Cold Snow

There's been a lot of interesting weather across Alaska in recent days - as demonstrated by the sudden uptick in blog posts - and here's another one that merits attention.

Back on Sunday, as the upper level trough pivoted over western Alaska, heavy snow developed in the frontal zone over Anchorage, even as temperatures at the surface remained uncharacteristically low.  Sunday's high temperature was only +2°F even while prolonged snowfall occurred in the afternoon and through the overnight hours, leading to an accumulation of 9.9" by midnight and another 6.7" the next day (adjacent to the airport).

Such a large snowfall at such a low temperature is unprecedented in the Anchorage climate data back to 1953, and not by a small margin: check out the following chart showing the joint distribution of daily high temperatures and daily snowfall.

The explanation for this outcome involves the rare combination of strong mid-atmospheric lift along the frontal zone, combined with an unusually cold airmass at the surface.  Here's the Anchorage airport sounding at 3pm on Sunday:

Notice the cold layer at the surface, with northerly winds of over 30mph at about 2500' above ground.  But higher up the winds were out of the southeast and south, and the sounding shows a deep layer of saturated (humid) air near or just below -10°C, which is a temperature conducive to growth of dendritic snow crystals.

Here's a map of 850mb temperatures (shaded) at the same time, showing the extremely cold air to the north and west, and much warmer air to the southeast of Anchorage; the snow occurred in the strong gradient between the two.

And the 500mb height analysis at the same time:

It's interesting to note that Fairbanks and Bettles also saw accumulating snow to an unusual degree for such cold weather.  In Fairbanks the daily total was only 0.6" - also on Sunday - but nevertheless this tied the record for most snow with such a cold high temperature (-27°F).  In Bettles 4.9" fell on Monday, with a high temperature of -13°F.