November’s temperatures averaged close to normal. Precipitation was generally above normal, although there were areas near the Nevada border where precipitation was less than 50 percent of normal. A strong upper level high pressure ridge over the Intermountain Region on the 1st and 2nd weakened on the 3rd, but above-normal temperatures continued through the 6th ahead of a Pacific cold front.
— NWS Boise (@NWSBoise) December 1, 2020
— NWS Boise (@NWSBoise) December 1, 2020
On the 2nd, the highs of 70 at Burns and Twin Falls tied their records for the date. The 74 at Jerome set a new record, as did the 68 at Baker City. On the 4th, new records were set at both Burns and Baker City with highs of 70 and 71 respectively. On the 5th, Boise’s high of 76 set a new record for the date, as did the highs of 75 at Burns, 69 at Ontario, and 72 at Baker City.
On the 6th, the high of 67 at Ontario set a new record for the date. The high of 71 at Mountain Home tied their record.
The cold front crossed southeast Oregon and southwest Idaho on the 6th with little if any precipitation. The cold upper-level low pressure trough behind the front drifted south over California before settling over the Great basin on the 7th. At the same time, a second trough from Alaska was deepening over western Canada.
On the 8th, light to moderate amounts of snow fell as the Canadian trough joined forces with the Great Basin trough. Snowfalls in the Boise area ranged from 1 to 4 inches. The 2.7 inches at the Boise Airport set a new record for the date.
Yet another trough, this time from the Gulf of Alaska, arrived over the northwest U.S. on the 10th with only light snow. Meanwhile, a band of westerly flow aloft, aka the jet stream, was strengthening south of the Aleutians. Its arrival over the northwest coast on the 13th was preceded by a very active weather disturbance.
Rainfall amounts with this system included over two-thirds of an inch at Jerome and McCall, nearly a half inch at Boise, and a third of an inch at Burns and Ontario.
Heavy snow fell in the mountains, including 9 inches east of New Meadows, 8 inches northwest of McCall, and 10 inches near Halfway and Sumpter in Baker County.
Snow amounts in the mountains have been impressive so far- here's a look at a few area snow stake cams as of 3 PM MDT. McCall up to 9 inches! Let us know how much snow has fallen at your location. #IDwx #ORwx pic.twitter.com/iYftS7t5yM
— NWS Boise (@NWSBoise) November 13, 2020
Also in eastern Oregon on the 13th, a wind gust of 66 mph was measured north of Wagontire, and a gust of 63 mph was measured north of Andrews.
On the 15th, copious Pacific moisture accompanying a warm front brought more rain. The 0.46 inch at Boise and the 0.28 at Twin Falls were new records for the date.
More snow fell in the mountains, including 4 inches east of New Meadows and 9 inches at Cuprum in western Adams County.
Temperatures had been below normal since the 7th, but an upper-level high pressure ridge following the warm front raised temperatures above normal on the 16th.
On the 17th, a cold upper-level low pressure trough, which had been deepening off the B.C. coast, generated a strong cold front which pushed rapidly inland. The front, energized by the jet stream, was accompanied by thunderstorms with abundant lightning as it roared across southwest Idaho during late afternoon on the 18th.
Quite a cold front that moved through the region today. There were over 4400 in cloud flashes (shown in green) and 1100 cloud to ground strikes (blue/red) across the region today. #idwx #orwx pic.twitter.com/2VNbIp8jqo
— NWS Boise (@NWSBoise) November 19, 2020
Storm reports in the Boise area included brief heavy showers which produced from a quarter to a half inch of precipitation, small hail covering the ground at some locations, and wind gusts exceeding 40 mph. An inch of rain fell at Placerville and three-quarters of an inch near Horseshoe Bend.
The cold front caused only slight cooling, and temperatures remained above normal through the 19th under southwest flow aloft.
The next several days were much less dramatic as a series of relatively weak troughs and ridges crossed our area. During the morning of the 25th, a better organized trough brought light snow. One inch was measured at the Boise airport. Most of it melted during the afternoon.
Enjoy this time lapse from the top of Squaw Butte northeast of Emmett looking toward Boise of the fog flowing through the valleys. Shafer Butte is in the center of the image. #idwx #orwx pic.twitter.com/IKZxnIDklC
— NWS Boise (@NWSBoise) November 26, 2020
By the 27th, a high pressure ridge had built over the northern Intermountain Region. As is typical of this time of year, the warm air aloft and nighttime cooling in the valleys resulted in an inversion which kept temperatures below normal from the 28th through the 30th.
On the 30th, a weather disturbance brought more light snow. It was not strong enough to break the inversion.
Snow across the mountains was well above normal for November standards.
All the rain and snow we've been getting lately has resulted in a great start to the mountain snowpack this winter. Snow water equivalent across Oregon and much of Idaho is well above normal for the date. #IDwx #ORwx pic.twitter.com/tiaQc0P8FQ
— NWS Boise (@NWSBoise) November 20, 2020