Temperatures were above normal in the mountains and below normal at lower elevations due to temperature inversions. The entire region was drier or much drier than normal. The driest areas were the central Snake River Valley, the Camas Prairie, and the Boise Mountains. An upper-level high pressure ridge dominated through the 10th, keeping our region dry. Burns was an exception, where a weak upper level low pressure trough brought light snow on the 6th. From the 11th through the 16th, a temperature inversion resulted in areas of night and morning fog. A trough from the Gulf of Alaska brought light snow on the 13th and 14th. It was too weak to mix out the inversion, as most of its energy crossed the Intermountain Region well south of our area.
— NWS Boise (@NWSBoise) December 15, 2020
The inversion finally broke as a stronger trough crossed the area on the 17th, bringing 5-10 inches of snow to the mountains, but only light precipitation in the valleys. An exception was the .55 inch rainfall at Jerome, which established a new record for the date. Mild westerly flow aloft brought a warming trend from the 18th through the 22nd. High temperature records were tied or broken at several locations on the 21st and 22nd.
|City||New Record||Old Record||Year|
|Twin Falls (21st)||59||53||2014|
|Twin Falls (22nd)||50||50||2005|
A strong Pacific cold front crossed the area on the 22nd, followed by a 10-20 degree temperature drop. Strong northwest winds gusting to 40-50 mph followed the front. A gust of 57 mph was measured north of Andrews in Harney County. The trough which followed the front produced light precipitation at lower elevations. Moderate amounts of snow fell in the mountains, including 5 inches at Brundage.
Following this system, a strong ridge kept skies mostly clear through Christmas eve.
A weak trough brought mainly light precipitation on the 25th and 26th. Boise was an exception, where a third of an inch of precipitation fell on the 26th, which included half an inch of snow. A few inches of snow fell in the mountains.
Another temperature inversion formed on the 26th, and locally dense fog returned to the valleys. With cold air trapped in the valleys, temperatures failed to reach the freezing mark at many of the usually warmer locations in the 28th and 29th.
The final trough of the month weakened as it moved inland on the 31st, but it was able to drop several inches of snow on the mountains, including 8 inches at McCall and 6 inches at Tamarack.
Temperatures were above normal across the region, and much above normal in the Snake River Valley and parts of southeast Oregon. Most of southern Idaho and parts of Baker and Harney Counties in Oregon were drier than normal, while the Treasure Valley and west central Idaho received above normal precipitation.
Strong westerly flow across the Pacific was charged with abundant moisture as it swept inland with a warm front on the 3rd, resulting in heavy precipitation. Totals from a half inch to an inch of water were common, falling as rain at lower elevations and snow in the mountains. Bogus Basin accumulated 7 inches. Strong west winds, mostly in the 40 to 50 mph range, followed a cold front on the 4th. A gust of 63 mph was measured at Wagontire in Harney County. On the 5th another warm front brought heavy snow to the mountains. Banner Summit got 14 inches. Bogus Basin and Tamarack received 10 inches. Atlanta also got 10 inches. Bogus Basin got 7 additional inches on the 7th as a trough crossed the area. Little if any precipitation fell at lower elevations from the 5th through the 7th. On the 12th a weak but moist trough embedded in the westerly flow brought more snow to the mountains, while relatively light precipitation fell at lower elevations. At Council 7 inches was measured, and 6 inches fell at Brundage and McCall. A cold front crossed the area on the 13th, followed by wind gusts of 30-45 mph.
— NWS Boise (@NWSBoise) January 13, 2021
High pressure kept the area mostly dry from the 14th through the 20th. From the 21st through the 26th, weak troughs moving down the coast from the Gulf of Alaska brought only light precipitation to our valleys and a few inches of snow to the higher elevations. A record low temperature of 10 degrees was set at Twin Falls on the 25th, breaking the old record of 12 set in 2008. On the 27th a stronger trough centered over southeast Alaska deepened southward just offshore. Southerly flow aloft on the east flank of the trough was felt at the surface as gusty south to southeast winds, generally in the 40 to 50 mph range. A gust of 68 mph was measured at Trail Gulch, 14 miles east-southeast of Hollister. Also on the 27th, moderate amounts of snow fell in eastern Oregon and west-central Idaho, due mainly to lifting of the air by the mountains. At McCall 9 inches was measured, and 7 inches fell at Midvale. At Huntington in Baker County 5 inches was reported.
February was a month of very active weather. Temperatures were near or above normal at most locations. February is normally warmer than January, but this year it was actually colder than January at Boise, McCall, Mountain Home, and Ontario. It’s not the first time this has happened, but at Boise it was the fourth year in a row.
On average, February is the driest winter month, but this year it was the wettest in the Treasure Valley and eastern Oregon. However, the Boise Mountains, Camas Prairie, and Magic Valley were drier than normal. The month began with above-normal temperatures under southwest flow aloft ahead of an offshore trough. As the trough moved inland, a cold front crossed our area during the afternoon and evening of the 2nd. This marked the beginning of a pattern change which would lead to long overdue winter weather for the valleys. Northwest flow aloft developed on the 4th and strengthened during the following days. On the 5th, a fast-moving trough from the Gulf of Alaska brought a few inches of snow to the mountains, but its main impact was strong west to northwest wind. Gusts of 45-55 mph were common, but by far the strongest wind measured was 92 mph at Soldier Mountain. Meanwhile on the 3rd, arctic air had plunged south across the Canadian border east of the Rockies. The mountains, and strong northwest winds aloft, kept it out of Idaho. A second invasion of even colder arctic air entered Montana on the 5th. By the 6th, its western margin had stalled along the Montana border. Meanwhile, the “milder” western portion of the same arctic airmass was drifting south through the mountains of British Columbia. It entered Washington on the 8th and northeast Oregon on the 9th. The front marking its leading edge reached eastern Oregon and southern Idaho on the 11th. Moist air streaming inland ahead of an approaching trough was lifted over the cold air north of the front, resulting in snow which became heavier as the trough drew closer on the 12th and crossed our area on the 13th. Snowfalls of 3 to 6 inches were common in the valleys on the 12th. Up to 6 more inches fell on the 13th, with heavier accumulations in the mountains. By the time the snow ended, 7 to 10 inches had accumulated in the Treasure Valley. McCall, which nearly always gets way more snow than Boise, also reported a storm total of 10 inches. Totals of 13 inches were measured at both Bogus Basin and Magic Mountain ski areas, while Twin Falls reported only 2 inches. The 4.4 inches which fell at the Boise airport on the 12th broke the old record for the date of 1.3 inches set in 1966, and the 5.5 inches on the 13th broke the old record of 4.3 inches set in 1995.
— NWS Boise (@NWSBoise) February 13, 2021
On the 15th another trough from the Pacific brought more snow, with 1 to 4 inches in the valleys and 5 to 10 inches in the mountains. Wind gusts of 40 to 55 mph were observed in Harney County and the Magic Valley. From the 16th through the 22nd, Pacific systems brought more snow to the mountains but generally light precipitation in the valleys. With afternoon temperatures above freezing, there was little if any snow accumulation in the lower valleys. On the 23rd, a cold front was followed by a wind event which mainly affected southeast Oregon and areas east of Boise. There were many reports of gusts in the 45 to 55 mph range, but a gust of 67 mph was measured at Twin Falls. On the 26th, another strong cold front swept across the region. One of the stronger gusts was 59 mph from the west-northwest at the Boise airport. A brief snow squall followed the front, but less than half an inch fell at the airport.
Here are the reported winds #gusts today across the area. Most of these wind gusts occurred with the intense band of #snow that moved through earlier this afternoon. #idwx #orwx pic.twitter.com/iwsbMmIhNg
— NWS Boise (@NWSBoise) February 27, 2021
On the 27th a trough from the Gulf of Alaska brought heavy snow to the mountains. Some of the heavier totals were 12 inches at Mores Creek Summit, 15 inches at Banner Summit, 20 inches at Tamarack, and 22 inches at Brundage.
— NWS Boise (@NWSBoise) February 27, 2021