April 2020 Climate Statistics

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Precipitation was near normal, and temperatures averaged slightly warmer than normal.

A cold upper level low pressure trough over western Canada and the Northwest states during the first three days of the month held temperatures below normal.

Southwest flow aloft ahead of a low pressure trough off the Pacific Northwest coast started a warming trend on the 4th.  As the trough moved south along the California coast on the 5th and 6th, showers spread inland, but the Boise area received only light precipitation.  The air was unstable enough for thunderstorms both days.

With our area between the low pressure system over the southwest states and the jet stream to our north and east, weak high pressure kept temperatures above normal from the 5th through the 11th.

A cool down was on the way as the flow over western Canada shifted into the north on the east flank of a high pressure ridge off the west coast of North America.

A cold front from British Columbia passed Boise during the evening of the 11th, accompanied by a wind gust of 45 mph from the north.  Temperatures the following day were 10 to 15 degrees cooler, and temperatures stayed below normal through the 14th.

Boise fared better than eastern Idaho, and much better than states to the east, where a late season arctic outbreak brought snow and winter temperatures.

After a brief warmup on the 15th, another cold front from British Columbia crossed the Boise area that evening. Compared to the previous front, the cool down was minor.  A convergence zone in the Snake River Basin near Mountain Home created show showers on the morning of the 16th.

A warming trend began on the 17th as the high pressure ridge began to move inland from the coast.

Increasing westerly flow aloft gradually weakened the ridge, and on the 22nd and 23rd a disturbance embedded in the flow brought the largest amount of precipitation for the month. Most of it fell on the 23rd with scattered convective showers, some of which produced brief heavy rain and small hail.

On the 25th and 27th a couple of weak disturbances dropped most of their precipitation over north Idaho, with only light showers at Boise.

On the 28th a strong high pressure ridge developed over the western U.S. as it amplified and drifted east, it provided the season’s warmest weather so far.

On the 29th strong southwest flow aloft on the west flank of the ridge, down valley flow at the surface, and mostly sunny skies helped boost the temperature at Boise to 88 degrees, one degree shy of the record set in 1992. Severe thunderstorms containing 1″ hail were reported in Baker City.

On the 30th the southwest flow carried an energetic weather disturbance across southeast Oregon and southwest Idaho. Late that morning a line of thunderstorms formed over Harney County.  The storms intensified rapidly as they approached Idaho. They crossed the border at about 1:45 pm, and strong outflow ahead of the storms arrived at the Boise airport at 2:30 pm.  Five minutes later a gust of 61 mph was measured just ahead of the rain, which was heavy at times.  Golf-ball size hail (1.75″) was reported at Oreana, ID (40 miles southwest of Boise).

 

The winds were likely stronger in some parts of town, as there were reports of power outages, downed trees, and other damage. Large trees across the Treasure Valley toward the Boise Mountains and Magic Valley were knocked down as a result of this storm.  Numerous large trees were knocked down from Prairie to Pine to Featherville with this storm.

 

Here are the rest of the climate graphics for southwest Idaho and southeast Oregon.  McCall was cooler than average due to a deep snow pack on the ground within the valley, which kept their temperatures lower than other areas.

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January 2020 Climate Statistics

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It was the fifth warmest January at the airport, where records go back to 1940, and the eighth warmest January in the Boise area in 156 years of temperature records.

A strong westerly circulation maintained a progressive pattern which brought pacific weather systems across our area at frequent intervals. This hindered the formation of long-lasting temperature inversions. It also prevented arctic air from pushing as far south and west as Boise.

The low for the month, and so far this winter, was 16 on the 15th and also on the 30th of October. The probability is slightly above 50 percent that the temperature will fall below 16 in February.

Precipitation was nearly one inch above normal.

Snowfall totaled 8.9 inches, compared to the normal 5.1 inches. The temperature rose above freezing every day, limiting accumulation. The longest period of snow cover (1 inch or more) was five days from the 13th through the 17th. The greatest depth was 4 inches on the 15th.  Here are other stations across southwest Idaho and southeast Oregon.

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December 2019 Climate Statistics

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It was one of the milder Decembers. Only eight Decembers were warmer in the 80 years of airport records.  No temperature records were broken or tied.

The temperature rose above freezing every day except the 29th, when the high at the airport was 32 degrees.  However, nearby Boise locations warmed into the mid-30s that day.  Normally there are 7 days in December when highs fail to exceed 32 degrees.

The relatively warm weather was due the absence of long lasting temperature inversions, and a pattern which kept arctic air east of The Rockies.

The 22nd was the warmest day with 61 degrees, under strong southwest flow aloft ahead of an upper level trough.  It was also the windiest day, with a gust of 36 miles per hour from the southeast.

Precipitation was half an inch below normal.  Snowfall was less than half normal.  The greatest snow depth was 1 inch on the 2nd.  The average December has 7 days with an inch or more on the ground.

The precipitation deficit was due to a tendency for storm systems to move south over California, rather than moving east over our area.

Nearly half of December’s precipitation fell from the 11th through the 14th, when a brief pattern change allowed moist westerly flow aloft to push inland over the northern intermountain region.

Across southwest Idaho and southeast Oregon, it was a common theme of above normal temperatures and below normal precipitation and snow.

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November 2019 Climate Statistics

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Temperatures at lower elevations averaged near normal, but over the higher terrain, temperatures averaged warmer or much warmer than normal.  This pattern reflects the temperature inversions which were prevalent during the month.

No temperature records were broken or tied.

Precipitation was sparse for the month as a whole. But ironically, a storm at the end of the month brought heavy snow to some areas.

Following a period of cool northwest flow aloft at the beginning of the month, an upper level high pressure ridge which resided over the west coast kept our weather mainly dry.

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

On the 19th and 20th another unremarkable system split as it moved inland, with the strongest portion heading south toward southern California and Arizona.  Following this 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. On the 25th a weather system from the Gulf of Alaska moved into the trough, dropping  traces of snow at a few valley locations before heading for the four corners area.

The next system was much more dramatic. As the storm intensified over the northwest coast on the 26th, it generated a steep pressure gradient across southeast Oregon and southwest Idaho, creating strong southeast surface winds. Gusts exceeded 50 mph at many locations. A spotter near Hollister in Twin Falls County measured a gust of 69 mph.

 

The storm also produced appreciable amounts of snow, not only at higher elevations, but also at a few valley locations. On the 27th there was an unofficial report of 10 inches at Baker City. A spotter at Burns measured 7 inches. At Weiser 5 inches fell, and Midvale got 7 inches. Even a few locations in the lower Treasure Valley got an inch or two. Amounts in the upper Treasure Valley were generally less than half an inch, leaving only traces on the ground.

Snow continued to fall before the trough jumped the Rockies and headed across the Midwest. Lower elevation reports on the 29th include 2 inches at Buhl and Glenns Ferry, 3 inches at Hammett and near Mountain Home, and 4 inches at Gooding. In Malheur County 5 inches fell at Rome and Jordan Valley.

The 30th brought partial clearing under a transitory high pressure ridge.

 

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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.

It was a dry month, with below normal precipitation across most of southeast Oregon and southwest Idaho.  But a few locations, mainly in Harney County, were actually wetter than normal.

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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 Boise Airport records. Measurable precipitation fell on only five days at Boise.

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, temperatures rose to summerlike values in southwest Idaho. Boise, Jerome, and Twin Falls all recorded highs of 80 degrees under warm 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 generate thunderstorms with strong gusty winds. There were numerous reports of gusts over 50 mph. At some locations in the Magic Valley, including Twin Falls and Hazelton, gusts exceeded 60 mph.

Power poles were knocked down near Wendell and at Caldwell. Around 30 poles were knocked over along Highway 93 between Twin Falls and Jerome. At Kuna, several large trees were down, fences were blown over, cars were damaged, and power lines were down. Northeast of Boise in Ada County, lightning struck a house, causing a fire. Two people were injured.

And it snowed. McCall got 4 inches and Tamarack 6 inches. A spotter northeast of Featherville reported 7 inches that day.

Even more snow fell on the 20th.  Tamarack got an additional 4 inches, making their storm total 10 inches. Other storm totals in the mountains ranged from 6 inches at Mores Creek Summit to 12 inches at Atlanta Summit.

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Real winter cold arrived at the end of the month.

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 through the 31st.  Here are some record temperatures during the cold spell.

October 29: lows…Burns 1, Ontario 15, Baker City 6.

October 30: high…Boise 35 (coldest high temperature for the date and so early in the season) lows…Boise 16 (tied 1991), McCall 4 (tied 1991), Ontario 9, Mountain Home AFB 9, Jerome 8, Baker City 4

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.

It was wet nearly everywhere, with parts of Malheur County getting over three times normal precipitation.

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.

A record high of 101 was set at Ontario on the 5th.

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.

Thunderstorms on the 5th was preceded by gusts over 55 mph near Swan Falls, in the Boise area, and at Glenns Ferry. In Boise, a microburst knocked down numerous trees from Broadway to Apple Street, some falling on houses. At Timberline High School fences were knocked down, and a batting cage was lifted over a baseball field fence and thrown into an adjacent parking lot.

On the 6th large tree branches were blown down near Buhl and at Glenns Ferry.

On the 8th a Pacific cold front dropped a third of an inch of rain at Boise. The .52 inch total at Mountain Home AFB set a record for the date.

On the 9th one-inch diameter hail fell at several locations near Twin Falls and Eden in Jerome County. Near Hansen in Twin Falls County 1.5 inch diameter hail was reported.

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. In the Treasure Valley the last 90-degree highs of the summer were recorded on the 14th and 15th. The 95 at Ontario on the 15th tied the record for the date set in 1956.

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 days of the month.

At Burns on the 30th the low of 23 tied the record set in 1981.

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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
The 9th.

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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