December 2016 was the 5th coldest at the Boise Airport, where records go back to 1940. Only 1983 through 1985 and 1990 had colder Decembers. For the greater Boise area, going back to 1864, it ranks seventh all time.
During the first 15 days of December, half the days had above normal temperatures. There was one cold spell from 5th through the 9th, when a brief invasion of arctic air resulted in lows in the teens and highs below freezing. This previewed what was to come during the last half of the month.
Starting on the 10th, westerly flow aloft brought milder Pacific air to the region. Highs reached the middle 30s to lower 40s.
On the morning of the 14th, a temperature inversion had developed, with lows in the middle 20s in the valley. Moist air flowing over the top of the inversion dropped around 2 inches of snow on the valley floor by the morning of the 15th.
By the 16th, northerly flow over western Canada sent colder air into the intermountain region, insuring that the snow in the Boise area would not melt. Snow cover hindered daytime warming and enhanced overnight cooling, resulting in temperatures around 20 degrees below normal.
By the 19th, the pattern had shifted to westerly flow aloft, but it brought only slight daytime warming to the valley from the 20th through the 23rd. Significant warming was prevented by persistently cold nights.
On the 23rd and 24th, a strong and very moist Pacific weather system dumped 3 inches of sn ow at the Boise Airport on the 23rd, and 4.7 inches on the 24th. This set new snowfall records for both days. The snow depth of 9 inches at the Boise Airport on the 25th was the most snow on the ground ever recorded on Christmas Day.
The snowfalls 1.9 inches on both the 14th and the 16th also set new daily snowfall records.
Following this storm, westerly flow aloft persisted. Without snow cover, this pattern usually brings mild temperatures, as it did earlier in the month. But the deep snow during the last half of the month maintained cold arctic-like conditions.