September 2017 Climate Stats

Summer abruptly changed to fall following a cold front on the 14th, a week before the fall equinox. Temperatures averaged nearly 10 degrees above normal during the first two weeks of the month, and nearly 6°F degrees below normal for the rest of the month.  Precipitation was close to normal.  Smoke from numerous wildfires plagued the region for the first half of the month.
Highs ranged from 99°F on the 3rd to 52°F on the 22nd. Lows ranged from 69°F on the 9th to 36 on the 23rd and 24th, when frost was observed at many locations around the valley. At Boise the average date of the first 36°F degree low is September 25, and the average date of the first 32°F degree low is October 10.  The low of 69°F on the 9th set a record for the date, breaking the old record of 68°F in 1969.  The lows of 67°F on the 4th and 8th, and 66°F on the 13th, tied record high minimum temperatures set in 1998, 1967, and 2011 respectively.

The first half of the month was dominated by a strong and very warm high pressure ridge over the Western U.S. and southwest Canada.  At its strongest, during the first week of September, the ridge extended as far north as Northern B.C. and Alberta.

By the 12th, westerly flow aloft across southern Canada had eroded the north portion of the ridge, and temperatures in our area cooled to near normal by the 14th, ahead of an upper level trough which was deepening and drifting south from B.C. As the trough continued to deepen, it pushed a cold front across eastern Oregon and southwest Idaho during the evening of the 14th.  The trough began a stretch of unseasonably cool fall-like weather which was to last through the 25th, interrupted briefly on the 17th by warmer air ahead of a second even colder trough which originated west of Alaska.

That system brought showers, gusty winds, and the coolest temperatures of the month.  As it exited to the east, temperatures moderated to near normal by the 26th.

From the 27th through the 29th an upper level ridge provided clear skies and slightly above-normal temperatures. But during the night of the 29th, another Pacific cold front crossed the area, followed by more cool, showery, and breezy weather to end the month.


July 2017 Climate Stats

Jul-2017

July was hot. The average temperature of 81.6 degrees ranked the second warmest month for all years back to 1877.  Only July 2007, which averaged 83.1 degrees, was hotter. On the other end of the scale, July 1993 averaged a rather chilly 65.0 degrees.

It was a dry month. Only 0.01 inch was measured at the airport. Dry summer months are not unusual. Since 1877, 33 July’s were as dry or drier.

A semi-permanent warm upper level ridge, which normally forms over the Colorado Plateau during late spring and early summer, kept most of the Northwest U. S. dry through the month. Weather systems from the Gulf of Alaska were blocked from penetrating very far south of the Canadian border, and were forced to move east over southern Canada.

On three occasions the ridge expanded north. From the 4th through the 9th, the 13th through the 15th, and the 29th through the 31st, Boise was directly under the ridge, resulting in triple digit highs.

Monsoon moisture drifting north from Nevada produced locally wet late afternoon and evening thunderstorms. On several days, thunderstorms formed over Owyhee County, but they weakened as they moved over the Owyhee Mountains. Any rain that reached the ground usually missed the National Weather Service rain gauges.

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June 2017 Climate Stats

Jun-2017
June was unsettled, with alternating cool and warm periods more characteristic of spring than summer.

Overall, June was slightly warmer than normal, but daily averages fluctuated from 15 degrees above normal on the 7th to 12 degrees below normal just a few days later on the 11th. No records were equaled or broken.

Precipitation was slightly more than twice normal, although it was a mostly dry month. The anomaly occurred on the 11th and 12th, when 1.15 inches of rainfall was measured at the airport. The total for the other 28 days was only a quarter inch.

The first week was warm and dry, dominated by an upper level high pressure ridge extending from the Colorado Plateau to the northern Plains.  The high for the month of 97°F on the 7th was repeated on the 25th.

On the 9th, a cold upper level low pressure area from the Gulf of Alaska began to invade the Pacific Northwest states.  By the 12th, it was centered over the Idaho-Nevada border.  The well developed counterclockwise circulation pulled in copious moisture which originated in the Gulf of Mexico and crossed the Rockies from eastern Montana.  This resulted in the heavy rain which fell mainly overnight between the 11th and 12th.

Despite the subtropical origin of this moisture, cold north Pacific air kept Boise’s temperature well below normal.

Following this system, the temperature rebounded to normal on the 15th.  And on the 19th and 20th the upper level ridge, having expanded north from the Desert Southwest, pushed the temperature up to 96°F both days.

Sojourns of westerly or northwesterly flow aloft kept temperatures close to normal from the 22nd through the 24th, and from the 28th through the 30th.  A brief incursion of the upper level ridge brought more hot weather on the 25th, with a high of 97°F for the second time during the month, and a high of 96°F on the 26th.

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May 2017 Climate Stats

May-2017

Although only half of the normal monthly precipitation fell at the Boise Airport, and measurable rain was recorded on only 7 days.  Nevertheless, strong thunderstorms did occur in our region on the 5th, 12th, and 16th.  Most of the severe weather missed Boise, but high winds, large hail, and brief heavy downpours hit a number of locations in eastern Oregon and southwest Idaho.

Temperatures averaged close to normal. The high of only 49°F on the 17th was the only record.  It was the coolest maximum temperature ever recorded on that date, breaking the old record of 51°F set in 1896. The last freezing temperature of the season was 31 on the 13th. The average date of the last 32 degree reading is may 8.

The first official 90 degree high of the season occurred on the 30th. The last time 90 degrees was reached was September 29, 2016.

Typical of spring, changing weather patterns brought a succession of warm and cool periods.  Upper level high pressure ridges with above normal temperatures were displaced about every 6 days by cold fronts followed by upper level troughs with unseasonably chilly north Pacific air, gusty northwest winds, and showers.

The most noteworthy of these weather systems arrived from the Gulf of Alaska on the 16th, passing directly over the Boise area that afternoon. It was ushered in by a thunderstorm which formed over the lower Treasure Valley and dropped half an inch of rain on the Boise Airport. Small hail was seen in some parts of town.

During the late afternoon of the 30th, a dry thunderstorm rolled off the Owyhee Mountains and arrived at the airport just before 7 pm, with gusty winds and blowing dust.  A peak gust of 45 mph was measured at 6:58 pm MDT.

On the 31st, isolated afternoon showers and early evening thunderstorms developed west of Boise and intensified as they moved north ahead of an approaching Pacific cold front. They had little effect on the upper Treasure Valley, but the front ended the five day stretch of summer-like weather.

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April 2017 Climate Stats

Apr-2017

April was characterized by changeable weather typical of spring.  Mild periods were interrupted by days with below normal temperatures. The only record was the unseasonably warm low of 52 on the 6th, which broke the old record of 51 set in 1898.

The unsettled weather was the result of a steady progression of upper level ridges and troughs across the inter mountain region.

Unseasonably cold sea surface temperatures and cold air aloft resided off the northwest coast for the entire month, contributing to several cool spells and frosty mornings in Boise.

It was the 16th wettest April since 1878, and the 9th wettest April since 1940, when record keeping began at the Boise airport.

The 0.51 inch of precipitation on the 8th eclipsed the old record for the date of .40 inch set in 1881. This event was caused by an increasingly moist and unstable southwest flow ahead of an upper level offshore trough. The scenario was aided by a relatively weak disturbance moving through the flow ahead of the main trough.

A tenth of an inch of snow fell on the 8th, but there was no accumulation.

It was a breezy month. Nearly half the days had gusts which reached or exceeded 30 mph.  The main event was the 53 mph gust from the southwest on the morning of the 7th. Convective showers were forming in moist and unstable
air ahead of a strong cold front. Southwest winds exceeded 40 mph as low as 5000 feet above the surface, and a downdraft from a shower approaching the airport added to this wind as it descended to ground level.

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March 2017 Climate Stats

Mar-2017

March 2017 was warm and wet, with a major storm to finish out the month.  Without that storm, march would actually have been slightly drier than average.  But with 2.86 inches, it turned out to be the second wettest march at the Boise airport, and the seventh wettest march going back to 1878 in the Boise area.  The 1.40 inches on the 30th tied January 16, 1896 as the tenth wettest day in the Boise area. At the airport it was the seventh wettest day.

There were showers each day from the 3rd through the 11th, and there was measurable precipitation on nearly half the days from the 12th through the 31st.

The average temperature of 48.9 degrees tied 1992 as the second warmest march at the Boise airport, and the fifth warmest march going back to 1878 in the Boise area.

There were no record high temperatures, but the lows of 52 on the 18th and 48 on the 20th broke the previous daily records for warmest lows. The 49 on the 19th tied the record high low for that date.  There were no freezing temperatures from the 8th through the 27th.

During the first 28 days of the month there were no significant storms. Predominantly westerly or southwesterly flow aloft and eastward migrating high pressure ridges kept temperatures mild.  Minor weather systems moving inland from the Pacific provided light precipitation.

On the morning of the 29th, an ordinary looking upper level trough off the British Columbia coast was approaching the northwest U.S. By evening it had noticeably deepened as it neared the Washington coast.  A third of an inch of rain fell at the airport that evening, way out ahead of the main storm.

The rain stopped before midnight, only to resume just before 7 am MDT on the 30th as the cold front approached. As the front passed Boise around 8:30 am, the rain became heavy, and west winds increased to around 25 mph. The rain changed to snow just before 10 am, but only a trace fell before the snow ended shortly after 11 am.

Just over one inch of precipitation fell in 6 hours on the morning of the 30th.  Interestingly, the amount of moisture available for precipitation, measured by radiosonde during the two hours before the rain started, was a mere half inch. So how could nearly three times that amount fall on Boise?

As the upper level trough rapidly intensified directly over southwest Idaho, more moisture was pulled in from outlying areas and lifted over the treasure valley.  At the same time, the strong cold front provided additional lift to turn that moisture into rain and snow. Also, a closed circulation developed aloft, slowing the storm movement. This allowed more time for precipitation to accumulate.

As the system strengthened at upper levels, surface low pressure to our east deepened, causing northwest winds to increase through the afternoon. A peak gust of 52 mph was measured at the airport at 4:06 pm.

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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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