2015 was the 2nd warmest on record at Boise. 2015 also had the warmest June on record, the warmest June day on record (110°F) and the latest 90 degree day ever recorded at Boise, on October 10th.
October 2015 was the 11th consecutive month of above normal temperatures at Boise, and the 2nd warmest in 76 years of airport records. The 90°F highs on the 9th and 10th were records for those
dates. The 90°F high on October 10 was also the latest 90°F ever recorded. Overnight lows were also unusually warm. The 56°F on the 8th set a new record for the date, and the 59°F on the 10th tied the record set in 1942.
The late summer warmth might have lasted another week had it not been for the Walker Fire, which blanketed the valley with smoke from the 11th through the 17th. The smoke filtered enough sunlight to prevent temperatures from warming much above the 70s.
At Boise, the average date of the first 32°F low is October 10th. This year, at the airport, the coldest reading for the month was 37°F on the 23rd. But that same morning, frost was seen around the Treasure Valley, some Citizen Weather Observer Program (CWOP) sites at Meridian and Eagle reported lows of 30°F and 32°F respectively.
At the Boise Airport, the latest occurrence of a freezing temperature was November 11, 1944. Last year the first freeze happened on November 10, ranking second all-time.
The above-average temperatures were partly a reflection of a warm upper level high pressure ridge which persisted over the western U.S. through most of the month.
Showery periods raised the precipitation total to slightly above average for the month. On the 1st and 2nd, the 17th through the 20th, and the 25th through the 28th, low pressure troughs managed to push inland through the ridge. Showers on the 30th came from a warm front which tapped very moist air from off the Washington coast.