Tuesday, November 19, 2019

Winter Rain History

After a nasty ice storm a couple of weeks ago, Bethel experienced blizzard conditions today, with temperatures below 10°F and 40+mph wind gusts combined with heavy snow this morning.  However, as the warm front approached this afternoon, warm air aloft caused the snow to change over to a brief period of freezing rain again before precipitation ended.

The (nominally) 3pm balloon sounding from Bethel shows the dramatic temperature gradient just above the surface.

At the time the balloon was launched (2pm), the surface air temperature was 16°F, but freezing rain was already occurring as there was enough warm air upstream to melt falling snow; the sounding profile shows the temperature just nudging above freezing at 400m above sea level.  Notice the veering (clockwise directional change) of the wind with height, indicative of the rapid approach of warm air.

As of 6pm this evening, the temperature is up to 36°F, and the wind has gone around to the southeast; it must feel positively tropical compared to the conditions only 8 hours earlier.  Check out the temperature gradient and wind shift across the Y-K delta region in the 6pm surface data (click to enlarge):

Here's the 3am surface analysis, courtesy of Environment Canada; note the strong pressure gradient over southwestern Alaska, with a cold high to the north (-28°F at Galena this morning, -26°F at Kaltag).

In light of the ice storm earlier in the month (see my earlier post), I took a quick look at the historical frequency of liquid precipitation (rain or drizzle) in winter at Bethel and a few other sites.  The chart below shows the annual percentage of winter (November-March) hourly observations that reported rain or drizzle, regardless of temperature, in Bethel.  Note that I've excluded supplementary observations that were not taken at the top of the hour.

I'd be surprised if there are not some systematic biases between the pre-ASOS era (pre-1998) and the modern era, so I think about all we can say is that there's no evidence of an increase in the frequency of winter rain in Bethel.  The frequency has been relatively high in the past decade, but it seems to have been high also in the 1980s and in many previous years.

Here's the same chart for McGrath and Fairbanks, with the same vertical scale to emphasize the drastic difference in climatological frequency of liquid precip in winter.  If the trends are reliable, McGrath seems to have seen a rather steady decline in frequency, whereas Fairbanks has seen an upward trend in the last 20-30 years (but perhaps not exceeding the levels of earlier decades).

For completeness, I include the chart for Anchorage below, which also shows nothing particularly exceptional.  However, the second chart below shows the frequency of liquid precipitation at sub-freezing temperatures, i.e. freezing rain or freezing drizzle, and this does seem to show a decline since the 1980s (the 2013-2014 spike notwithstanding).  It seems possible that the warming trend in the more maritime zone of south-central Alaska has tended to produce more plain rain at the expense of freezing rain in winter.

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