Summer was, in a word, hot…the hottest on record at several locations. Many monthly and daily temperature records were broken or tied. It was also drier than normal overall, with the notable exception of the Treasure Valley.
June-August 2021 3-Month Averages vs Previous Warmest
Summer 2021 Previous warmest
Boise 78.0 76.6/2015 Baker City 67.2 69.0/1961 Burns 69.6 69.2/1961 Jerome 74.2 74.2/1961 McCall 65.6 65.3/1961 Ontario 79.4 78.5/1961 Rome 71.0 72.4/2007 Twin Falls 73.9 75.1/2013
June-August 2021 Monthly Averages vs Previous Warmest
June July August
Boise 75.9 70.5/2007 83.8 83.1/2007 74.4 78.7/2001 Baker City 65.4 66.0/1961 71.3 72.3/1985 64.9 71.3/1961 Burns 66.9 66.9/2015 74.5 73.2/1960 67.3 72.3/1967 Jerome 73.5 71.8/1974 79.0 77.9/1985 70.1 77.3/1967 McCall 62.9 63.3/2015 71.1 68.7/2017 62.8 67.9/1961 Ontario 75.9 80.0/1971 85.1 82.3/2007 75.9 80.0/1971 Rome 68.9 72.4/2015 75.2 78.1/2007 68.8 74.2/1967 Twin Falls 72.2 73.5/2015 78.8 79.6/2007 70.6 77.1/2013
2021 Monthly Highs vs Previous Monthly Highs
June July August
Boise 105 110/2015 107 111/1960 105 110/2018 Baker City 103 102/1961 101 105/2020 98 109/2018 Burns 103 102/2015 102 107/2002 101 103/2018 Jerome 100 110/1940 100 108/1973 99 107/1940 McCall 96 97/2015 95 102/1928 93 104/1928 Ontario 107 109/2015 107 113/1967 106 113/1961 Rome 101 107/2015 105 110/2002 103 107/2018 Twin Falls 100 103/2013 100 107/2003 96 102/2013
A persistent and very warm upper level high pressure ridge resulted in a number of record highs. It was the warmest June on record at Boise, Burns (tied), and Jerome. The high of 103 at Boise on the 3rd was not only a new record for the date, it was also the highest for so early in the season.
A dry pacific cold front crossed the Boise area on the 4th, and temperatures were back to near normal from the 6th through the 9th.
A cold upper level low pressure trough from the Aleutians arrived at the northwest coast on the 7th. After deepening southward, it moved inland over the Pacific Northwest and northern California on the 9th and crossed the Intermountain Region on the 10th. The 0.71 inch of rain at Boise that day was only 0.04 inch short of Boise’s normal precipitation for the entire month of June.
Temperatures rose from the 11th through the 13th under southwest flow aloft ahead of an offshore trough.
As the trough moved inland on the 14th and 15th, the coolest air was diverted north of our area by an expanding upper level high pressure ridge over the four corners. So the trough brought only slight cooling, lowering temperatures to near normal on the 16th.
As the ridge amplified, temperatures rose again.
A low pressure trough formed off the California coast on the 20th. A disturbance generated by the trough lifted north, heading for southeast Oregon and southwest Idaho. It spawned strong, but mostly dry, thunderstorms in southeast Oregon on the 22nd. Cloud cover and cool air in the trough kept temperatures in the 60s and lower 70s in Harney County, while Boise`s high soared to 98 degrees. The increased pressure gradient caused by the temperature difference enhanced outflow from the storms as they drifted north, resulting in strong west winds in the Treasure Valley. A gust of 59 mph was measured at the Boise Airport, but there were reports of even stronger gusts elsewhere in the valley.
On the 23rd, an upper level ridge began to build off the coast. By the 26th it had moved inland over B.C. and the northwest U.S., unusually far north for such a strong and very warm ridge. Temperatures responded with a number of record highs from the 28th through the 30th.
During the hottest part of the day, humidities were low, falling into the teens and single digits.
Moisture circulating around the ridge resulted in numerous showers and thunderstorms around the region on the 30th, but many locations reported only sparse precipitation.
July 2021 was the warmest on record at several locations, including Boise, Burns, Jerome, McCall, and Ontario. It could have been even warmer. Persistent smoke from wildfires restricted sunlight from reaching the surface, so highs were slightly lower than they would have been under clear skies.
At Boise, several other temperature records were broken, tied, or approached.
highs >= 90...44(6/17 - 7/31) ranks 2nd (50 in 1875 ranks 1st)
>= 100...9(6/28 - 7/6) ties 2015, 2006, and 2003
lows >= 60...43(6/19 - 7/31) ranks 1st (37 in 2007 ranks 2nd)
>= 70...11(6/27 - 7/7) ranks 1st (5 in 2015 ranks 2nd)
Number of days
highs >= 90...31...ties July 2017 (30-year average is 22)
>= 100...12...ranks 4th (15 in 2003 ranks 1st)
lows >= 60...31...ties July 2007 (30-year average is 20)
>= 70...16...ranks 1st (9 in 2007 ranks 2nd)
(30-year average is 3)
It was a dry month overall, but a storm on the 31st brought significant rain to many Idaho locations.
An upper level warm high pressure ridge was responsible for the heat. Weak cold fronts associated with Pacific weather systems crossing western Canada occasionally pushed far enough south to lower temperatures by a few degrees. But other than breezy northwest winds, they had little noticeable effect.
By the 20th the ridge had shifted east and was centered over Colorado. Southwest flow between the ridge and an upper level trough centered over the B.C. coast transported monsoon moisture north of the Nevada border. The resulting thunderstorms brought generally light precipitation, along with gusty winds.
On the morning of the 22nd, a cold front passed our area as the upper level flow began to shift into the west, carrying the monsoon moisture east. Cooler drier air following the front resulted in the below normal average daily temperatures from the 22nd through the 24th.
On the 28th a brief influx of monsoon moisture generated early morning showers and thunderstorms, but precipitation was sparse. Skies cleared, and it was another hot afternoon and evening. Drier southerly flow brought even higher temperatures on the 29th and 30th.
By the 26th the center of the upper level ridge had migrated to the central Great Plains. Easterly flow south of the ridge picked up very moist air from the Gulf of Mexico and carried it west to New Mexico and Arizona. Southerly flow on the west side of the ridge then transported it north to the northern Intermountain Region.
On the 31st a low pressure system formed over Nevada. As it moved north, it interacted with the Gulf moisture, setting off numerous showers and thunderstorms with locally heavy rain. The Boise and Ontario areas, the Magic Valley, southern Twin Falls County, and the central Idaho mountains got the brunt of the rain.
In contrast to June and July, August was a relatively wet month with near normal temperatures, although it certainly had its share of record daily high temperatures. But smoky skies prevented highs from reaching their full potential. Temperatures were mostly above normal for the first half of the month and mostly below normal for the last half.
An upper level high pressure ridge kept temperatures hot from the 2nd through the 5th.
On the 6th a cold front and weak upper level trough crossed our area. Temperatures warmed briefly on the 7th ahead of a stronger trough from the Gulf of Alaska. That trough, and the northwest flow that followed, cleared the smoke and kept temperatures below normal through the 10th.
The heat returned on the 11th as an upper level ridge centered off the coast built inland. The heat persisted through the 16th. By the 14th the smoke had overspread our area again.
A major pattern change on the 17th ended the heat. An upper level trough deepened south from Canada over the Pacific Northwest and the Intermountain Region as an upper level ridge between Alaska and Hawaii strengthened and expanded north. The resulting northwest flow aloft putus in the path of more troughs coming out of Alaska. One of these systems brought light but measurable rain on the 20th and 21st, plus another brief respite from the smoke. The air was unstable enough for a thunderstorm on the 21st.
The next trough in the series crossed the Canadian border on the 22nd, and the last trough of the month settled over southwest Canada and the Pacific Northwest on the 31st. The smoke cleared again that afternoon following a dry cold front.