Bering Sea Cold

Alaska's west coast and southwestern peninsula have been remarkably chilly in the past couple of days - the coldest in many years for this date on the calendar.

But before I get into details on that, it's worth noting that the Fairbanks NWS office has issued 4 severe thunderstorm warnings (so far) today: one early this afternoon to the east of Fairbanks, and then 3 more for severe storms west of Nenana and up towards Manley Hot Springs.  Historically, many years pass with less than 4 severe thunderstorm warnings issued for the whole summer in the Fairbanks office.  Here's a photo taken around 4:20pm from UAF, courtesy of the NWS via X/Twitter:

A very strong and cold upper-level trough to the west and south provides part of the explanation for the stormy weather: there is unusually strong convective instability caused by the cold air aloft. 

The trough has also produced extremely chilly conditions for the Bering Sea and southwestern Alaska yesterday and today.  Bethel's high temperature was only 40°F yesterday, and snow was observed briefly in the morning.  Bethel hasn't seen a daily high temperature that low in June since 1972.  Similarly, yesterday's high in Cold Bay was only 39°F - again with some snow - the coldest this late in the spring since 1985.

Out in the Bering Sea, St Paul Island reported several lengthy periods of light snow earlier today, with visibility down to 1.5 miles and the temperature hovering at or just below freezing.  The FAA webcam showed some accumulation, as illustrated below:

Remarkably, this morning's balloon sounding from St Paul Island reported a temperature of -9.7°C at 850mb, which is the lowest this late in the season since 1985.  This is about the same as the normal 850mb temperature in January.

The unseasonable cold is related to a suddenly very intense negative phase of the PDO.  Check out the SST anomaly pattern across the North Pacific: extreme warmth extends eastward from Japan, but the Bering Sea has become much colder than normal.

The daily PDO index managed to reach -3.0 for the first time since September 2012, and it's only the second time it's happened in the history of daily data since 1982.

What's driving the anomaly?  Well, the negative PDO regime has been more or less locked in for several years now; but the current amplification is happening as the perennial North Pacific ridge has joined forces with Arctic high pressure via a ridge over eastern Russia, allowing the Bering Sea trough to deepen and intensify.  As for the Arctic ridging, it's a "blocking pattern" that's conducive to extremes at lower latitudes - for example, flooding in central Europe a couple of days ago, and snow in upland Scotland yesterday.

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.