Temperatures were above or much above normal across most of the region. Precipitation was below or much below normal across Baker County Oregon and most of southwest Idaho. Only Harney and Malheur Counties, the lower Treasure Valley, and the southwest corner of Owyhee County were wetter than normal due to Pacific storms during the first half of June and monsoon thunderstorms in Harney, Baker, and southwest Owyhee Counties in August.
June
The first half of the month was dominated by westerly winds across the North Pacific which brought changeable weather to the northwestern U.S. Weather systems carried inland nearly daily showers from the 4th through the 8th.
On the 5th and 6th an upper level trough weakened as it moved inland, but it retained enough moisture and energy to generate strong thunderstorms across much of southwest Idaho. Hail up to 1.25 inches was reported at a number of locations. Rainfalls up to 1.25 inches were also measured.
On the 10th warm air spreading north from the Great Basin raised temperatures above normal.
On the 12th a deep upper level trough, which had tapped abundant subtropical moisture, pushed inland. The cold front in advance of the trough was followed by a second front as the trough approached. The fronts triggered thunderstorms and heavy showers which dumped over an inch of rain at several locations. In Harney County north of Andrews a Mesonet station at Little McCoy Creek measured 2.23 inches. A Mesonet stations near Featherville in Elmore County measured nearly 3 inches. Local flooding was common around the region. Wind gusts from 40 mph to around 50 mph were observed at several locations.
The trough lingered through the 15th, keeping temperatures around 15 degrees below normal. Twin Falls recorded a record low of 37 on the 14th. On the 15th a record low of 26 was set at Burns.
Temperatures quickly rebounded on the 16th under southwest flow aloft ahead of the next trough.
The cold front ahead of the trough moved into western Idaho late on the 16th. It stalled west of the Magic Valley, and Twin Falls recorded a record high of 94 on the 17th. Behind the front that same day highs were 10-15 degrees cooler. The front finally exited east of our area late on the 17th, and the trough following the front kept temperatures 5-10 degrees below normal through the 20th.
By the 21st westerly flow aloft along the Canadian border had carried the trough east, allowing temperatures to recover to near normal.
On the 23rd a trough from the Gulf of Alaska had drifted south to southern B.C. and the Idaho panhandle. The cold front crossed the Boise area that afternoon, resulting in below-normal temperatures on the 24th and 25th. A record low of 41 was set at Twin Falls on the 25th.
The trough rapidly exited eastward across southern Canada , allowing a very warm high amplitude ridge to build over the Intermountain Region, and the first really hot weather of the summer ensued. Highs from the mid 90s to a few degrees over 100 were common at lower elevations from the 26th through the 28th.
A weak upper level trough moved over the area on the 29th, lowering temperatures to near normal through the 30th.
July
On the 1st and 2nd a weak upper-level ridge and southwest flow aloft supported highs 5-10 degrees above normal.
On the 3rd an upper-level trough, which had been centered over Vancouver Island, began to edge inland. Cooler Pacific air associated with the trough lowered temperatures to near normal on the 3rd and below normal on the 4th. Following the cold front on the 3rd, a wind gust of 67 mph at the Boise Airport knocked out power to the NIFC (National Interagency Fire Center).
Blocked by the ridge, the trough retreated back offshore on the 5th, allowing highs to rebound from the 5th through the 9th.
The trough moved back inland on the 9th, weakening as it encountered the strengthening ridge. The trough retained enough energy to spark thunderstorms. One storm produced three-quarter inch hail at Council.
The ridge continued to strengthen and expand, and by the 12th it covered all of the western and south-central contiguous states. Highs reached or exceeded 100 at lower elevations each day from the 12th through the 17th.
A downburst from a thunderstorm at Burns on the 12th generated a wind gust of 67 mph.
Meanwhile, another trough had migrated from the Aleutians across the Gulf of Alaska before crossing Washington and northern Idaho on the 18th. Cool air filtering south prevented temperatures from rising above 90 at most locations.
The trough continued on its eastward trek, allowing the ridge to expand northward again. Highs reached or surpassed 100 on the 20th in the lower valleys.
Highs were around 5 degrees cooler from the 22nd through the 25th under the influence of a weak trough.
A new ridge developed off the coast and began to work its way inland on the 26th, resulting in another series of 100+ highs from the 26th through the 31st.
Date Location Record high 7/17 Burns 100 Twin Falls 100 tied record set in 2010 7/25 Burns 98 tied record set in 2021 7/26 Burns 102 7/27 Burns 101 tied record set in 1939 Ontario 107 tied record set in 1964 7/28 Burns 103 Ontario 109 7/29 Boise 104 tied record set in 1934 Burns 103 McCall 95 tied record set in 1994 Ontario 111 national high! 7/30 Burns 103 tied record set in 2003 McCall 96 Baker 101 tied record set in 2003 7/31 Burns 104 Ontario 102
On the 31st at Baker a thunderstorm downburst produced a gust of 62 mph.
August
A persistent upper level high pressure ridge was responsible for the record-setting heat. On the 2nd as a Pacific cold front trailing south from Canada lowered highs by 10-15 degrees. Temperatures rebounded on the 3rd and 4th ahead of the next Pacific cold front. That front passed on the 5th and lowered highs a few degrees on the 5th and 6th.
Warmer air returned on the 7th, and on the 8th highs exceeded 100 in the lower valleys.
On the 9th the ridge was centered over Wyoming and Colorado. Southerly flow between the ridge and an upper level low pressure center off the northern California coast began to pull monsoon moisture north over our area, resulting in scattered thunderstorms from the 10th through the 12th.
On the 10th there were reports of thunderstorm outflow gusts to around 60 mph in Malheur and Owyhee Counties. Heavy rain on the slopes of the Owyhee Mountains caused Rabbit Creek near Murphy to flood, with water over roads in places. A trained spotter at Murphy measured .78 of an inch of rain in 40 minutes. A spotter four miles southwest of Murphy measured 1.2 inches.
On the 11th hail .5 to 1.5 inches in diameter fell at several locations in Canyon, Payette, and Owyhee Counties. On the 12th a trained spotter reported 1.5 inch hail at Oxbow Dam in Baker County, and a spotter at Yellow Pine reported 1.75 inch hail.
On the 13th the low pressure center finally moved inland over southwest Canada. This allowed the upper level flow to shift into the southwest, bringing much drier air.
From the 14th through the 24th highs in the valleys ranged from mid 90s to just over 100.
Monsoon moisture returned late in the day on the 18th as a weak low pressure trough approached from northern California. No unusual rainfalls were reported from thunderstorms on the 19th, but reports on social media described trees and power lines knocked down two miles northwest of McCall. The strongest gust measured at McCall airport was 45 mph.
The heat persisted through the 26th.
By the 27th the upper level ridge center had relocated westward between Alaska and Hawaii, so flow across the North Pacific was no longer blocked from entering the northwest U.S. On the 27th a dry cold front crossed our area, resulting in a very pleasant weekend with highs near normal on the 27th and 28th.
On the 29th the upper level ridge over the Intermountain Region was already rebuilding, and temperatures were on their way up again. The 106 at Boise on the 31st was the warmest ever so late in the season.
Date Location Record high 8/1 Burns 100 tied record set in 2015 McCall 94 Ontario 109 8/13 Twin Falls 96 8/17 Boise 103 Burns 103 McCall 97 Twin Falls 99 Baker 99 tied record set in 2020 8/29 Burns 95 tied record set in 2017 8/31 Boise 106 Burns 101 McCall 95 Ontario 102 Mountain Home 104 Jerome 99 tied record set in 1916 Twin Falls 101 Baker 102
