Overall, fall was warmer and wetter than normal. Temperatures were generally above normal in September and October, and above or much above normal in November. September precipitation ranged from above to below normal. In October the entire area was wetter or much wetter than normal. The trend reversed in November, with below normal precipitation nearly everywhere.
Temperatures were above normal over more than half of the area. Precipitation presented a patchwork pattern, reflecting the predominantly convective nature of the heavier precipitation events.
A weak upper level low pressure trough hanging over the northwest U.S. kept temperatures 5-10 degrees below normal through the 3rd.
A warming trend commenced on the 4th as an upper level high pressure ridge built over the Intermountain Region. Temperatures topped out at 10-15 degrees above normal on the 9th. Wildfire smoke limited heating to some extent, but temperatures still managed to reach the 93 at Baker City, Burns, and Twin Falls, 94 at Ontario, 95 at Jerome, and 98 at Boise. Except for Ontario, these were all records for the date.
The high the following day was 20 degrees cooler as a Pacific cold front and upper level trough crossed our area. Many locations experienced wind gusts in the 40-50 mph range as the front passed.
Yes the fog is making it difficult to see the mostly smoke free sky this morning. But we checked the mountain cameras (below courtesy of Bogus Basin) and can confirm once the fog dissipates it will be nice clear day! #Idwx #orwx pic.twitter.com/isQCzLYPrJ
— NWS Boise (@NWSBoise) September 11, 2021
Following the trough, temperatures remained cool on the 11th. Slightly warmer westerly flow aloft from the 12th through the 15th provided highs in the lower 80s at lower elevations…near normal for mid September.
— NWS Boise (@NWSBoise) September 14, 2021
Another Pacific cold front passed on the evening of the 15th, lowering highs on the 16th by around 10 degrees.
Southwest flow aloft ahead of a much stronger Pacific cold front added 10-15 degrees to highs on the 17th and 18th. As the front passed, gusts of 40 to 55 mph were measured at many locations. Precipitation with the front was generally light, but a bit more fell on the 19th under the upper level trough which followed the cold front. Highs on the 19th and 20th were mostly in the 50s and 60s, the coolest of the season so far.
A high pressure ridge brought a warming trend on the 21st and 22nd. Warming was briefly interrupted on the 23rd as a weak trough crossed our area. The ridge strengthened behind the trough, and highs rose into the 80s at lower elevations from the 25th through the 27th.
During the afternoon of the 27th, desert winds from the south brought dry air with humidity dropping into the single digits at several locations.
On the 27th gusts of 45-50 mph were reported at many locations following a strong cold front. On the 28th the winterlike trough following the front lowered snow levels to around 5000 feet. At Bogus Basin, nearly 3 inches had accumulated at the snow stake just before 11 am MDT.
— NWS Boise (@NWSBoise) September 28, 2021
Temperatures in the north Pacific air that afternoon were as much as 30 degrees cooler than the previous day`s highs.
By sunrise on the 29th, cool air and clear skies allowed the temperature to drop into the mid and upper 30s in the Treasure and Magic Valleys. Since winds were light, that was cold enough for patchy frost. McCall froze with 29 degrees, but the coldest readings were in eastern Oregon, with 23 at Baker City and 20 at Burns.
The month ended under a high pressure ridge which brought sunny skies and near normal temperatures.
October was noteworthy for heavy rainfall events. Precipitation was over 150 percent of normal across wide areas of southeast Oregon and southwest Idaho. Significant areas had over 300 percent of normal.
During the first five days of the month, temperatures rose to summerlike values under a high pressure ridge. On the 4th, southwest flow aloft ahead of a low pressure trough raised the temperatures into the 80s at many valley locations.
After reaching the coast, the trough deepened far enough south on the 6th and 7th to pick up tropical moisture. As the system moved inland on the 8th, precipitation was hit and miss. Ontario reported no rain, only .04 inch was measured at Baker City, and Burns got only a trace. But Boise picked up .46 inch, Jerome .65 inch, McCall .63 inch, and Twin Falls received a total of .71 inch on the 7th and 8th.
Cool showery weather persisted through the 10th.
A colder trough moved inland on the 11th. Temperatures averaged 10-15 degrees below normal through the 14th. There was only light of precipitation.
During the next couple of days, a ridge brought a warming trend. As it shifted east, southwest flow ahead of the next Pacific cold front raised the temperature from the upper 60s to the mid 75s on the 17th. Cool air behind the front lowered highs on the 18th by 10-15 degrees.
By the 19th a weak ridge had again built over the Intermountain Region.
Southwest flow aloft intensified as the next trough approached the coast. Like the system earlier in the month, it had entrained tropical moisture. As it moved inland over the northwest states on the 22nd, rainfall totals ranged from a quarter inch to over half an inch in the valleys. McCall got .88 inch.
Another even deeper trough from the Aleutians intensified as it approached the coast on the 24th, resulting in even stronger southwest flow from the tropics to the Pacific Northwest and Northern Rockies by way of California.
This cool-season phenomenon is known as an “atmospheric river” or AR. It is a narrow region of poleward transport of moisture, where strong flow taps very moist air from the tropics and carries it to higher latitudes. An intense low pressure system developing on the west side of the AR, plus the associated cold front and warm front, lend lift to the extremely moist and potentially unstable air. So the northern Sierra Nevada, Cascade, and Owyhee ranges, which usually create “rain shadows”, have limited effect on the amount of rain falling downwind.
Heavy precipitation was widespread on the 24th and 25th. By far the heaviest amounts fell in the Boise mountains, where totals exceeding 2 inches were common. The winning totals were 3.05 inches 4 miles southwest of Featherville, and 4.83 inches 1 mile south-southeast of Featherville.
— NWS Boise (@NWSBoise) October 24, 2021
The 24th was not only wet, but very windy. Gusts over 40 mph were common. Several locations experienced gusts over 50 mph. A gust of 65 mph was measured at a location 6 miles north of Wagontire in Harney County, Oregon. In Washington County, Idaho, a gust of 64 mph was measured 15 miles west-northwest of Cambridge.
The ridge warmed temperatures to 10-15 degrees above normal on the 28th and 29th. Then another Pacific cold front dropped temperatures to near normal on the the 30th.
November was unseasonably warm and dry. The month began with above normal temperatures under an upper level high pressure ridge. Disturbances moving into the ridge brought only light precipitation.
— Danielle Strollo (@dstrollo) November 4, 2021
Southwest flow aloft brought more light showers on the 5th and 6th, and temperatures cooled to near normal on the 7th following another weak Pacific cold front.
⛅️A beautiful morning on top of Bogus Basin with snow capped mountains and valley stratus. Today will be clear and dry ahead of incoming rain and mountain snow on Tuesday.
#IDwx #ORwx pic.twitter.com/yWeTPWgkD2
— NWS Boise (@NWSBoise) November 7, 2021
On the 9th moderate amounts of precipitation accompanied a Pacific cold front and upper level low pressure trough. Totals from a quarter to a half inch were common, the largest one day precipitation totals of the month at many locations.
On the 11th a high pressure ridge initiated a warming trend, and temperatures averaged 10-15 degrees above normal through the 15th. Disturbances moving around the northern periphery of the ridge brought light showers from the 11th through the 14th.
During the night of the 16th, a strong but dry cold front from the Gulf of Alaska was followed by the coldest air so far this season, with lows below freezing at nearly all locations. Both Burns and McCall registered lows of 15 degrees.
On the 19th a Pacific cold front triggered scattered thunderstorms. In the Boise area, small hail was reported at a few locations, and there were a couple of reports of wind gusts over 50 mph.
A high pressure ridge built in from the southwest on the 20th and kept our area dry through the 22nd.
On the 23rd another pacific cold front brought showers and cooler temperatures.
A high pressure ridge centered off the California coast expanded north on the 24th. It kept the area dry through the end of the month, but it created a temperature inversion, resulting in air stagnation in the lower valleys. Despite the inversion, afternoon heating under mostly sunny skies pushed temperatures above normal during the final week of the month.