Alaska's Changing Environment

Last week UAF released an updated version of their summary report on "Alaska's Changing Environment", documenting trends and recent events across many aspects of the natural environment in Alaska.  I recommend taking a look at the latest report, which updates and extends the original 2019 publication:

https://uaf-accap.org/alaskas-changing-environment/

There's a wealth of information in the document.  Extreme events of recent years are highlighted (e.g. ex-Typhoon Merbok, landslides), and the discussion goes far beyond traditional weather and climate metrics to explore ocean and wildlife changes that I knew nothing about.

Coming back to today's weather, my late November comment about the negative PDO phase favoring colder weather in southern Alaska is being challenged in dramatic fashion, as western and southern regions have been overwhelmed by warm Pacific air in the last couple of days.  Anchorage reached 47°F last night, the highest December temperature since 2019, when it reached 51°F (the monthly record).  Nearby Merrill Field and Elmendorf AFB both reached 51°F last night, and again that's the warmest since 2019.  It was even warmer a bit farther north:

There'll be no prizes for guessing the Pacific weather pattern responsible for this: widespread low pressure across the Bering Sea and Aleution region, and a prominent ridge over western Canada.  Here are maps from 3pm AKST yesterday:

Haines Ice Storm

A major winter storm has affected Southeast Alaska in the last few days, with very heavy rain in the south, well over a foot of snow in Juneau, and freezing rain in the northern Panhandle yesterday and today.

Juneau saw six hours of freezing rain yesterday morning, with the ASOS instrumentation suggesting that ice accretion may have reached 0.2", and then Haines suffered significant icing today.  Hourly data from Haines shows freezing rain for about seven hours, and the ASOS algorithm indicated ice accretion of 0.3".  The National Weather Service relayed reports of as much as 0.5" of ice, and they issued an Ice Storm Warning - apparently the first ever to be issued by the Juneau NWS office.

Freezing rain isn't particularly unusual for communities in the northern Southeast, of course, because the ingredients are quite easy to come by: cold low-level air drawn from the frigid interior, and warm air aloft from the Pacific.  Haines ASOS data since 1996 indicates that freezing rain is reported at least once in just about every winter, and it's not uncommon to have a half-dozen days or more with freezing rain: for example, the winters of 2018-19, 2022-23, and 2023-24.

However, the data also suggests freezing rain isn't typically prolonged in Haines, as warm Pacific air usually wins out rather quickly.  The hourly Haines data indicates that the record for consecutive hours with freezing rain is 9 hours - only a little more than today's event - and that occurred in December 2019.  The NWS discussion highlighted the unusual nature of the current storm: "In fact, more freezing rain has been observed during this one event than some of the staff here at the office have seen over the past 18 years."

As is typically the case, the freezing rain eventually changed to plain rain for both Juneau and Haines, as temperatures rose above freezing.  This is the normal course of events, as warm air typically erodes and displaces the surface-based cold air, at least in a coastal setting like Southeast Alaska.  Here's the NWS graphic for how precipitation type depends on the vertical temperature profile:

Dawson Follow-Up

Following up on last week's post about Yukon River freeze-up at Dawson City (Yukon Territory), it's worth looking at whether excessive warm season precipitation may have been a factor in preventing a complete freeze-up in some recent years.  To recap, the Yukon failed to freeze over properly at Dawson in the winters of 2016-17 though 2018-19, and then again last winter; but freeze-up did succeed in 2019-20 through 2022-23, and again this winter.

Here are the May-September precipitation estimates from ERA5 for the months prior to the "no freeze" winters:

There's no evidence here that the southern half of the Yukon Territory (much of which is in the Yukon drainage) was consistently wetter than normal in these years, and in fact the summer of 2023 was somewhat dry.

As for recent "normal freeze" years, 2019 was dry, but others years tended to be more on the wet side; and this summer wasn't particularly unusual in either direction.

How about water levels at Dawson?  Here are the September mean water levels in the no-freeze years, for the Yukon River (left) and the tributary Klondike River (right), which meets the Yukon immediately upstream of downtown Dawson:

2016   2.02m   1.36m

2017   1.56m   1.23m

2018   1.85m   1.10m

2023   1.60m   1.30m

And the normal freeze-up years:

2019   1.03m   0.85m

2020   2.65m   1.39m

2021   2.30m   1.28m

2022   2.16m   1.65m

2024   2.12m   1.50m

With the exception of 2019, the normal freeze-up years all had higher Yukon River levels in September than the no-freeze years.  The tendency is the same for the Klondike River, with 2022 and 2024 both having higher levels than the no-freeze 2023.

I think we can therefore dispense with the idea that the freeze-up failures occurred because of higher flows - if anything, the reverse seems to be more likely - but I won't indulge in any more speculation now.  Previous posts and discussions in the blog comments can be accessed by searching the blog archives for "Dawson".