Saturday, July 8, 2023

Severe Thunderstorms

The east-central interior saw a remarkable and rare outbreak of strong to severe thunderstorms yesterday evening, and the NWS meteorologists in Fairbanks had their hands full issuing warnings for the numerous storms.  The first severe thunderstorm warning was issued at 5:47pm AKDT for a storm about 30 miles west of Fairbanks, and 12 more warnings were issued over the next two and a half hours.

Here's the Fairbanks radar image at the time the first warning was issued (click to enlarge).  The storm cell in question is the one just to the north of the Parks Highway (the red line) to the west of Fairbanks.  The storm just to the southwest of that, closer to Nenana, dropped half an inch of rain at the airport there in 22 minutes.

Two particularly severe storms developed a bit later well to the northeast of Fairbanks, near Central, and eventually crossing the Yukon River.  Here are the radar snapshots from 7:08 and 7:24pm: note the very high radar reflectivity values, which are strongly indicative of large hail.  The Weather Service issued warnings for "half dollar size" hail, i.e. 1.2 inches in diameter.

The ingredients for the storms involved very high humidity at low levels and a powerful impulse (widespread lifting/ascent) in association with a strong cold front approaching from the west.  The dewpoint reached 61°F in Fairbanks in the early evening, which is about as high as humidity typically gets in summer there, and Fort Yukon's dewpoint made it up to 63°F.  Low-level humidity greatly adds to convective instability, which is the fuel for deep, vigorous overturning in thunderstorms.

Here's the 500mb analysis at 4pm yesterday.  Note the very sharp trough over southwestern Alaska and the strong pressure gradient to its east, with strong southerly flow between the trough and a ridge downstream over northwestern Canada.


That southerly flow and the strong Canadian ridge produced excessive heat over Alaska's far eastern interior, western Canada, and southeastern Alaska: here are maximum temperatures yesterday (click to enlarge).

I imagine Rick Thoman will provide more expansive historical context for the heat wave on his Substack channel, but the 91°F reported by the co-op observer at Eagle is pretty unusual: before 2012 it was about a once or twice a decade occurrence.  Father east, the 95°F at Carmacks is within a degree of the all-time high in 2004.  Canada has been breaking all sorts of heat records this summer.

Finally, here are simple radar animations from yesterday evening: split into two halves for the sake of file size (i.e. the second animation picks up where the first one ends).  What a remarkable event!  I'd be very glad to hear of any reports from those on the ground - feel free to share in the comments.

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