Thursday, January 16, 2014

Weak Inversion

The brief cold spells in interior Alaska over Christmas and earlier this week brought strong low-level temperature inversions to the Fairbanks area, as is typical for the time of year; but since the beginning of the climatological inversion season (October 19), far more days have had weak or non-existent inversions than strong inversions.  The chart below shows the daily inversion strength based on the daily mean temperatures at Keystone Ridge and Fairbanks International Airport.  (Note the missing data from Keystone Ridge in early December.)  Remarkably, there were seven consecutive days in December with a positive mean temperature lapse rate (no inversion); the average number of such days in December is only two.

Looking at the mean inversion strength from October 19 through January 13, this winter so far has the weakest mean inversion since records began on Keystone Ridge in 1996 (see chart below).  Over the same dates, airport temperatures have averaged 4.7 °F above normal while Keystone Ridge has been 1.8 °F above normal; and so the inversion has been 2.9 °F weaker than normal.


  1. The analog forecast called for a cold Jan. While it feels like the temps have been above normal, the inversions have been strong. This brings up two possible research topics:

    1. What is the analog forecast for Fairbank inversions?
    2. Based on history, what is the probability of another -30 cold snap?

    1. Eric,

      It certainly looks like the analog forecast is a bust, as the month appears sure to end warmer than normal. Some of the forecasts are showing quite extreme warmth now in the medium-range. I'll discuss possible reasons for the analog failure when it's all said and done.

      Keystone Ridge data only go back to 1996, so it's not possible to examine the PAFA-KERA inversion strength in all the analog years. Of the years since 1996, 4 of 7 had stronger than normal inversions in January, 3 of 7 were weaker. Looking at the inversion strength from the surface to 850 mb at the airport, 7 of 10 analog years showed stronger than normal inversions in January; this is consistent with the idea of a cold month overall.

      Historical data from the 1930-2013 and 1981-2013 periods both suggest about an 85% chance of reaching -30 F again this winter, after accounting for the fact that it probably won't happen in the next 10 days. Plenty more winter lies ahead!

  2. Today the CPC offered up their February and 3-mo outlooks. Not bad. We might not freeze too muchl.


    30 day:

    90 day:

    Gary (getting his Harley ready for riding)

    1. Link to their improved interface: