Year and poor snowpack has put much of southern Idaho in the abnormally dry category. Weather and precipitation for the remainder of spring will determine whether or not conditions improve or deteriorate for areas experiencing the dryness. Good reservoir storage will ease drought concerns for those served by major storage projects.

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Long Range and Seasonal Outlooks

The outlook for May favors above normal temperatures across the state. The May precipitation outlook favors above normal precipitation for southeast Idaho and below normal precipitation for the northwest half of the state. The seasonal outlook for June through August favors above normal temperatures and below normal precipitation.

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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/

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/

 

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

Spring Snowpack and Flood Outlook – April Update

The potential for spring flooding due to snowmelt in 2018 is well above average across most of northern and eastern Idaho and Montana. Meanwhile, the spring flood potential is slightly below average for southwest Idaho and southeast Oregon.

The storm track over much of the winter brought above normal precipitation values to much of the region. However, it was a tale of two cities on the northern end (cooler) and the southern end of the storm track. Cooler conditions on the northern portion of the storm track led to above normal snow values across Washington, northern Idaho, and western Montana, while the southern end of the storm track brought warmer conditions to Oregon, southwest Idaho, Nevada and Utah, leading to a below normal snow pack. The following graphic illustrates the amount of snow pack compared to normal for mid April.

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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.