October 2019 coldest on record in some places across SE Oregon and SW Idaho

Temperatures were well below normal during October 2019. Jerome, ID, Fairfield, ID, Burns, OR, Halfway, OR and Ontario, OR recorded the coldest October on record.  Fairfield, ID had snow cover at the end of the month, and shattered their previous October record by 5.6 degrees, finishing 10.3 degrees below normal for the month.  Idaho as a whole, ranked as the coldest October on record.

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Baker City, OR ranked as 3rd coldest October on record.
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McCall, ID ranked as 4th coldest October on record.

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It was the third coldest October in 80 years of airport records, and 6th coldest on record at Boise. Temperatures averaged above normal on only five days.

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The low of 23 on the 10th eclipsed the old record for the date of 26 set in 1985.

The high of 35 on the 29th broke the previous record for the date of 37 set in 1991. It was also the coldest daily high temperature for so early in the season.

The low of 16 on the 30th tied the old record set in 1991. It was the coldest temperature since the 10 degrees on February 22, which was last winter’s low.

It was a dry month, tying 1965 as the 12th driest October in airport records. Measurable precipitation fell on only five days.

The unseasonable cold can be blamed on the persistent pattern of cold upper level low pressure troughs interspersed with cool northwest flow aloft.  The northwest flow was a result of a strong high pressure ridge off western North America, directing the storm track into Alaska.  Since most of the troughs came from northwest Canada and the cold waters of the Gulf of Alaska, they brought little moisture.

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There were a couple of exceptions to the predominately cold and dry weather.

On the 15th an upper level high pressure ridge built over the western U.S. and southwest Canada.  As the ridge drifted east on the 16th, the temperature maxed out at 80 degrees under southwest flow aloft.

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On the 19th a broad jet stream over the north pacific pushed a fast-moving upper level trough inland across Idaho. There was enough moisture and energy with this system to produce a thunderstorm and nearly a quarter inch of rain.

Winter weather arrived very early during the final week of the month.  By the 26th northwest flow aloft had transported cooler air from British Columbia to southwest Idaho.  From the 26th through the 28th, A very cold upper level low pressure trough moved from north of Alaska through central Canada to the U.S. Border, causing the flow over western Canada to strengthen and shift into the north.  Arctic air was on the way.  The arctic front passed Boise around 4 am on the 29th. Behind the front, very cold high pressure centers moving south on both sides of the Rockies caused northwest and northeast surface winds to converge over the Snake River valley.

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The convergence zone approached Boise from the east later that morning, generating the first snow flurries of the season at the airport and in southeast Boise, while the sun shone on the rest of the city.  At the airport nearly half an inch of very dry snow briefly covered the ground.

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Temperatures stayed well below normal on the 30th and 31st, although there was gradual warming.

Below are graphics of mean temperature percentiles and departure from normal across the Pacific Northwest for October 2019.

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Idaho as a whole ranked as the coldest October on record.

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For more information about the U.S. Climate in October 2019, see Assessing the U.S. Climate in October 2019.

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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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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October 2016 Climate Stats

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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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October 2015 Weather Stats at Boise

Oct2015October 2015 was the 11th consecutive month of above normal temperatures at Boise, and the 2nd warmest in 76 years of airport records.  The 90°F highs on the 9th and 10th were records for those
dates. The 90°F high on October 10 was also the latest 90°F ever recorded.  Overnight lows were also unusually warm. The 56°F on the 8th set a new record for the date, and the 59°F on the 10th tied the record set in 1942.

The late summer warmth might have lasted another week had it not been for the Walker Fire, which blanketed the valley with smoke from the 11th through the 17th.  The smoke filtered enough sunlight to prevent temperatures from warming much above the 70s.

At Boise, the average date of the first 32°F low is October 10th.  This year, at the airport, the coldest reading for the month was 37°F on the 23rd.  But that same morning, frost was seen around the Treasure Valley, some Citizen Weather Observer Program (CWOP) sites at Meridian and Eagle reported lows of 30°F and 32°F respectively.

At the Boise Airport, the latest occurrence of a freezing temperature was November 11, 1944.  Last year the first freeze happened on November 10, ranking second all-time.

The above-average temperatures were partly a reflection of a warm upper level high pressure ridge which persisted over the western U.S. through most of the month.

Showery periods raised the precipitation total to slightly above average for the month.  On the 1st and 2nd, the 17th through the 20th, and the 25th through the 28th, low pressure troughs managed to push inland through the ridge.  Showers on the 30th came from a warm front which tapped very moist air from off the Washington coast.