No More Wind Chill Warnings

On Friday the National Weather Service in Fairbanks issued an "Extreme Cold Warning" for two eastern interior forecast zones: the Fortymile country and the White Mountains.  This warning has rarely, if ever, been issued by the NWS in Alaska, so one might think conditions must have been extreme indeed.

But no: in a recent change of procedure, the Extreme Cold Warning simply replaces the old Wind Chill Warning for the NWS nationwide.  The goal is to simplify communication of weather hazards:

"Social science research indicates users of NWS products can become confused by the number of hazard messages NWS issues before and during active weather. Especially while dangerous weather is unfolding, it is critical that NWS hazard messages are simple, short and direct."

So how cold did it get over the weekend?  Well, it was certainly very chilly, with temperatures dropping below -40° in the usual cold spots of the eastern interior, and wind chills of -50°F in some places.  The lowest wind chill measurement I saw was -52°F at Delta Junction early on Sunday morning.  Here's a map of observed wind chill in the region surrounding Fairbanks at about that time (click to enlarge):

The following chart shows the 850mb (approx 5000 feet elevation) temperature and wind at 10pm Saturday.  As is often the case, the cold air mass arrived from the northeast as it traveled clockwise around a high pressure ridge to the north.  (The blank areas on the map are locations where the surface pressure is less than 850mb in the model.)

Severe Thunderstorm Warnings

This summer the National Weather Service in Fairbanks has issued a lot of severe thunderstorm warnings, including three in the last couple of days, and another one just now.  Remarkably, yesterday evening's warning was for a storm near Eagle - a very long way from Fairbanks.

The total of 28 severe thunderstorm warnings this summer is far and away the most issued by the Fairbanks office, based on an archive back to 2004.

What's going on?  Has this summer actually produced far more severe thunderstorms across the central interior than any other summer in the last 20 years, or has there been a change of procedure?

It would be very interesting to work with the radar data to create an objective measure of the number of intense storms each summer, but it would be a lot of work - a good project for a Masters degree student, perhaps.  Lacking such an analysis, we can look at the lightning data, and it turns out that this summer is still on the low side of normal, although activity has been extremely high in the last two and a half weeks.  (But note that this pertains to the entire ALDN region west of 129°W, i.e. including a good portion of NW Canada.)

Have all the severe warnings been issued since late July?  Not at all: 18 of 28 were issued on or before July 17, when lightning activity was unusually low.  Here's a look at the date distribution of all severe thunderstorm warnings issued by Fairbanks; note that sometimes several are issued on the same day.

This summer has seen warnings issued periodically since mid June.

There has in fact been no change to the criteria for issuing these warnings, and so the most likely explanation is that the NWS meteorologists are much less hesitant to issue them this year.  It's probably not a good thing to see such a drastic change in practice; but there may be a few people receiving benefit from the warnings.

One other note, and this is a comment on the lightning chart, reproduced below.  It's quite interesting to see a bifurcation between active and inactive years from mid-July onward: the slopes of the cumulative lightning counts through the end of July are quite distinct.  And this highlights how unusual summer 2023 has been: it started out very inactive indeed for lightning, but now at the eleventh hour the lightning activity has ramped up at a rate worthy of a very active year, but a full month later on the calendar.

Notice too that none of the last 11 years has seen any really significant lightning activity after mid-August; so it would be extraordinary indeed if anything like the current pace continues for more than another few days.