Historic Winter Rain

Extraordinary and historic winter rain has been occurring today across a large swath of interior Alaska extending all the way up to the Yukon Flats and the southern edge of the Brooks Range.

This blog has documented many instances of winter rain in Fairbanks, but this is something else.  In data extending back to 1948 (albeit with one missing decade from 1964-1974), Fort Yukon has only reported plain rain (i.e. with a temperature above freezing) in November through February on two occasions, and that briefly: a single report of light rain and 33°F on January 22, 1962, and a report of light rain with 35°F on Christmas Eve, 1985.  But today it rained from 4:15am through 10:30am, and then it began again at 3pm; it's still ongoing as of 6pm, and the temperature has been as high as 39°F.

Rain has also been reported in Bettles, Fairbanks, Tanana, and of course points to the southwest, where the moisture is coming from.  The following map shows 12-hour precipitation totals (inches) as of 5:30pm, and it appears these represent mostly or entirely rain, at least at lower elevations.  Click to enlarge:

Two of the sites with higher totals are the Livengood (1.24") and Chalkyitsik (1.06") RAWS, and the hourly observations look very plausible:

Webcam images from Chalkyitsik show that it certainly wasn't snowing, and snow can be seen melting off a roof in the course of 3 hours (lower right, click to enlarge):

Incredibly, it was simply a rainy day across the central and northeastern interior.  And NWS Fairbanks expects it to continue; they say "an additional 0.5 inches is expected to fall through Sunday" (in Fairbanks).

What can explain such an amazing departure from normal?  Here are the 500mb and MSLP charts from 3am this morning:

These show a very pronounced pressure gradient and intense southwesterly flow between the deep trough over far eastern Russia and a powerful ridge over the northeastern Pacific.  Warmth and moisture have been transported quickly to interior Alaska from deep in the subtropics.  We also see that the flow aloft is oriented from southwest to northwest, rather than south to north, so the moisture makes it into the interior without hindrance from the southern coastal mountains and Alaska Range.

Winds across the North Slope have been extraordinary in their own right: here are the peak wind gusts in mph.

And maximum temperatures across the interior:

In the Fairbanks region, the international airport reached 42°F, Eielson reached 46°F, and Nenana saw 44°F.

For future reference, here are several surface observation maps showing snapshots of weather conditions throughout the day (two or three purple dots to the left of the station indicate rain):