Temperatures were well below normal during October 2019. Jerome, ID, Fairfield, ID, Burns, OR, Halfway, OR and Ontario, OR recorded the coldest October on record. Fairfield, ID had snow cover at the end of the month, and shattered their previous October record by 5.6 degrees, finishing 10.3 degrees below normal for the month. Idaho as a whole, ranked as the coldest October on record.
It was a dry month, with below normal precipitation across most of southeast Oregon and southwest Idaho. But a few locations, mainly in Harney County, were actually wetter than normal.
It was the third coldest October in 80 years of airport records, and 6th coldest on record at Boise. Temperatures averaged above normal on only five days.
The low of 23 on the 10th eclipsed the old record for the date of 26 set in 1985.
The high of 35 on the 29th broke the previous record for the date of 37 set in 1991. It was also the coldest daily high temperature for so early in the season.
The low of 16 on the 30th tied the old record set in 1991. It was the coldest temperature since the 10 degrees on February 22, which was last winter’s low.
It was a dry month, tying 1965 as the 12th driest October in Boise Airport records. Measurable precipitation fell on only five days at Boise.
The unseasonable cold can be blamed on the persistent pattern of cold upper level low pressure troughs interspersed with cool northwest flow aloft. The northwest flow was a result of a strong high pressure ridge off western North America, directing the storm track into Alaska. Since most of the troughs came from northwest Canada and the cold waters of the Gulf of Alaska, they brought little moisture.
There were a couple of exceptions to the predominately cold and dry weather.
On the 15th an upper level high pressure ridge built over the western U.S. and southwest Canada. As the ridge drifted east on the 16th, temperatures rose to summerlike values in southwest Idaho. Boise, Jerome, and Twin Falls all recorded highs of 80 degrees under warm southwest flow aloft.
On the 19th a broad jet stream over the north Pacific pushed a fast-moving upper level trough inland across Idaho. There was enough moisture and energy with this system to generate thunderstorms with strong gusty winds. There were numerous reports of gusts over 50 mph. At some locations in the Magic Valley, including Twin Falls and Hazelton, gusts exceeded 60 mph.
Power poles were knocked down near Wendell and at Caldwell. Around 30 poles were knocked over along Highway 93 between Twin Falls and Jerome. At Kuna, several large trees were down, fences were blown over, cars were damaged, and power lines were down. Northeast of Boise in Ada County, lightning struck a house, causing a fire. Two people were injured.
Widespread severe wind speeds across SE Oregon and SW Idaho. Here are some of the preliminary max wind gusts today. Several large trees down and powerlines damaged from earlier this afternoon. #IDwx #ORwx pic.twitter.com/gGLUHuzNFA
— NWS Boise (@NWSBoise) October 19, 2019
And it snowed. McCall got 4 inches and Tamarack 6 inches. A spotter northeast of Featherville reported 7 inches that day.
Even more snow fell on the 20th. Tamarack got an additional 4 inches, making their storm total 10 inches. Other storm totals in the mountains ranged from 6 inches at Mores Creek Summit to 12 inches at Atlanta Summit.
— NWS Boise (@NWSBoise) October 20, 2019
Real winter cold arrived at the end of the month.
Winter weather arrived very early during the final week of the month. By the 26th northwest flow aloft had transported cooler air from British Columbia to southwest Idaho. From the 26th through the 28th, A very cold upper level low pressure trough moved from north of Alaska through central Canada to the U.S. Border, causing the flow over western Canada to strengthen and shift into the north. Arctic air was on the way. The arctic front passed Boise around 4 am on the 29th. Behind the front, very cold high pressure centers moving south on both sides of the Rockies caused northwest and northeast surface winds to converge over the Snake River valley.
The convergence zone approached Boise from the east later that morning, generating the first snow flurries of the season at the airport and in southeast Boise, while the sun shone on the rest of the city. At the airport nearly half an inch of very dry snow briefly covered the ground.
Temperatures stayed well below normal through the 31st. Here are some record temperatures during the cold spell.
October 29: lows…Burns 1, Ontario 15, Baker City 6.
October 30: high…Boise 35 (coldest high temperature for the date and so early in the season) lows…Boise 16 (tied 1991), McCall 4 (tied 1991), Ontario 9, Mountain Home AFB 9, Jerome 8, Baker City 4
— NWS Boise (@NWSBoise) October 30, 2019
Below are graphics of mean temperature percentiles and departure from normal across the Pacific Northwest for October 2019.
For more information about the U.S. Climate in October 2019, see Assessing the U.S. Climate in October 2019.