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.

September 2019 Climate Statistics

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Unseasonably cool weather at the end of the month compensated for hot weather at the beginning and middle of the month, so the average temperature was exactly normal. Total precipitation was a quarter inch above normal.

Summer weather persisted through the first week of September, with a strong upper level high pressure ridge anchored over the western half of the nation.

The 99 degrees on the 5th was the culmination of a streak of 9 consecutive days with highs over 90.

Airflow from the Desert Southwest brought enough monsoon moisture for isolated thunderstorms on the 5th and 6th. On the 6th, 0.15 inch of rain fell, exceeding our total summer rainfall by a hundredth of an inch.

A Pacific cold front moving through the northern edge of the ridge generated a third of an inch of rain on the 8th. The upper level trough following the front kept temperatures a few degrees below normal through the 11th.

After the trough exited on the 12th, warm dry air south of the jet stream raised temperatures above normal. The last 90-degree highs of the summer were recorded on the 14th and 15th.

On the 16th an upper level trough from the Gulf of Alaska, followed by a second trough on the 19th, maintained cool showery weather through the 21st.

After the trough departed, temperatures were slightly above normal from the 22nd through the 27th.

During the early morning hours of the 28th, a strong cold front imported much cooler air from Alaska as a major upper level trough deepened over the northwest U.S. Temperatures averaged nearly 15 degrees below normal for the final three cays of the month.

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May 2019 Climate Statistics

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It was the wettest May since 2005, and the 6th wettest on record, with a total of 3.98 inches. In addition, the period from January 1 through May 31 is the wettest on record at the Boise Airport, at 12.07 inches. It ranks in second place for the entire period of record in the Boise area (behind 1896 at 14.27 inches), and even exceeds Boise’s average annual precipitation of 11.73 inches.

No rain fell during the first two weeks of the month. During the rest of the month most days had measurable rain.

Northwest flow aloft kept temperatures below normal for the first two days of the month. A warming trend began on the 3rd as a warm upper level high pressure ridge centered offshore began to extend its influence inland. The ridge kept our area warm and dry as it moved inland over the next several days. By the 15th it was east of the Rockies, and a Pacific cold front was approaching.

Unseasonably cool and wet weather followed the front on the 16th as an upper level trough settled over the inter mountain region, where it remained for the rest of the month. The moist and unstable air provided favorable conditions for daily rounds of showers and thunderstorms, some of which produced locally heavy showers.

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September 2018 Climate Statistics

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With only a trace of rain, September 2018 was one of the driest Septembers on record. There were only 9 others with no measurable precipitation in 152 years of Boise area records, and only 5 others in 78 years of airport records.

Total precipitation for the four months from June through September was only 0.36 inch. June-September periods with so little precipitation are unusual, but not unique. For example:

1994 .44
1901 .39
1898 .37
2018 .36
1957 .31
2012 .31
1933 .27
1966 .27
1935 .19

Temperatures averaged only slightly above normal.  September began with near normal temperatures under dry westerly flow aloft. The flow backed into the southwest on the 5th as a high-pressure ridge built over the northern Rockies. This resulted in the last 90-degree highs of the summer on the 5th, 6th, and 7th.

A cooling trend commenced on the 8th as an upper-level low-pressure trough from the Gulf of Alaska approached the British Columbia Coast. As the trough edged closer, a dry cold front passed Boise on the 10th, followed by cooler north Pacific air. Highs were only in the low 70s from the 11th through the 13th.

The trough remained parked over the west coast through the 18th, keeping temperatures a few degrees below normal. It finally moved inland on the 19th, then continued east out of our area on the 20th.

Brief warming followed on the 21st and 22nd, with highs in the low 80s, but another north Pacific trough was on the way.

Rather than stalling over the coast, this trough continued on an eastward track, pushed along by a building upper-level high-pressure ridge offshore. The trough was east of our area on the 24th. Like its predecessors, it produced no rain in the Boise area.

East of the ridge, northwest flow aloft over the Pacific Northwest states kept temperatures 5 to 10 degrees below normal, with highs of only 69 on the 24th and 25th, the first highs in the 60s since June 17. The low of 38 on the 24th was the first low in the 30s since May 2.

As the eastern portion of the ridge edged inland, warmer air raised temperatures above normal from the 27th through the 29th, with highs in the low 80s.

During that time, an upper-level low was drifting slowly eastward toward California. By the morning of the 30th, it was centered over the Oregon-northern California coast. Its only effect on Boise was a few clouds and slightly cooler temperatures.

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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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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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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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