Friday, September 29, 2017

First Flakes

The first snow of the season was reported at Fairbanks airport early this morning, but so far it's only a trace amount and mixed with rain.  According to long-term records, more than 80% of years bring at least a trace of snow to Fairbanks in September, and in recent years it's been more like 90%; the only years in the past two decades without any snow in September were 2007 and 2010.  Of course two years ago 11 inches fell on this date:

Up in the hills, rain turned to snow yesterday afternoon as colder air moved in aloft.  Here are a couple of webcam shots from 3pm and 7pm yesterday at 2200' on the Steese Highway.

Temperatures in Fairbanks have now entered the 6-week period of most rapid decline, with peak cooling of around 6°F per week occurring near October 20.  It's interesting to note, however, that daily maximum temperatures cool more quickly at first, and daily minimum temperatures then catch up; the difference in cooling rates is most pronounced from mid-September to early October.

This nuance in the seasonal transition is related to snow cover, because having snow on the ground makes a bigger difference for minimum temperatures than maximum temperatures; with snow on the ground, the temperature can drop very rapidly at night with clear skies and light winds.  Most years do not have snow on the ground in Fairbanks at the beginning of October, but snow cover becomes 30% likely by October 9 and 50% likely by October 14.  So the establishment of snow cover plays an important role in reinforcing the overall downward temperature trend after about the first week of October.

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