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 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 airport records. Measurable precipitation fell on only five days.
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, the temperature maxed out at 80 degrees under 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 produce a thunderstorm and nearly a quarter inch of rain.
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 on the 30th and 31st, although there was gradual warming.
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.