February 2017 Climate Stats

Feb-2017

Spring-like weather paid Boise and the rest of southeast Oregon and southwest Idaho an extended visit in February.  Starting the day after Groundhog Day, temperatures were above normal most days for almost three weeks, and eleven nights had lows above freezing at Boise.

By the 5th the snow had melted down to a trace, ending 52 days of continuous snow cover of an inch or more. This was the 5th longest period on record, exceeded only during four of the infamous winters of the 1980s.

February`s mild weather was the result of upper level winds from the wouthwest and west. This pattern transported enough moisture inland from the Pacific for almost daily showers, including 2 inches of snow on the 7th, which quickly melted.

Offshore, an upper level trough was poised to move inland, and it finally did on the 11th. It brought cooler air, but no precipitation for Boise as it headed south to California and northwest Mexico.

Following this trough, an upper level high pressure ridge built over the northwest states.  Cool air left behind by the trough was capped by warmer air aloft in the ridge, forming a shallow temperature inversion.

Enough moisture was present in the valley for the formation of late night and morning fog from the 13th through the 16th.

By the 16th the ridge had moved east, leaving strong southwest flow aloft ahead of yet another upper level trough. A weak disturbance moving through this flow brought enough instability and wind to break the inversion, and by afternoon the temperature had rebounded to above normal. The high of 58 degrees at the Boise Airport that day was the warmest reading of the month.

Before the trough moved inland on the 22nd, more weak disturbances brought daily showers, and the first thunder since October 2016 was heard on the 16th and 19th.

The trough lingered over the inter mountain region from the 22nd through the end of the month, keeping temperatures below normal.

Highs failed to rise above the 30s from the 23rd through the 27th, and snow flurries were an almost daily occurrence. On the 28th a disturbance from the Gulf of Alaska strengthened as it moved into the trough. It generated an inch of snow which covered the ground early that morning, but it was gone by the end of the day.

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January 2017 Climate Stats

Jan-2017

January was cold, snowy, and unusually foggy.

It ranked as the 9th coldest January at the Boise Airport and the 14th coldest since records began. Highs reached 32 degrees or higher on only 12 days. Lows fell below zero on 5 nights. The high temperature of 6 above and the low of 11 below on the 6th  set new records for that date.

The lows of 11 below on the 6th and 10 below on the 7th were the coldest temperatures since the low temperature of 25 below on December 22, 1990. Since then temperatures dipped to zero or below only five times. The coldest reading during the 1991-2016 period was 7 below on December 9, 2013.

It was the snowiest month since December 1983, when 26.2 inches fell. The snowiest month on record was January 1929, with 27.0 inches.

There was fog on 26 days, 6 of which had dense fog, with visibilities a quarter mile or less.

On the 1st an upper level high pressure ridge over the Gulf of Alaska extended north over Alaska, enabling northerly flow aloft to transport weather systems and cold air south from Alaska and the western Canadian arctic.

From the 1st through the 5th an upper level trough, which originated over the gulf of Alaska, presided over the Pacific Northwest states.  It brought Boise 9 inches of snow, increasing the snow depth to 15 inches on the 5th, the most since snow depth records began in 1940, as arctic air moved into the region from British Columbia.  The 0.45 inch precipitation on the 4th set a new daily precipitation record for the date. It came in the form of 6.5 inches of snow, the 15th highest calendar day snow total since 1892.

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The colder air, along with clearing skies and fresh deep snow, provided ideal conditions for radiational cooling and subzero temperatures.

On the 8th southwest flow brought a period of relatively mild weather. Winds were strong enough at all levels to prevent an inversion from forming, and temperatures actually averaged above normal from the 8th through the 11th.

The cold pattern returned on the 12th as an upper level trough deepened over the region. The trough departed for California and Baja on the 13th, but cold air remained trapped in the Treasure Valley as an upper level ridge brought warming aloft, forming a temperature inversion. The 5 inches of snow cover hindered daytime warming and guaranteed cold overnight temperatures. In this “Homemade” arctic air, highs were only in the teens and lows were near zero from the 15th through the 17th.

Milder weather returned from the 19th through the 22nd under southwest flow aloft, thanks to an upper level low pressure system off the northwest coast.

As the low moved inland on the 23rd, it brought Boise 3 more inches of snow along with colder air, which as usual became trapped in the valley.  Another temperature inversion formed and intensified as an upper level ridge built over the northwest U.S.  Valley temperatures were not at cold as earlier in the month, but still averaged about 10 degrees below normal.

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December 2016 Climate Stats

Dec-2016

December 2016 was the 5th coldest at the Boise Airport, where records go back to 1940. Only 1983 through 1985 and 1990 had colder Decembers.  For the greater Boise area, going back to 1864, it ranks seventh all time.

During the first 15 days of December, half the days had above normal temperatures. There was one cold spell from 5th through the 9th, when a brief invasion of arctic air resulted in lows in the teens and highs below freezing. This previewed what was to come during the last half of the month.

Starting on the 10th, westerly flow aloft brought milder Pacific air to the region. Highs reached the middle 30s to lower 40s.

On the morning of the 14th, a temperature inversion had developed, with lows in the middle 20s in the valley. Moist air flowing over the top of the inversion dropped around 2 inches of snow on the valley floor by the morning of the 15th.

By the 16th, northerly flow over western Canada sent colder air into the intermountain region, insuring that the snow in the Boise area would not melt. Snow cover hindered daytime warming and enhanced overnight cooling, resulting in temperatures around 20 degrees below normal.

By the 19th, the pattern had shifted to westerly flow aloft, but it brought only slight daytime warming to the valley from the 20th through the 23rd.  Significant warming was prevented by persistently cold nights.

On the 23rd and 24th, a strong and very moist Pacific weather system dumped 3 inches of sn ow at the Boise Airport on the 23rd, and 4.7 inches on the 24th. This set new snowfall records for both days.  The snow depth of 9 inches at the Boise Airport on the 25th was the most snow on the ground ever recorded on Christmas Day.

The snowfalls 1.9 inches on both the 14th and the 16th also set new daily snowfall records.

Following this storm, westerly flow aloft persisted. Without snow cover, this pattern usually brings mild temperatures, as it did earlier in the month. But the deep snow during the last half of the month maintained cold arctic-like conditions.

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November 2016 Climate Stats

Nov2016

November was the second warmest and second driest in 76 years of Boise Airport records.

During most of the month the inter mountain region was dominated by a warm upper-level ridge which kept temperatures above normal. Pacific weather systems weakened as they approached the ridge, so they brought little of any precipitation to the Treasure Valley.

Starting on the 15th, a series of deep upper-level troughs crossed the region, but the ridge always managed to rebound between them.

On the 26th an even deeper trough crossed the western U.S.  On the 27th it was centered over the northern plains states, leaving Boise under northwest flow aloft between the trough and a high pressure ridge over the northeastern Pacific Ocean.

This pattern change paved the way for a winter-like weather system to drop in from the Gulf of Alaska. The storm brought Boise the first trace of snow of the season, which melted as it fell.  The average date of the first trace or more of snow is November 22nd.

A weaker system from the Gulf of Alaska moved into southwest Idaho on the 30th. Like its predecessor, it brought a mix of rain and snow, but precipitation amounts in the valley amounts were light.

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October 2016 Climate Stats

Oct2016

October 2016 brought an end to a long period of little rain. It was the first month since March 2016 with above-normal precipitation, although Boise missed out on record precipitation experienced by most of the Pacific Northwest.

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Like other places in the Pacific Northwest, Twin Falls had their most precipitation on record in October unofficially, while other places were slightly above average. The general unsettled weather kept temperatures at night above average for the month, with average low temperatures at McCall the 2nd warmest on record in October.

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September 2016 Climate Stats at Boise

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September’s average temperature was slightly below normal due to cool upper-level troughs which dominated the inter-mountain region for most of the month.

Highs were only the 60s on six days.  The high of 63°F on the 14th was the coolest day since June 15.

A very warm upper-level ridge built over the region on the 25th and 26th, bringing Boise the highest temperatures of the month.  The 93°F on the 28th was only two degrees shy of the record for that date.

As the ridge moved east on the 29th and 30th, southwest flow aloft on its western flank, combined with southeast surface winds, continued the warm weather.

Precipitation was one third of normal. For most of the month, Boise was on the western margin of moist air moving up from the south, so upper-level troughs interacting with this moisture dumped most of their rain on south-central and southeast Idaho.

Locations east of Boise received substantial monthly precipitation totals, ranging from just under an inch at Mountain Home to over two inches at Twin Falls and Pocatello, much of that from the 21st through the 23rd.

August 2016 Climate Stats at Boise

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August 2016 was dry and seasonably warm, with the average temperature within one degree of normal. There were no record high or lows set.  Only traces of rain fell at the airport. That’s not unusual, because August is generally the driest month at Boise.  Historically at Boise, 25% of Augusts had no measurable rainfall and 50% had no more than a 0.10″.

Weather patterns were typical of this time of year. Pacific systems were relatively weak, mainly moving inland north of our area with precipitation confined to northern and central Idaho and adjacent sections of eastern Washington, northern Oregon, and western Montana.  But even those areas were drier than normal, and little if any rain fell on southwest Idaho.

Incursions of monsoon moisture were usually deflected to the east ahead of trailing pacific cold fronts, so measurable rain from that source came no closer than the Nevada border and eastern Idaho.

At the Boise Airport, where records go back to 1940, June-August 2016 ranked among the driest 15% of summers.

July 2016 Weather Stats at Boise

JUL2016

July 2016 was the first month with below normal temperatures since November 2015. Although July’s average temperature and total precipitation were close to normal in Boise, it was an unusual month.  The average temperature of 75.4°F was the lowest July average since 2001.  The low temperature of 47°F on July 11th was the coolest July temperature in Boise since 2000.  The high temperature of 69°F on July 10th was the coolest July daily high temperature since 2001.  The hottest day of July 2016, 101°F on the 27th, was the lowest monthly July maximum since 1997. There were no record daily high or low temperatures set in July 2016.  A daily precipitation record was set on July 10th, when the total rainfall of the month fell (0.27in) in one day. Even heavier amounts were measured elsewhere in the Boise area, including 0.76in at Eagle and 0.61in at Meridian and over an inch was measured in surrounding areas.

June 2016 Weather Stats at Boise

June2016

June 2016 was the fourth warmest at the Boise Airport, and seventh warmest all-time, but still around 4 degrees cooler than June 2015, the warmest on record.  Temperatures reached 90°F on 13 days in June 2016 (June average is 5 days).  It was a dry month, ranking among the driest 20 percent of Junes on record.  New record daily highs were set on the 5th (97°F) and the 8th (101°F). Record high daily minimum temperatures were set on the 6th (68°F) and the 8th (69°F).

May 2016 Climate Stats at Boise

May2016May was unsettled, with showery periods interspersed with warm dry weather.  June-like temperatures alternated with cool blustery days more typical of March. There were no record highs or lows.

During the first week, temperatures averaged as much as 15 degrees above normal, thanks to an upper level ridge and southwest flow aloft.  Highs reached 81°F on the 3rd and 85°F on the 4th.  On the 5th and 6th, a low pressure system which soaked up plenty of moisture off the southern California coast, moved inland over the Desert Southwest. Some of this moisture reached southern Idaho, which was under an unusual easterly flow on the northern periphery of the low. Showers and thunderstorms crossed the Boise area on the 6th, dumping heavy rain on a few spots. One observer in southwest Boise measured 1.36 inches of rain in 20 minutes, resulting in local flooding across the Boise Metro area. Another observer in the foothills north of Boise reported 0.64 inches in 30 minutes!

The low was kicked east out of the intermountain region on the 7th and 8th by a cold upper level trough which moved over western Canada from the Gulf of Alaska. Cooler drier air associated with the trough drifted into the Treasure Valley and kept temperatures a few degrees below normal from the 9th through the 11th.

Temperatures soon rebounded, with highs from 80°F to 85°F from the 12th through the 14th under a temporary upper level ridge.  A weak trough followed the ridge, with cooler air and showers on the 15th.

A ridge building inland from the northwest coast brought a warming trend from the 16th through the 18th. It was abruptly ended by another Gulf of Alaska trough, which arrived over the Pacific Northwest on the 19th and covered most of the Intermountain West by the 21st.

Unseasonably cool, moist and unstable air associated with the trough generated scattered showers and isolated thunderstorms each day from the 19th through the 22nd.  The high of 55°F at Boise on the 20th was the coolest day since April 14.

Although the trough remained over the western U.S., temperatures moderated to near normal by the 23rd due mainly to long days and widespread sunshine.

The next trough in the series had less effect on our area. Initially centered over southeast Alaska and the British Columbia coast, it sent a dry cold front across the Boise area on the 27th, lowering high temperatures by around 5 degrees.

The trough moved east along the Canadian border over the Memorial Day Weekend, with little effect on Boise other than breezy northwest winds

A warming trend began on the 31st as an upper level ridge built offshore.