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".

Frozen Over at Dawson

A couple of interesting items came to my attention today.  First, the Yukon appears to be properly frozen over at Dawson City in the Yukon:

This is big news over there, because the lack of freeze-up next to town has been a huge problem in recent years; freeze-up has often been much delayed and sometimes hasn't occurred all winter.  I've commented on it occasionally, most recently last March:

https://ak-wx.blogspot.com/2024/03/yukon-at-dawson-still-open.html

The change this year isn't caused by colder weather - it hasn't been unusually cold - and the lack of freeze-up in recent years was generally not caused by unusual warmth.  When I have a chance, it will be worth looking at regional streamflow and precipitation data from recent months to see if anything stands out.

Here's an animation of webcam images from today, courtesy of http://dawson.meteomac.com/

On a completely different note, I ran some calculations today to establish whether the Northern Hemisphere circulation patterns have been unusually volatile this month; it sure seems like it.  The answer is yes: the following chart shows a metric of 500mb volatility for 45-90°N around the globe.

To be precise, the figure shows the area-average of the standard deviation of 500mb heights for November 1-26, expressed as a fraction of the 1991-2020 normal (which differs from place to place).  Here's a map of the departure from normal: 500mb heights have been much more variable than normal over the eastern Bering Sea and southwestern Alaska, as well as near Baffin Island and over northern Europe.

The region of high variance near Alaska largely reflects the dramatic change from this a few weeks ago:

to this last week:

The big ridge over the eastern Aleutians and southwestern Alaska about a week ago was bumping up against records for the month of November.

What could explain the volatile patterns around the higher latitudes of the Northern Hemisphere?  I think it's the unusual combination of sea surface temperature anomalies across the tropics and the mid-latitude oceans.  We have a weak La Niña, which tends to favor meandering "blocking" patterns in the high latitudes in early winter (November in particular), but we also have tremendous warmth in the northwestern North Pacific, which favors mid-latitude ridging (high pressure) and a stronger, more west-east jet stream to the north of the ridge.  These two Pacific ocean anomalies therefore currently have opposing influences on the jet stream pattern, as I see it, and volatility has resulted.

Looking ahead, as winter settles in, La Niña's influence will very much shift towards favoring a stronger jet stream with a strong polar vortex, and that in turn will tend to favor unusual cold in Alaska, particularly in the south.  This is very consistent, of course, with a negative PDO phase, which is locked in because of the warmth to the east of Japan.  Here's the SST anomaly from the past month:

In conclusion - bundle up; and happy Thanksgiving to U.S. readers.

High Wind History

After mentioning strong winds in Wednesday's post, I thought it would be interesting to look at the top high wind events of recent decades - at least according to ERA5 reanalysis data.  I did something similar for the lower 48 the other day, showing that September's Hurricane Helene ranked #5 for area of the contiguous U.S. that experienced hurricane force wind gusts.  What are the top events in Alaska's modern history?

The ERA5 data goes back to 1940, although pre-1950 the uncertainty is very high, so we won't look at anything quite that far back.  The event with the greatest land area coverage of hurricane force wind gusts (64 knots or higher), based on peak wind speed in a 3-day window, was in early 2000:

This event pops up in the Deep Cold archives, finding a mention as the date with the highest sustained wind speed (58 mph) at Delta Junction airport in the 1998-present ASOS era.

Second on the list since 1950 is a North Slope wind storm at the end of 1951.  Hourly observations from Utqiaġvik confirm a peak sustained wind speed of 56 mph (gusts not reported).

Number 3 was in February 1989: the North Slope again.  Gusts were measured to 60 knots (69 mph) at Utqiaġvik.

And in fourth place, a more recent event that brought damage and power outages to the Anchorage area:

Events #5-#8 highlight southern and especially southwestern Alaska as a hot spot for these widespread strong wind events - as we might expect with the favored Aleutian storm track.

I'm a little surprised not to see a west coast event in this list; even ex-typhoon Merbok doesn't rank anywhere near the top:

Active Weather

The weather pattern has been very energetic and changeable across Alaska in recent days, leading to sharp cold in some areas, and wind and storm damage in others.

Coastal erosion and damage occurred down in Homer over the weekend, and far to the northwest Point Hope is without power today, owing to high wind.  Winds gusted to 68mph in Point Hope yesterday, and 93mph at nearby Cape Lisburne.  Today the high winds spread across the North Slope, with Utqiaġvik gusting to 62mph and temperatures rising above freezing.

Here's this morning's 3am MSLP analysis, courtesy of Environment Canada:

Several things can be noted here: the axis of high pressure across the southern interior, associated with cold weather at the surface; the strong pressure gradient across northwestern Alaska, producing the high winds; and the extraordinarily deep low pressure system to the west of Seattle, causing severe wind damage in Washington last night.

The cold yesterday morning was notable across the interior: -40° was reached on the Yukon Flats at Beaver, and Chicken saw -42°F.  Fairbanks reached -29°F at the airport, the coldest this early since 2020 - and we have to look back to 2011 to find colder conditions this early in the season.  The Salcha RAWS reached -37°F, the coldest this early since 2011.

Cold in the Brooks Range

The past several weeks have been notably cold in the Brooks Range as a result of persistent northeasterly flow; and this has been caused by an unusual gradient between low pressure over southern Alaska and atypical high pressure over the Arctic Ocean.  Sea-level pressure has been 15mb higher than normal this month at about 80°N on the Date Line:

Here are the daily mean temperatures this autumn compared to normal at Anaktuvuk Pass, 2100 feet elevation in the heart of the Brooks Range:

From last Sunday through yesterday - six days in a row - daily average temperatures were below -15°F in Anaktuvuk Pass, which is pretty chilly for the time of year; we have to go back to 2005 to find as many days this cold in the first half of November.

It's also the first year since 2012 with an average temperature below 0°F for the first half of November, and in fact it's a notable break from the remarkably persistent warmth of 2013-2023:

The Arctic as a whole isn't noticeably cooler this year, so this seems to be a case of an unusual and persistent flow pattern bringing an unseasonably cold air mass to the region.

High-quality data from the Toolik Lake CRN site confirms the anomaly: this is also (easily) the coldest first half of November in the short history since 2017 at that location.

NWS Fairbanks expects no improvement in the short term: here's an excerpt from their latest forecast discussion.

"North Slope and Brooks Range...

- Cold temperatures (around -30F) become more widespread through Tuesday across the Arctic Plains and Brooks Range. North winds around 15 mph in the Brooks Range Sunday night into Monday morning will result in very cold wind chills near 55 below just barely short of a Cold Weather Advisory."

October Climate Data

Looking back at October, there was a lot of active weather in Alaska, especially in the second half of the month, and so it's not surprising that temperatures were highly variable.  However, the statewide average temperature (as calculated by UAF) didn't see any extremes of either sign; neither the warm nor the cold anomalies were all that unusual - see the right-hand portion of the figure below.

The "near-normal" characterization of the monthly average temperature extended to most parts of the state, with the exception of somewhat significant cool in the north-central interior and the northern Panhandle, and warmth on the North Slope.  The southern Yukon Territory was pretty chilly, however.

Warmth on the North Slope is a given these days in October, because of the lack of sea ice.  Every October since 2002 has been significantly warmer than the earlier multi-decadal normal, as illustrated so clearly in Rick Thoman's signature chart for Utqiaġvik - check out his latest post:

https://alaskaclimate.substack.com/p/alaska-october-2024-climate-summary

October precipitation was significantly above normal for the West Coast north of the Y-K Delta and across to the central and northern interior, as well as the Brooks Range.  But I'm not sure the ERA5 data (see below) fully captures the magnitude of the precipitation anomaly in the Fairbanks area, where it was the wettest October in many decades.

Not surprisingly, it was a windier month than normal for much of the state, and especially so for the West Coast.  Every month this year except February and September has been notably windier than normal for a majority of the West Coast.

The mid-atmosphere circulation pattern involved a strong ridge over and to the south of the central Aleutians, and a trough from northwestern Canada to the Gulf of Alaska.

This is very much a negative PDO pattern, with the ridge corresponding to above-normal sea surface temperatures to the east of Japan, and the Gulf of Alaska trough corresponding to cool ocean temperatures.  The warm anomaly is much more extensive and far more intense than the cool anomaly - see below.  It's interesting to see that the dividing line between warm and cold lies right along the western Aleutian chain.

Speaking of the PDO, the monthly PDO index for November was the most negative of any month since 1955.

The negative phase is being driven mostly by the extreme warmth in the northwestern Pacific, and the latest seasonal forecast models show no reprieve at all in the coming months.  Here's the NMME multi-model forecast all the way out in late spring:

And here's an ensemble of model forecasts of the PDO index:

If this forecast is correct, and I have high confidence that it is, then Alaska will continue to be strongly affected by this PDO anomaly for the foreseeable future.  We can expect an enhanced probability of unusual cold in southern Alaska this winter, and perhaps farther north as well.

More Snow in Anchorage

Anchorage is off to a remarkably snowy start to winter, with 22" reported since the beginning of the week, and a snow depth of 19" on yesterday's climate observation.  Only a handful of years have seen greater snow accumulation by this date, and none has reached a snow depth of 19" this early.  With consistent climate data back to 1953 (at the international airport), the previous earliest date with 19" or more on the ground was... last year, November 10.  Prior to that, the earliest was November 21, 1994.

This marks the third year in a row with significantly unusual heavy snow in early winter.  Last year it was the snow onslaught of November 5-13, and in 2022 the city was buried in two major events in the first half of December.

https://ak-wx.blogspot.com/2023/11/south-coast-snowstorm.html

https://ak-wx.blogspot.com/2023/11/a-few-notes.html

https://ak-wx.blogspot.com/2022/12/anchorage-snow.html

With a snow pack of 10" in Fairbanks, this also marks the second earliest that both cities have recorded at least 10" on the ground at the same time.  The earliest was in the notorious autumn of 1992, but even then Anchorage didn't have sustained snow cover until October 27.

Here's a chart of snowfall through November 15 in Anchorage, with this year's total included through November 1.

The variability is enormous, and the change in 2020 really stands out compared to 2012-2019.  The only other period with 5 consecutive years of at least 10" in this early winter period was 1979-1983.

It seems unlikely to be coincidental that 2020 is when the PDO phase switched to negative, after 6 years of a mostly positive phase.

Here's a comparison of early winter snowfall with the PDO index in November:

There are some notable PDO extremes that coincide with snowfall extremes, e.g. high snow and negative PDO in 1955-56 and 2011, and low snow with positive PDO in 1957, 1986, 2002, 2014, and 2016.  However, the overall correlation seems to have been modest at best before the turn of the century - see below.  Since 2000, there's been a much better correlation.

One hypothesis to explain this might be that in earlier decades the positive PDO didn't prevent early winter snow, because it was still cold enough, but in recent years a positive PDO has brought greater warmth that has led to rain rather than snow.  More investigation would be required to explore this; but without doubt, the negative PDO regime is a snow-lover's friend these days in Anchorage.

Flood Follow-Up

A few different items of comment today.  First, a significant snowfall is under way across the south-central region today, signaling the start of winter proper for many valley locations in that area.  If Anchorage's snow cover hangs around in the coming weeks, it will mark an earlier than normal onset of permanent snow pack; the normal date is about November 9th.  Of course Anchorage occasionally lacks snow cover even in the depths of winter, but it's unusual, occurring most recently in 2015-16 and for a few days in late December 2019.

As for Fairbanks, the normal date for establishing permanent snow cover is October 18, and it was right on time this year.  However, the most recently reported snow depth of 10" is the highest for the date since 1992, and more fell today.  (But 11" was on the ground in September 2015.)

Looking back at last week's historic west coast storm, it's worth highlighting blog comments by Rick and Gary.  Rick Thoman confirmed that long-time Kotzebue residents reported the highest water in living memory, which is why it was such a devastating event for some in the community:

https://www.adn.com/alaska-news/rural-alaska/2024/10/26/after-flood-kotzebue-residents-take-stock-of-damaged-homes-and-property-and-take-care-of-one-another/

Although the meteorology was different, we might say this was Kotzebue's version of ex-Typhoon Merbok, which caused such havoc for coastal locations farther south about two years ago:

https://ak-wx.blogspot.com/2022/09/more-on-ex-merbok.html

The article highlighted by Gary indicates that flooding was not quite as bad this time around from Nome down to Unalakleet:

http://www.nomenugget.net/news/high-winds-cause-erosion-and-flooding-region

Rick also commented that storms in November 1973 and 1974 may be the most analogous events in the modern climate record.  Certainly the storm of November 10-11, 1973, shows a rather similar sequence of events: low pressure over the Chukchi Sea created strong winds out of the south early on the 10th, and then the winds went around to the west and blew fiercely with a strong north-south pressure gradient, pushing water into Kotzebue Sound.  Here are MSLP maps:

Compare to the recent event:

Interestingly, the wind speed itself was not particularly unusual with last week's storm, and there have been many past events with equally strong, or stronger, winds from a westerly direction in Kotzebue.  I think therefore it was the very low pressure, the large size of the storm, and the long duration of southerly fetch that allowed such a large surge to develop before the winds turned westerly and pushed the water onto the coast.