February 2018 can be described as three seasons in one month.
It began with early spring. The 1st through the 9th had the same average temperature as the last half of a normal march. The 59°F on the 2nd tied the record for the date set in 1881. The 61°F on the 3rd eclipsed the old record of 60°F in 1953.
Temperatures from the 10th through the 18th were near normal for late winter.
It was back to mid-winter from the 19th through the 28th, which averaged colder than a normal December.
The average temperature for the entire month was deceptively close to normal.
February is usually the driest winter month at Boise. This February had only 56 percent of normal precipitation, despite heavier than average snowfall. Measurable precipitation fell on only 4 days.
Snowfall totaled 7.4 inches, over twice the normal 2.8 inches. There was at least patchy snow cover each day during the last week of the month. The 4.9 inch snowfall on the 22nd set a record for the date. The old record was 4 inches in 1912. The greatest snow depth at the Boise Airport was 5 inches on the 22nd.
A warm upper level high pressure ridge off the coast was responsible for the spring-like weather from the 1st through the 9th. It also kept most of the precipitation associated with Pacific weather systems east of our area.
The ridge gradually shifted west, allowing northwest flow aloft to transport cooler but not unseasonably cold air into the Intermountain region from the 10th through the 18th. One weather system embedded in the flow was strong enough to bring Boise a quarter inch of precipitation on the 14th, including half an inch of snow.
The ridge continued to drift west as a very cold upper level trough deepened over western Canada. Strong northerly flow on the west side of an upper level low over Hudson Bay pushed the trough south of the border. The cold front ahead of the trough passed Boise at about 9 pm on the 17th. Following the front, the wind increased from the northwest, and a peak gust of 43 mph was recorded at the airport just after midnight.
Temperatures initially were not unseasonably cold, but by the 19th modified arctic air had begun to filter into the Treasure Valley, while the true arctic front was stalled in the central Idaho mountains.
Although the coldest air stayed well to our north and east, clear skies, light winds, and very dry air allowed the temperature to drop into the single digits in the Boise area by sunrise on the 20th. The low of 9°F at the airport was the coldest since the 8 degrees on December 24, 2017.
The cold pattern persisted through the end of the month, with a deep upper level trough anchored over the western U.S. and western Canada. Temperatures remained unseasonably cold, and weather disturbances moving into the trough from Alaska brought periods of snow.