Monday, May 6, 2024

Late Snowfall

Green-up was declared in Fairbanks on Saturday, but that distinctive first shade of green on the hillsides was obscured by white today, as light but steady snow moved into the area mid-morning.  With temperatures dropping slightly below freezing for a while, there was some minor accumulation on grassy and elevated surfaces; the NWS posted this photo:

Accumulating snow in early May in Fairbanks isn't too unusual.  There have been three May snows of more than half an inch since 2013; but of course it quickly becomes a more uncommon event as the month advances.  May 1992 was the exceptional outlier, with 9.4" on the 12th and another several inches in the subsequent days.  1992 also produced an absurdly cold and snowy September in Fairbanks, and it seems rather likely that this "coincidence" was somehow linked to the Pinatubo eruption (1991) and temporary global cooling.

But back to more recent events: read about the 2022 and 2013 May snows here:

In the northern interior, Bettles saw a daily high temperature below freezing yesterday, marking the coldest day so late in the season since the (much colder) conditions of 2013.  Bettles also saw a bit of snow, but that's less uncommon: accumulating snow occurs in May in more than half of years up there.

And in Nome it's a very chilly day for the time of year: the temperature hasn't yet risen above 20°F today.  Again the last time that happened in May was in 2013.

The immediate cause of the cold weather is a trough that dropped down from the Canadian Arctic Ocean over the weekend and "joined forces" with another trough moving across the Aleutians into the Gulf of Alaska.  Here's the 500mb analysis from 4am this morning, courtesy of Environment Canada:

Arguably the chain of events was set in motion by a strong ridge stretching across the central Arctic Ocean late last week; and that in turn reflects the lingering influence of a "sudden stratospheric warming" event way back in early March.  The map below shows last Thursday's 500mb height anomaly, with generally above-normal heights from the Arctic Ocean to the northern North Atlantic, i.e. a negative Arctic Oscillation.  This is a typical outcome in the wake of a wintertime stratospheric disruption, although the timing and persistence of the anomaly varies widely from case to case.  Meteorologists watch for these events closely, because they tend to presage mid-latitude cold outbreaks and generally volatile weather owing to the Arctic "blocking" patterns.

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