Tuesday, March 31, 2020

Cold and Snow

As is only fitting, winter is coming to an end with another spell of unusual cold in much of Alaska; here are a few notable reports from the past several days.

-44°F  Umiat RAWS
-36°F  Eagle COOP
-34°F  Chicken COOP
-34°F  Tok 70SE CRN
-32°F  Salcha RAWS
-23°F  Goldstream Creek COOP

In Fairbanks, March is ending as the fourth month in a row with average temperature below the 1981-2010 normal, and it's the coldest first quarter of the year since 2007.

March was also a very snowy month in Fairbanks, as Rick Thoman illustrated nicely in the following plot (via Rick's Twitter feed); click to enlarge.

With 27.5 inches of snow, March was the snowiest month of the winter in Fairbanks.  Since the winter of 1929-30, this has happened 8 times before, so it's unusual but not rare.  Average (median) March snowfall is less than 5 inches, but it's a very skewed distribution, with occasionally much more occurring.

April is even more skewed, of course; the median is only 1.7", and yet even April has been the snowiest month of the winter on 4 occasions - most recently in 2008 (14.7").

Here's a frequency histogram of the snowiest month of the winter in Fairbanks, Bettles, and Anchorage.  Fairbanks stands out in terms of having October as the snowiest month rather frequently; I find this particularly surprising as rain is more common than snow in the first week or so of October in Fairbanks.

Other interesting features of the chart include the notable November-December peak in Bettles, and the enhanced frequency in late winter (February through April) in Anchorage.  Surprisingly, March has more often been the snowiest month than January in Anchorage, and average March snowfall lags only very slightly behind January.  I suspect part of this is an artifact of sampling variability, but there may well be a physical reason why January tends to underperform for snow; I don't think it's as simple as "it's too cold to snow", but if readers have any ideas, I'm all ears.


  1. I'm going to guess some here based upon 55 years of observations...during late September to October there's often a prolonged "loading" of the Interior's atmosphere with moisture. The ground's wet from fall rain events and same for low clouds with close dew points. All it takes in one cold front from Siberia or general NW and down the snow comes, at least near Fairbanks due to orographic lifting.

    January...generally not much moisture action from stormy seas due to the slowing of the tropical typhoon season in December. Sort of a hiatus for moisture influx in January. And cold dense air likes to sleep. By late meteorological winter action in the North Pacific picks up some and the jet stream gets rooting around in the cold Polar Vortex and snow happens some years.


    1. Thanks Gary. I do think moisture availability is important, together with large-scale temperature gradients. Both diminish from early to late winter as the N Hem oceans cool (with large lag relative to the atmosphere). And I agree there are links to the N Pacific typhoon season. But February is interesting, I'm not sure about that one.

  2. There's always the usual suspect drivers in spring like the increase in solar forcing, meandering rossby waves, and vertical wave activity flux (WAFz) indices. Still learning about their source and influence.

    It seems this winter what was asleep in Dec-Jan woke up in February. Once we got some insolation in the Northern Hemisphere North Pacific action resumed and then more frequent wetter flow from the SW and residual cold combined to make wind and snow.

    Maybe a focus on February weather from a historical perspective would inform? Fairbanks may lag traditionally in snow due to the immediate drying effect of the Alaska Range (providing the incoming flow direction allows), whereas Anchorage gets hit more directly due to ocean proximity while Bettles/Brooks Range receives sufficient moisture overrun that passes over or to the west of the Alaska Range.