January was cold, snowy, and unusually foggy.
It ranked as the 9th coldest January at the Boise Airport and the 14th coldest since records began. Highs reached 32 degrees or higher on only 12 days. Lows fell below zero on 5 nights. The high temperature of 6 above and the low of 11 below on the 6th set new records for that date.
The lows of 11 below on the 6th and 10 below on the 7th were the coldest temperatures since the low temperature of 25 below on December 22, 1990. Since then temperatures dipped to zero or below only five times. The coldest reading during the 1991-2016 period was 7 below on December 9, 2013.
It was the snowiest month since December 1983, when 26.2 inches fell. The snowiest month on record was January 1929, with 27.0 inches.
There was fog on 26 days, 6 of which had dense fog, with visibilities a quarter mile or less.
On the 1st an upper level high pressure ridge over the Gulf of Alaska extended north over Alaska, enabling northerly flow aloft to transport weather systems and cold air south from Alaska and the western Canadian arctic.
From the 1st through the 5th an upper level trough, which originated over the gulf of Alaska, presided over the Pacific Northwest states. It brought Boise 9 inches of snow, increasing the snow depth to 15 inches on the 5th, the most since snow depth records began in 1940, as arctic air moved into the region from British Columbia. The 0.45 inch precipitation on the 4th set a new daily precipitation record for the date. It came in the form of 6.5 inches of snow, the 15th highest calendar day snow total since 1892.
The colder air, along with clearing skies and fresh deep snow, provided ideal conditions for radiational cooling and subzero temperatures.
On the 8th southwest flow brought a period of relatively mild weather. Winds were strong enough at all levels to prevent an inversion from forming, and temperatures actually averaged above normal from the 8th through the 11th.
The cold pattern returned on the 12th as an upper level trough deepened over the region. The trough departed for California and Baja on the 13th, but cold air remained trapped in the Treasure Valley as an upper level ridge brought warming aloft, forming a temperature inversion. The 5 inches of snow cover hindered daytime warming and guaranteed cold overnight temperatures. In this “Homemade” arctic air, highs were only in the teens and lows were near zero from the 15th through the 17th.
Milder weather returned from the 19th through the 22nd under southwest flow aloft, thanks to an upper level low pressure system off the northwest coast.
As the low moved inland on the 23rd, it brought Boise 3 more inches of snow along with colder air, which as usual became trapped in the valley. Another temperature inversion formed and intensified as an upper level ridge built over the northwest U.S. Valley temperatures were not at cold as earlier in the month, but still averaged about 10 degrees below normal.