More Anchorage Snow

Anchorage has seen another big snow event this week, occurring three weeks after the record (for January) snowstorm earlier in the month.  The new snow accumulation of 11 inches takes the January total to just over 40 inches, which is a record for the monthly total.  It's also the fifth snowiest calendar month in the modern Anchorage climate history (1953-present).

Here's a visualization of calendar month snow totals in Anchorage, with the top 10 events for the main snow months marked in red.  February has two huge outliers - 1996 and 1955 - but the most favored month for big totals is clearly December.

The climatological drop-off in January is actually rather significant; December reaches 20" over a third of the time (26 of 73 years), while January sees 20" in less than 15% of years (10 of 73, including 2026).  So this month's big snow total is a notable outlier for the time of year.

Another interesting tidbit is the preponderance of high monthly totals in recent years: 2023 saw the snowiest November, 2022 and 2023 had the 2nd and 3rd snowiest Decembers, and now we have the snowiest January.  February 2023 gets an honorable mention with the 4th snowiest February (32").  These are good times for snow lovers in Anchorage.

The statistics are at least as striking when we look at liquid equivalent precipitation in Anchorage, i.e. it's been wet in recent winters.

2022 - 2nd wettest February

2022 - wettest December

2023 - wettest November

2025 - 3rd wettest January

2026 - wettest January

What changed in 2022?  The PDO (Pacific Decadal Oscillation), for one thing:

See this 2022 post for some comments on the correlation between the PDO and Anchorage snowfall:

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

First Snow, and North Pacific Warmth in Winter

Fairbanks saw its first snowflakes of the season today, just a few days earlier than normal.  Temperatures were above freezing and no accumulation occurred in town, but the scene looked a lot like "late autumn" in the hills.  Here are a couple of webcam views from Cleary Summit on the Steese Highway shortly after 9am today:

On another topic, reader AlexG asked whether this summer's extreme North Pacific warmth is likely to have an effect on winter in Alaska.  It just so happens that I gave a talk related to this question 10 years ago at UAF, and here's a link to the slides from that occasion:

https://s2s.worldclimateservice.com/wcs/uaf_sep2015_north_pacific_sst_blob.pdf

The winters of 2013-14 and 2014-15 both saw a tremendous amount of warmth in the northeastern Pacific, and that sea surface temperature anomaly became known colloquially as the North Pacific "blob".  In a 2015 paper, Dennis Hartmann of the University of Washington discussed a mode of natural variability - the North Pacific Mode - that resembled the "blob" pattern, and my 2015 talk looked into its connection to Alaska winter climate.

The bottom line from this work of a decade ago is that a positive North Pacific Mode (warm ocean) is linked to unusual winter warmth in most of Alaska except the Southeast, and it also favors wet rather than dry conditions in most areas.  The link with winter precipitation is strong in interior Alaska, because a positive NPM is associated with stronger westerly flow and less chinook flow from the south.

The other major mode of North Pacific temperature variability is of course the Pacific Decadal Oscillation (PDO), which is also closely linked to precipitation across interior Alaska and to temperatures statewide.  A negative PDO phase, which is what we have at the moment, tends to bring wetter winters to the interior (like the +NPM) but colder temperatures statewide (generally opposite to the +NPM).

Here are the canonical sea surface temperature (SST) patterns associated with positive NPM and PDO phases:

Note that the PDO and NPM are independent of each other, so a positive NPM can readily coincide with a negative PDO, as this year.  Also, a negative PDO doesn't necessarily imply cooler than normal SSTs in the northeastern Pacific; the mode expresses the spatial contrasts/gradients in SST rather than the absolute values.  This year's negative PDO phase has been very much driven by extreme warmth near and east of Japan, rather than cool water along the coast of North America.

If we look at previous winters with the +NPM/-PDO combination in the North Pacific, we see - as expected - a strong indication of above-normal precipitation across most of Alaska to the north of the southern coastal regions (where it tends to be dry), and there's also a fairly robust cold signal that apparently reflects the dominance of the PDO influence.  The fundamental circulation anomaly involves unusual high pressure (or equivalently, less low pressure) over the Aleutians, producing a stronger component of westerly flow across central Alaska.

Of course we don't yet know whether the North Pacific SST patterns will remain similar until winter and throughout winter, and indeed there's a good chance they won't.  It's therefore worth examining past years when the +NPM/-PDO combination occurred in summer, rather than assuming it will persist unto winter.  Here's the result:

Interestingly, the precipitation signal is very similar, but the cold signal is much weaker in the summer "analog".  This seems to be because several of the summer matching years ended up with El Niño and/or a near-neutral PDO phase during the subsequent winter, thereby changing the remote influences on the circulation patterns near Alaska.  In contrast, most of the winter +NPM/-PDO matches involved La Niña during winter, reinforcing the cold signal.

And for this winter?  Best estimates are that La Niña will probably prevail during autumn and early winter before fading over the course of the winter; El Niño is very unlikely.  But that doesn't mean it will be cold.  Arguably the most similar recent year in terms of the tropical and North Pacific setup is 2022-23, and that was a warm winter for Alaska (except for Southeast), while the precipitation anomalies matched the +NPM/-PDO pattern very well.

https://ak-wx.blogspot.com/2023/04/another-warm-and-wet-winter.html

June Climate Data

June was an interesting month for global climate, and Alaska's weather reflected the larger-scale patterns that unfolded.

One of the most striking aspects was that unusually strong ridges developed in the mid-latitudes of the Northern Hemisphere - particularly across the North Pacific from Japan to the US West Coast, and across the North Atlantic and southern Europe.  Record heat waves occurred in western Europe and Japan, and it was the hottest June on record in both regions.

In contrast to the mid-latitude ridging, the atmospheric pressure was generally lower than normal in the Arctic, and especially from eastern Russia to the Bering Sea and across the northern North Atlantic and northern Europe.  This "see-saw" of MSLP is typical of the Arctic Oscillation, and June characterized a positive AO phase.

The Arctic Oscillation typically refers to conditions in the troposphere and is not always directly linked to the stratospheric flow above, but in this case the circulation anomaly extended well up into the stratosphere.  In the lower stratosphere, for example at 100mb pressure, there was a similar ring of above-normal heights (pressure) in the mid-latitudes and low heights near Greenland:

In tandem with the pressure anomaly, the 100mb circumpolar westerly winds at 60°N were the strongest on record for June.  Think of it as a vortex of counter-clockwise rotating air aloft that was moving faster than ever observed before (in June) up at about 50-60,000 feet.

What relevance does this have for Alaska?  Well, the low pressure over the Bering Sea involved unusual storminess, so it was a very windy, cloudy, and wet June for Aleutian and Bering Sea communities.  Also, with southern Alaska located squarely in the zone of enhanced westerly winds to the north of the Pacific ridge, the wet weather traveled eastward across the Gulf Coast to the northern Panhandle.  It was also unusually cool in southern and especially Southeast Alaska, with warm air unable to migrate up from the south.  Here are my usual monthly percentile rank maps based on NCEI data (top two maps) and ERA5 data (maps below):

One might expect that low pressure in the Arctic (positive AO phase) would also produce windy and perhaps wet weather for Arctic Alaska, but in fact the classical positive AO setup at this time of year tends to allow a localized ridge to the north of Alaska, and that's what we saw in June (see the first map in this article).  As a result, the North Slope was mostly calm, sunny, and generally warm and dry.  Sunny, warm, and calm conditions also extended over most of the northern and eastern interior - but this refers to monthly averages, and there was also a tremendous amount of lightning during the month.  Here's a map of lightning strikes during the month, showing very widespread activity:

With over 93,000 ground strikes, this was the second most active June recorded by the ALDN (2012-present); the most active June was in 2015, with about 103,000 strikes; but that year the activity was concentrated farther south, and especially in the southwestern interior:

I'll be doing more analysis of this year's lightning activity in a subsequent post.

It's also worth commenting on North Pacific SSTs, which have reached extreme levels of unusual warmth from Japan eastward in recent days (responding to the strong ridge across the North Pacific).  Check out the development of extreme warmth between early June and early July:

This is an enormous marine heat wave, perhaps the most expansive and anomalous on record in recent decades; I'll have to do some objective analysis to quantify this.  The warmth in this region is characteristic of a negative PDO phase, and so the PDO index has plummeted to record negative values in recent days (see below).  We might say this is also part of the reason for the cool June in southern Alaska.

Autumn Climate Data

Autumn seems like a long time ago for much of Alaska, but it's worth making a mention of the autumn climate anomalies, if only for future reference.  I'll use "autumn" here to refer to the standard climatological season of September through November.

Statewide all three months were slightly warmer than the 1991-2020 average, and the North Slope was considerably warmer than the 30-year baseline, but overall it was a slightly cooler autumn than the last two years (but nowhere near as cold as 2021, when November was very cold).  The North Slope was the only region with a large departure from normal temperature, although parts of Southeast were relatively cool according to the consensus of ERA5 and NCEI data:

More significantly perhaps, September through November was the driest such period in Alaska since 2016, according to NCEI, and nearly all western and southern coastal areas encountered this dryness.  All three months were drier than average on a statewide basis, and that's the first string of three consecutive dry months (relative to normal) since spring 2022.  And yet in contrast, the central interior was very wet, with Fairbanks observing its wettest autumn since 2017; only five autumns have been wetter since 1930.  The wet weather occurred mostly in October.

November was easily the driest month of the autumn, as the Aleutian ridge of October shifted north and took up a dominant position over the Bering Sea and western Alaska - see below.

With Pacific storms held at bay, winds were lighter than normal for most of western Alaska in November, and autumn wind overall wasn't dramatically different from normal for most of the state.  That's a big difference from summer, which was exceptionally windy in the western half of the state.

The calm weather in the Bering Sea allowed SSTs to return to near-normal there, eliminating the cool anomaly that developed back in the summer.  However, exceptional and widespread warmth persisted to the south of the Aleutians, and the Gulf of Alaska remained quite cool throughout autumn.

As a reflection of the extreme temperature anomaly differences across the North Pacific, the PDO index became extremely negative throughout the autumn.  Before the past decade or so of exceptional warmth in the North Pacific and in the Arctic, it was commonly observed that (at least southern) Alaska had a strong tendency for unusual cold during pronounced negative PDO episodes, but we don't see the same robust connection these days.  Consider the map below, showing the average temperature departure from trend in 10 past years with a significantly negative PDO index in autumn:

Compare this to the temperature anomaly map for autumn 2024:

The general orientation of the anomaly patterns is approximately the same, but the 2024 map is considerably warmer overall.  I think this illustrates that even with a favorable SST configuration, cold in Alaska these days tends to be very muted by historical standards.

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.