southwest idaho

July 2020 Climate

The temperature, when averaged across the area, was slightly above normal. The greatest departures were at Baker City (+3.5 degrees), McCall (+2.5 degrees), and Rome (+2.1 degrees).
July 2020 temperature
Precipitation was below normal nearly everywhere. July is one of the two driest months of the year (the other being August), so dry is actually normal.
July 2020 precipitation
During most of the month, upper-level low pressure troughs moving inland over western Canada would play a role in our weather, as they vied for dominance with the very warm seasonal upper-level high pressure ridge over the southwest U.S.

Precipitation is usually associated with troughs, but during July they produced little if any rain south of the Canadian border, bringing only brief periods of slightly cooler weather.

Between troughs, the high pressure ridge to our south made its presence felt as it expanded northward.

Increasing amounts of monsoon moisture circulating within the ridge eventually managed to reach our area on the 22nd and 23rd, and again on the 27th and 28th, carried north ahead of weak low pressure troughs over California.

On the 23rd, thunderstorms brought locally strong gusty winds. At Rome, numerous tree branches were blown down and garbage cans were knocked over. A gust of 64 mph was measured 21 miles north of Murphy Hot Springs. Numerous other locations reported gusts in excess of 50 mph.

On the evening of the 28th, thunderstorms crossed the Owyhee Mountains, bringing spotty but locally heavy showers to the Boise area.

The ridge made a major northward expansion on the 30th and 31st, bringing the hottest weather of the summer.

At Baker City on the 30th, the high temperature of 101 tied the daily record set in 2003. And the 105 on the 31st set a new record for the date.

On the 31st at Burns, the high of 99 tied the record set in 1949. At Ontario, the high of 107 tied the record set in 1971.


Pocatello tied their all time record high temperature of 104 degrees as well.

 

 

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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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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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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July 2019 Climate Statistics

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The average temperature for the month was less than one degree above
normal. No temperature records were broken, but the low of 76 on the
23rd tied the record high low for that date set in 1994.

At the airport, the only triple digit high was the 102 on the 22nd.
The normal for July is 4 days of 100 or higher. The normal for
summer (June-August) is 7. Last summer had 14, with 9 in July and
the rest in August, including the 110 on August 10.

Only 0.01 inch of rain fell at the National Weather Service office in Boise,
which ranks in the driest 15 percent of Julys.

July and August are normally the driest months, averaging 0.33 inch
and 0.24 inch respectively.

As is common at this time of year, upper level troughs from the
north Pacific weakened as they encountered the strong summer high
pressure ridge over the western U.S. The ridge diverted these
systems too far north to bring measurable rain to the Boise area.

Upper-level winds were usually from the southwest, with enough
westerly component to shunt monsoon moisture east before it reached
the Treasure Valley. When it did manage to drift this far north and
west, it usually brought only cloud cover, which hindered the
heating that drives convection, so only a few sprinkles resulted.

Weak cold fronts accompanying the Pacific systems were followed by
slightly cooler and much drier air. For example, the afternoon
humidity dropped to 7 percent on the 19th and 8 percent on the 24th
after frontal passages. On the other days minimum humidities were in
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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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Record February Precipitation in 2019

February 2019 precipitation ranged 200-300% of normal across southern and central Idaho, southeast Washington, and across eastern Oregon. Several places recorded their February precipitation record.

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McCall set the all-time precipitation record for February at 7.57″ (normal 2.42″). McCall set the all-time snowfall record for Feb at 74.0″ (normal 22.5″), the most in Feb since 1975 when 70.0″ fell. Records began in 1906. Courtesy of COOP observer.

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Boise set the all-time precipitation record for February at 3.71″ (Normal is 0.99″). Boise received 11.6″ of snow in February, the most in February since 1949 when 25.2″ fell. Records began in 1877.

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Baker City, OR and Ontario, OR both broke their all-time February records for precipitation. Burns, OR came in at 4th most all-time for February.

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Mores Creek Summit and Bogus Basin snow surveys reported the biggest change in snow depth and snow water equivalent in February since records began in 1942. 360-370% of normal.

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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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