Cooling Off

The last two nights have brought hard freezes to many interior and northern locations, and temperatures below 20°F were widespread this morning across the northwestern interior and Brooks Range.  Arctic Village reached 15°F with a bit of snow on the ground; the Norutak Lake RAWS and Huslia CRN sites saw 16°F; and yesterday Tanana dropped to 18°F.  It is slightly early for cold like this, but not dramatically so.  With a substantial trough entrenched over Alaska, we would expect nothing less.

A bigger surprise is that the Fairbanks airport hasn't reached the freezing point yet.  Last year set a new record for latest first freeze (September 28), and only one other year (1974) went past September 25 without a freeze.  According to the NWS it may happen on Saturday or Sunday, but if not the record will be broken again.

Visible satellite imagery from today shows extensive snow cover in the central and eastern Brooks Range, but apparently not across most of the North Slope - and notice the complete lack of sea ice to the north (click to enlarge).  Of course we're only a couple of weeks past the annual minimum for Arctic sea ice extent.

The next few weeks have the most rapid rate of seasonal temperature decline for interior Alaska, and one way of thinking about this is to consider the probability that any one week will be colder than the previous one.  In Fairbanks this probability gets as high as 80% in the first half of October: there's no going back at this point.

If we compare this probability curve to the annual seasonal cycle of "normal" temperature, there's one curious feature (at least to data nerds like me): the aforementioned 80% probability occurs around October 10-15, but the decline in normal temperature is greatest about 10-15 days later.  The reverse occurs in the spring: the probability of one week being warmer than the last peaks in late April, but the normal temperature rises most quickly around April 10-15.  (Note that I'm using the entire history of Fairbanks data here, not just recent decades.)

I think what's happening here is that the variance of temperature is much higher in winter, and that tends to reduce the probability of "predictable" week-to-week changes as the normal changes in late autumn or early spring.  Imagine if the temperature variance were extremely low from day to day, so that temperatures simply followed the normal curve; then the sign of the weekly changes would be highly predictable.  But if variance were extremely high, the changes in the normal would be rather insignificant compared to the variance, and the probability would be much closer to 50%.

So in terms of the predictable, relentless grind lower in temperature, the next few weeks are as bad as it gets.  There's still a long way to go after that, of course, but with more ups and downs along the way.

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

Summer Review

I've been meaning to post a few comments on the climate patterns and anomalies during summer - better late than never, I trust.  On a statewide basis it was a fairly warm and fairly dry summer, but neither aspect was highly unusual in terms of the state averages: according to NOAA it was the 10th warmest summer in the last 30 years (warmer than 2024 but cooler than 2023), and statewide precipitation was the fifth lowest in 30 years (driest since 2019).

However, regional differences were significant, especially for precipitation.  The summer was much drier than normal along the central and eastern Gulf Coast, but much wetter than normal for most of western Alaska as well as the northern interior and western Brooks Range.  Here are the ERA5 (top) and NOAA (bottom) percentile rank graphics for the three-month period:

Here's a chart of accumulated precipitation at Yakutat, to illustrate the dryness (click to enlarge).  The June through August period was the second driest on record, following 2007, with total rainfall amounting to only 37% of the long-term median (9.6" versus 25.8").

Precipitation deficits were very pronounced in July along the northern Gulf Coast, but August really exacerbated the drought (relative to normal) and also set up the big contrast with excessive rainfall in northwestern Alaska:

We covered the late August stormy pattern for western and northern Alaska here, and the August 500mb height anomaly highlights the pressure gradient across western Alaska: the ridge over southern Alaska deflected the jet stream northward into western and northern Alaska.

In terms of sunshine and wind, it was a very poor summer for the Y-K Delta region, with all three months bearing part of the blame.  However, only June was cooler than normal in the southwest.  Eastern and northern Alaska were generally sunnier and less windy than normal, although not in August.

Looking back at lightning and wildfire activity, both were above the long-term median, but lightning much more so.  For a time in late June and early July, lightning strike counts were running second only to 2015 (with data back to 2012), and the seasonal total ended up in 4th place, similar to 2019 and 2022.  In contrast, wildfire acreage ended very close to 1 million acres, which is about twice the long-term median but well below 2019, 2022, and of course 2015.

It seems likely that Alaska's very wet spring was largely responsible for the relatively subdued fire season.  The statewide average for the April-May total precipitation was easily the highest on record, with both April and May ranking in second place statewide; and importantly, the wet anomaly included large parts of the fire-prone interior.

 

May was also cooler than normal, and June was only a little warmer than normal in the interior.  This is a big contrast with, say, the enormous warmth of May 2015 that dried out the state in advance of that huge fire season.

Here's a map to illustrate the distribution of lightning activity relative to normal.  There was far more lightning than normal in northern Alaska, and the more typically active southern interior also had more lightning than normal.

A plot of actual strike count density shows that the bulk of the activity occurred across the interior, as always, but it was very unusual to see the high strike counts extending so far north into the Brooks Range.

I suspect that the combination of fairly high temperatures and high humidity in the northern interior was part of the reason for the very active lightning season there (warm, humid air is fuel for thunderstorms).  Here's a map of dewpoint percentile relative to the 30-year climate:

From a broader climate perspective, widespread and extremely unusual warmth in the North Pacific this summer is one of the biggest stories in climate science at the moment.  June-August sea surface temperature anomalies were as much as 3°C above normal in the northwestern Pacific, leading Japan to have its third record-hot summer in a row.

Remarkably, the August area-average SST reached 20°C for the Pacific basin north of 30°N, according to NOAA's ERSST data.  The acceleration of warming in the past few years is quite something:

I'm out of time for this post, but I'll append below the rest of my August climate percentile maps for the sake of completeness.

August 1974

Here's a quick follow-up on the question I posed a few days ago: has anywhere in Alaska ever previously observed a temperature rebound to high summer warmth/heat after a really severe early autumn freeze (e.g. less than 20°F)?  This recently happened at Chicken: they dropped to 14°F on August 30, but rebounded to 80°F only 5 days later.

I sent the question over to former blog contributors Rick Thoman and Brian Brettschneider, both of whom have encyclopedic knowledge of Alaska's climate history.  Among the various candidate events they proposed, here's one that looks legitimate and was even more extreme than the recent Chicken temperature swing: in August 1974, the Mile 41 Steese co-op site reported a low of 16°F on the 22nd, followed by a high of 85°F on the 30th.  That's a rebound of 69°F, exceeding the recent 66°F change in Chicken.

Here's a chart of the data, courtesy of xmACIS2:

There's no doubt the freeze was severe in parts of the interior; the 17°F in Allakaket was the earliest date on record for a low below 20°F (data from 1908 through 1982).  At Bettles the 22°F still stands as the earliest below 25°F, and in the Fairbanks area there was a 22°F at North Pole (only 1°F shy of the record minimum for August).  Here are maps of low temperatures reported on the 22nd and 23rd (click to enlarge; the Mile 41 Steese site is in the middle of the maps, just to the north of Fairbanks):

The heat wave less than 10 days later was the real deal too; Fairbanks airport reached 83°F, and the university farm reached 84°F; both stand as the second latest on record for such warmth (after early September 1957).  Here are the regional maxima on August 30 and 31:

The sequence of mid-atmosphere circulation anomalies is quite interesting: a cold upper-level low dropped into eastern Alaska from northwestern Canada, but as it swung back into Canada, a strong ridge developed in its place.

The recent event at Chicken was different: the severe freeze was more localized, being a "Chicken special" in terms of how extremely cold it was in the valley.  The cold air mass wasn't all that extreme for the time of year (see my post here).  Here's the map sequence: