January 2018 was the fourth warmest on record at the Boise Airport. For all Boise locations it tied January 1914 for sixth place. The monthly average of 37.8 degrees was 6.5 degrees above normal, and it even exceeded February’s normal of 36.5 degrees. Only the first four days of the month were below normal.
It is noteworthy that, prior to this year, the top five warmest Januarys at the airport occurred during El Niño winters, and three of those were strong El Niños. January 2018 didn’t fit the pattern, as a weak La Niña was in effect.
There were two record highs, each occurring ahead of strong cold fronts. The 58°F on the 18th and the 59°F on the 30th broke the previous daily records of 57°F in 1998 and 56°F in 1992.
The unusually mild temperatures resulted from a combination of a progressive pattern and a persistent warm upper level high pressure ridge over the Western U.S.
Westerly flow aloft brought mild moist Pacific weather systems inland across the northern intermountain region, temporarily displacing the ridge, which always managed to rebuild. The active pattern hindered the formation of temperatures inversions and blocked invasions of cold air from western Canada.
Precipitation totaled 1.36 inches, close to the January normal of 1.39 inches. Most of it fell during two episodes.
On the 9th a cold front produced nearly half an inch of rain at the Boise Airport. During the afternoon of the 19th another cold front generated nearly a quarter inch of rain. The upper level low pressure trough which followed the front brought an additional two tenths of an inch of rain.
On the 30th the last cold front of the month passed the airport at 1:40 pm MST, with a wind gust of 34 mph from the northwest, but no precipitation.
The only measurable snowfall was 0.1 inch on the 25th. The total for January 2017 was 21.5 inches. Normal is 5.1 inches. There have been Januarys with less snow. Only traces fell in 1934, 1961, and 2003.
McCall, Idaho recorded its 4th warmest January on record in 2018. Daily average minimum temperatures in January 2018 averaged 21.4°F, 3rd warmest on record.
Burns, Oregon recorded their 6th warmest January on record in 2018.
Baker City, Oregon had their 3rd warmest January on record in 2018. Average maximum temperatures reached 42.9°F, tied for the warmest on record in January since records began in 1928.
Jerome, Idaho had their 3rd warmest January on record since 1916. Two record highs were set in the month of January. Average minimum temperatures tied for the second warmest on record in January at 28.5°F.
Ontario, Oregon had their 4th warmest January on record. A daily record high temperature of 54°F was recorded on January 12.
With the upper level high pressure ridge over the Great Basin,the storm track was directed to the north across British Columbia, Washington, northern Idaho, and western Montana, leading to warmer than normal temperatures.