Wet, windy warm front sweeps Southeast Alaska

It’s not often that the warmest time of the day is early in the morning, but that was the surprising fact at many locations in Alaska’s panhandle today. A small but potent low with a defined push of warm air on its eastern flank (the warm front) moved north last night and during the early morning hours today (Monday 9/26/2022), bringing plenty of rain, winds of 25 to 35 mph (40-55 km/hr) in most areas with gusts to around 60 mph in exposed locations, and temperatures reaching the upper 50s to mid 60s F (12-19 C). Here’s the NWS surface analysis maps from 10 am yesterday to 4 am this morning. Click the arrow on the right to loop through them. Look at the small low on the east side of the map right off the panhandle.

Rainfall began or intensified last night, ahead of the surface front location, as is characteristic of warm fronts. Rainfall overnight varied from under 0.5 inches to almost 2 inches (~10-50 mm), the heaviest in the northwest part of the panhandle from Yakutat on the west, to Pelican, Haines and Skagway on the east.

When the surface front arrived at various points, there was, in most cases, an impressive jump in both wind and temperature, especially in the northern locales. At Sitka the winds rose less abruptly, but on the shift to southerly winds at about 7 pm, the temperature, already a balmy 55 F (13 C), began an amazing climb, hitting an astounding 67 F (19 C) at around 5 am! If the temperature stays above 58 F (14 C) till midnight tonight they could have a new record for the highest low temperature set between the fall and spring equinoxes. (Didn’t know that existed as a record? Meteorologists are known for sculpting statistical contests out of thin air, and by golly with Sitka’s long and quite complete climatological data — for Alaska anyway — it’s a meaningful record).

These next two graphs (courtesy of Marine Exchange of Alaska) illustrate a more dramatic frontal passage at Eldred Rock, located in highly channeled Lynn Canal:

There were plenty of similar examples from Juneau, where the warm front went through a little before 10pm bringing wind gusts to 44 mph, and on up Lynn Canal to Skagway where the abrupt change hit at 6am, and even in Whitehorse, YT where the south winds built to around 30 mph with gusts to 40 (50g65 km/hr) by 8 am, and the temperature rose to 61 F (16 C) by 2pm AK time.

There’s more to come…

As this east-west oriented warm front moves north there remains a strong southerly flow (from the south) with a frontal zone oriented more north-south just off the coast. This setup is going to allow the strong south winds to pump warm, moist air into the region over a sustained duration, the feared atmospheric river.

The serious concern is the possibility of flooding and landslides. In fact there has already been some minor flooding and a couple of landslides, affecting highways only, thankfully. But the atmospheric river is still active, and where exactly it wobbles in response to small scale waves in the upper atmosphere will determine subsequent rainfall.

The models are predicting another wet night tonight then a slowdown till Friday, but again, it’s hard to know how well they’ve called it until it happens. Here’s a model composite of expected precipitation totals from 4pm Sunday through 7pm Wednesday. Keep in mind there are already differing updates since I borrowed this from The NWS Juneau.

The graph below is for Sitka, and the solid black dots showing actual rainfall track closely (so far) to the average of the model forecasts. But look at the 2nd graph, for Haines. The two main models (blue/black and red lines) don’t agree well, and the actual has been tracking at about twice the model average.

Why warm fronts aren’t so common in Alaska

Well, you might say, this is Alaska, the Frozen North, of course there aren’t many warm fronts… its cold country!

There is a grain of truth to that, and it has to do with the wimpy nature of warm fronts. All other things being equal, warm air is less dense than cooler, and especially downright cold air. When two masses of air with differing densities meet, which is basically what a front is, a battle for territory takes place. Colder air has the advantage of density, and when it advances, such as in a cold front, it easily displaces the warm air and shoves it up and out of the way. But when the winds are trying to push the warm air against a cold opponent, the warm air tends to wimp out and ride up over the cold air. When things work out just right, the warm air does advance at the surface, such as in the case I just described, but in Alaska this is not the norm. It happens often over the ocean, but even there a process called occlusion is usually taking place when the warm air in an entire sector of the low is pushed aloft, leaving an occluded front at the surface between the low center and shortened cold and warm fronts.

Since much of the action has shifted aloft, an occluded front typically has less abrupt weather changes at the surface than either a cold or warm front. A good example is on the surface maps at the top of this post. Out west there is a large low with an occluded front over the Bering Sea meeting the juncture of a cold front and warm front over the Aleutian Islands. When this common type of system makes landfall, more often than not it’s an occluded front that affects most parts of Alaska, with the more southerly positioned cold front impacting the panhandle or often just the southern panhandle. That does not mean there cannot be a strong flow of warm air ahead of a cold or occluded front, but It’s unusual to see a strongly defined warm front ushering it in. In the interior, warm air trying to invade from another region is even more likely to be forced aloft and in addition to occluded fronts you have the subtly different warm fronts aloft.

Thanks for reading and remember, I’d love to hear any comments or questions you might have.