Wednesday, October 8, 2014

Rapid Autumn Transition

The hardy residents of the Fairbanks area are used to rapid seasonal changes and dramatic temperature swings, but the change in the past 3-4 weeks has been remarkable even for the usually volatile interior climate.

As we noted here and here, a strong chinook event raised the temperature to 76 °F at Fairbanks airport on September 14, which was close to the warmest on record so late in the season, but only 3 weeks later there was 4 inches of snow on the ground with a high temperature of 31 °F.  Given that the peak normal high temperature in early July is 74 °F, we might call this a transition from summer-like to wintry conditions in only 3 weeks.

Is this a record for rate of transition?  Based on these specific criteria (last 75+ °F day, first sub-freezing day with 2+" of snow on the ground), yes.  See the chart below; click for a larger image.  It's interesting to note the break from the relatively uniform results of the past 18 years.

We can show a similar result for Keystone Ridge.  The highest September temperature of 66 °F did not quite reach the peak normal for summer, but the daily mean temperature on the 13th was above peak normal for summer.  Looking at the time from last 65+ °F day to first wintry day (same definition), we again see a transition that was shorter than in recent years (only 16 days this year).

We don't yet know for sure if the snow cover in Fairbanks is the permanent winter snowpack (although it's looking increasingly likely), but if it is, then we would tie the record for shortest time from last 70+ °F to arrival of the snowpack.  The record is 20 days and was set in 1974 and 1992.  The record for shortest time from last 75+ °F to snowpack is 25 days, set in 1965.


  1. Depending upon species and elevation, there's still green leaves and plants covered with snow. The streets are finally giving up any residual heat and ice covered roads are common, although there was some early afternoon melting today. Small ponds are ice covered, and the International Airport float pond was half covered with ice today about a week early.

    Hardy will turn to crabby if the mild winter earlier forecasted by the CPC fails to impress.


  2. I think that since most septembers don't have chinooks that you've introduced a bias and thus have an orange among apples. What is the rate of transition if we ignore the days with the Chinook?

    1. Eric, I understand your point from a certain perspective, but my point was that this year IS unusual - because of the chinook warmth followed by early snowpack. Such a rapid change from summerlike warmth to wintry conditions is a rare occurrence; so indeed it is an orange among apples.

      We could remove the warmest days of mid-September and then come up with a temperature transition rate, but I'm not sure which days would qualify or how to consistently remove similar events in past years; it seems that any decision would be rather arbitrary. But I'll give it some more thought.