November 2019 Climate Statistics

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Although temperatures averaged above normal for most of the month, below normal temperatures at the beginning and end of the month brought the November averages down to near normal.  No daily records were tied or broken.

Precipitation was sparse, totaling only .07 inch.  It was the driest November ever at the airport, where records go back to 1940. Going back to 1868, the start of continuous precipitation records in the Boise area, there have been only two drier Novembers.

Following a period of cool northwest flow aloft at the beginning of the month, an upper level high pressure ridge which resided near the west coast kept our weather mainly dry with above normal temperatures on all but two days from the 4th through the 24th.

On the 16th a low pressure system weakened as it moved through the ridge.  It produced only a trace of rain at the airport.

On the 19th and 20th a more robust system split as it moved inland, with the strongest portion heading south toward southern California and Arizona. Before going south, it managed to squeeze out a hundredth of an inch of rain at the Boise Airport.  Following the system, the ridge rapidly rebuilt over the Northwest U.S.

By the 24th a pattern change was underway as the ridge shifted west over the Pacific, allowing an upper level cold low pressure trough to deepen over the Intermountain region.  Weather systems from the Gulf of Alaska strengthened as they moved into the trough. On the 25th the first system dropped a trace of snow on Boise before heading for the four corners area.

The next system was more dramatic in terms of wind. As the trough intensified over northwest coast on the 26th, it generated a steep west-to-east pressure gradient across southeast Oregon and southwest Idaho.  This resulted in strong southeast surface winds. At the airport the wind peaked at 47 mph on the 26th and 46 mph on the 27th.

Nearly half an inch of snow fell at the airport on the 27th. Only traces of snow fell on the 28th, 29th, and early on the 30th before the trough jumped the Rockies and headed across the Midwest.

On the 30th skies cleared under a transitory high pressure ridge. Abundant sunshine melted the light snow which fell the previous night and temperatures warmed into the upper 30s.

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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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August 2019 Climate Statistics

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Temperatures averaged slightly above normal. Measurable rain fell on 3 days, totaling 0.09 inch. August is normally the driest month of the year, averaging 0.24 inch.

Precipitation for June, July, and August totaled only .14 inch, making this the second driest summer on record at the airport.

A massive upper level high pressure ridge covering the west kept the first week of August hot. Temperatures at Boise topped out at 100 on the 5th and 101 on the 6th.

Southerly flow aloft brought enough monsoon moisture for light showers on the 2nd and again on the 8th, plus enough cloud cover on the 8th to prevent the temperature from reaching 90.

On the 9th and 10th, a low pressure trough which had resided off the West coast since late July moved inland. As the associated cold front interacted with the monsoon moisture, it triggered thunderstorms which produced light but measurable precipitation and gusty outflow winds. A gust of 40 mph was measured at the airport on
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With a high of only 78, the 11th was the coolest day of the month, thanks to the north Pacific air which accompanied the trough.

Following the trough, highs were back in the 90s from the 13th through the 15th, and dry westerly flow aloft blocked monsoon moisture from invading southwest Idaho.

On the 16th and 17th, a weak cold front trailing south from a low pressure system over Canada lowered temperatures a few degrees.

The high pressure ridge expanded north again, and highs were in the 90s from the 18th through the 21st.

On the 22nd and 23rd, a Pacific cold front, followed by an upper Level low pressure trough from the the Gulf of Alaska, brought a trace of rain and kept highs from exceeding the 80s. A dry cold front followed on the 25th, keeping temperatures below normal through the 26th.

A warming trend commenced on the 27th as the high pressure ridge rebuilt and persisted through the end of the month, with temperatures around 10 degrees above normal.

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June 2019 Climate Statistics

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It was the second driest June at the Boise Airport, where records go back to 1940. It was the fourth driest June at all Boise locations going back to 1864. Measurable rain fell on only two days.

The average temperature for the month was very close to normal.

June began with above normal temperatures, with highs in the 80s under an upper level high pressure ridge.

On the 7th a pacific cold front, followed by an upper level low pressure trough, brought a brief break in the warm weather. Highs were only in the lower 60s on the 7th and 8th.

By the 10th temperatures had risen above normal again as another high pressure ridge began to build over the west coast. Highs were in the 80s from the 10th through the 19th, with the exception of the 91 on the 12th. That was the first 90 degree reading since September 7.

Following the 80 degrees on the 19th, much cooler air spread south following a cold front as an upper level trough deepened over southwest Canada and the northwest U.S. Highs were only in the mid 60s on the 20th and 21st.

After a chilly 42 degree low on the 22nd, temperatures began a slow recovery as the trough exited to the east. By the 25th milder west-southwest flow aloft had returned temperatures to near normal.

On the 26th an upper level trough from the Gulf of Alaska had arrived off the Washington-Oregon coast. Coming from the cold waters of the northeast Pacific, it contained little moisture.

Ahead of the trough, southwest flow aloft helped the temperature warm to 89 on the 26th.

As the trough edged further east that day, thunderstorms developed in northeast Oregon along a weak cold front. But they never got as far as Boise, and that night the front produced no precipitation as it crossed southwest Idaho.

The trough gradually weakened and moved offshore. It continued to have little effect on our area, although on the 28th a stationary band of dense high clouds prevented temperatures from rising above the lower 70s.

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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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