Despite cold weather during the first couple of days and the last week of the month, the average temperature for December was 1.1 degrees above normal. Abnormally mild weather from the 3rd through the 23rd more than compensated for the two cold spells.
Unsettled weather predominated. Precipitation was observed on all but 6 days, with measurable precipitation on 16 days, yet the monthly total was only slightly above normal.
What seemed like an endless series of pacific weather systems plowed into the northwest states, dropping most of their rain and snow on the west side of the Cascades. These systems tended to split as they moved inland, sending most of their energy south, further limiting precipitation totals at Boise. But there were a couple of exceptions. The storm on the 7th tapped into subtropical moisture, resulting in around half an inch of rain in the Boise area.
On the 13th, a storm with formed off the northwest coast. As it moved inland, a strong cold front and unusually unstable air for this time of year dumped over a third of an inch of precipitation in the form of rain and half an inch of snow.
A pattern change which started on the 23rd cut off the flow of moist Pacific air. An upper level trough began to form over the Pacific Northwest coast. As it deepened, it pushed a cold front across the Boise area depositing an inch of snow, enough for a white Christmas.
By the 26th, the trough was entrenched over the western states, as far south as southern California and Arizona. Cold air aloft with fresh snow cover and clearing skies provided ideal conditions for radiational cooling, filling the Treasure Valley with cold stagnant air.
By the 27th, a temperature inversion had been created as a warm upper level ridge built over the area.