Temperature inversions plagued the Treasure Valley most of the month, resulting in below normal temperatures at Boise on all but eight days. Although it ranked among the coldest 25 percent of Decembers since airport records began in 1940, it averaged 5 degrees warmer than December 2016.
A persistent high pressure ridge kept the Boise area dry through much of the month. Precipitation was nearly half an inch below normal, and half the December 2016 total.
The month started out fairly mild under west-southwest flow aloft ahead of an upper level low pressure trough. The trough crossed the Boise area on the 3rd, bringing the largest one-day precipitation of the month, mainly in the form of rain.
As this system exited to the east, an upper level high pressure ridge built offshore and expanded inland.
By the 5th an inversion had become established in the Treasure Valley, and temperatures remained below freezing from the 7th through the 15th.
On the 16th an upper level trough destabilized the atmosphere enough to break the inversion, and temperatures continued to warm as the upper level flow shifted into the west and southwest in advance of tet another upper level trough. The 19th and 20th were the warmest days of the month, with highs of 52 and 51.
By the 21st the trough had moved east of our area. It was followed by moist northwest flow aloft and a cold front which brought snow totaling 6 inches on the 22nd and 23rd. The accumulation of 5 inches was the first measurable snow cover since February 4.
A combination of modified arctic air, overnight clearing, fresh snow cover, and calm winds caused the airport temperature to fall to 8 degrees by sunrise on the 24th, the coldest reading since the 7 degrees on January 18.
More snow was on the way. It started at about 4 pm Christmas eve as a warm front approached from the west. By Christmas morning 3 inches of new snow had fallen.
Warming continued at higher elevations, ensuring that cold air would remain trapped in the valley for a few more days.
On the 30th a cold front passed through the area. It was felt as a warm front in the valley, as colder air aloft behind the front broke the inversion, allowing the airport temperature to climb to 43 degrees that afternoon.
On the 31st strong high pressure provided mostly clear skies. Although a shallow inversion had formed overnight, the sun warmed the surface enough during the day to mix the air, and temperatures around Boise warmed into the 35-40 degree range.
Precipitation across much of southern Idaho and Oregon was well below normal as a persistent upper level high pressure led to an inversion with cool conditions in the valleys and warm conditions in the mountains.