September was warmer than normal. On the 4th, record highs of 102 were set at Boise and Burns. The 99 at Ontario tied the record for the date. On the 5th, the high of 100 at Boise tied the record for the date, and the 98 at Twin Falls set a new record.
It would have been even warmer had it not been for widespread smoke and haze from wildfires.
Latest Geocolor satellite image shows the extent of the wildfire smoke (brownish color), which stretches across the southern and western U.S. and the Pacific Ocean where it's getting wrapped into a low pressure system. #idwx #orwx pic.twitter.com/NuP9yhF5FG
— NWS Boise (@NWSBoise) September 12, 2020
— NWS Boise (@NWSBoise) September 17, 2020
It was a dry month, especially in southcentral and southeast Oregon, where totals were less than 5 percent of normal across a wide area.
A very warm upper-level high pressure ridge anchored over the western region kept temperatures above normal during the first week of the month. By the 8th the ridge had migrated far enough west to allow a very cool upper-level low pressure trough to drop south from Canada.
The cold front from the storm system over UT/CO has pushed an incredible amount of smoke from the Oregon wildfires offshore. Yellow and red spots are satellite detected fires over the past 24 hours. Expect chilly temperatures tonight under northeasterly flow. #idwx #orwx pic.twitter.com/rt9A7qviwG
— NWS Boise (@NWSBoise) September 8, 2020
The cold front ahead of the trough crossed our area on the 7th. Following the front, northwest winds in the 40-50 mph range were common. A gust of 54 mph from the northwest was measured near Boise.
The trough kept temperatures below normal. Highs failed to reach 70 at a number of locations on the 8th.
Following the trough, the ridge built inland, and temperatures were above normal again on the 11th. The ridge dominated through the 17th, with highs in the 80s at many lower valley locations.
The northern part of the ridge gave way to a Pacific cold front on the 18th. Most locations received only light precipitation. Isolated thunderstorms generated outflow winds over 40 mph.
From the 20th through the 25th, the ridge was alive and well over the southwest U.S., while southwest flow aloft on its northern periphery kept temperatures in our region above normal.
Meanwhile, a more fall-like pattern was developing as westerly flow aloft strengthened across the North Pacific.
The upper-level winds carried a fast-moving weather disturbance inland on the 25th. While rainfall was generally light in the valleys, McCall set a record for the date with .54 inch.
— NWS Boise (@NWSBoise) September 26, 2020
Temperatures stayed below normal through the 27th. On the 28th the ridge, now resurrected over the west coast, brought a warming trend. It would guarantee above-normal temperatures through the 30th and beyond.
High pressure brings ☀️ to southwest Idaho and eastern Oregon today, with clouds rotating clockwise around the Pacific Northwest. Also visible is smoke from wildfires in northern California, driven out to sea by offshore winds. #IDwx #ORwx pic.twitter.com/yOcepOoZ4A
— NWS Boise (@NWSBoise) September 28, 2020