Saturday, July 12, 2025

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.


Tuesday, July 8, 2025

More Heat and Lightning

[Update July 10]

Rick Thoman pointed out that a SNOTEL site near Kobuk reached 93°F on Saturday, a remarkable temperature for this typically (slightly) more maritime area in the northwestern interior.  (Note that SNOTEL thermometers are more reliable than the warm-biased RAWS instruments.)

But it was back down to 30°F yesterday morning - that's quite a change.  Click to enlarge:


[end of update]

The second big heat wave of the summer for central and northern Alaska came to an end with a tremendous amount of lightning in the past couple of days.  The ALDN recorded over 12,000 cloud-to-ground lightning strikes in the 24 hour period ending at 6am today, with several regions of strong activity:


Yesterday was the 5th day this summer with over 10,000 ground strikes recorded, and this is the most of any summer in the modern ALDN history since 2012 (note that this calculation includes northwestern Canada to the west of 129°W).  Previously the record was 4 such days in both 2016 and 2017; but some summers don't see a single day this active - for example, the peak lightning day last summer was 6500 strikes on June 23.

In terms of total year-to-date strikes, 2025 remains in second place behind 2015.


Many new fires will have been started by this latest round of lightning, but the weather is now much cooler and more humid, which will greatly aid fire control efforts.  There are two substantial fires between Fairbanks and Nenana, collectively called the Nenana Ridge Complex, with expansive evacuation orders in place.  These fires have been burning since around June 20.


Looking back at the heat wave, it peaked over the holiday weekend, and was focused over the northern and northwestern interior, as well as the Brooks Range.  After seeing its latest measurable snow on record (again) only 4 weeks earlier, Bettles reached 90°F on Saturday, and that's only the second 90°F this century at Bettles - the other was in 2019.  Bettles is now running in second place for number of 80°F and 85°F days this summer (15 and 8 respectively).  What a turn-around from the cold start to summer:


Up at Anaktuvuk Pass, the temperature reached or exceeded 80°F for 4 days in a row, which is quite extraordinary for the location (2100' elevation at 68°N).  Even a single 80°F day is very unusual up there, but the count has now reached 6 days this summer; the previous record from recent decades (since the mid-1980s) is just 2 such days in 2016.

Friday, July 4, 2025

Northern Heat

The heat is back for interior Alaska, and this time with a focus on the north, especially the northwest.  A strong high pressure ridge has set up squarely over the North Slope and will intensify even a bit more into tomorrow morning before fading.  Here's the 500mb analysis from 4pm AKDT yesterday, courtesy of Environment Canada:


Mid to high 80s Fahrenheit have been reported from many sites around the northwestern interior and the southern slopes of the Brooks Range, but most of these are RAWS sites that are known to run hot on sunny summer days.  There was a credible 88°F at Huslia airport, however, on Wednesday.

More significantly, I think, there was a daily minimum temperature of 72°F at an elevated RAWS site about 40 miles north of Kotzebue, i.e. well above the Arctic Circle.  I think there's no reason to believe this temperature is too high, as there is no solar insolation in the early morning hours: the "midnight" sun is blocked by terrain to the north.


Click to enlarge the following map (July 3 minimum temperatures):



It is rare to see a 70+ daily minimum in Alaska, but this site's elevated location is quite prone to warm summer "nights" under similar heatwave conditions: 71°F daily minima were reported in 2013 and 2019.

It's harder to prevent "overnight" cooling in valley locations, so for example the record warmest daily minimum in Fairbanks is 70°F in 2013.  (The university farm reported a low of 76°F on June 26, 1915 - the same day as Fort Yukon's infamous 100°F - but that 76°F seems implausible because the Fairbanks observer also recorded "hail the size of marbles", which would surely have cooled the air more significantly.)

Here's a contender for perhaps the highest low-elevation daily minimum in Alaska: 72°F at Umiat in late June 1982.


Saturday, June 28, 2025

Lightning and Fire

It's been an eventful two weeks of Alaska weather since my last post.  After a long trend towards relatively cooler conditions during spring (i.e. relative to normal), culminating in a very chilly start to June, the pattern flipped and summer heat took over for about 10 days.  The UAF statewide temperature index nicely illustrates the change:

In Fairbanks, after the latest "first 70°F" in over 100 years, the thermometer then exceeded 80°F for 7 days in a row.  Such a stretch of "over 80°F" days is itself quite unusual: it has only happened in 12 previous years since 1930, with the record being 13 consecutive days in 1991; so about 85% of summers never see such a sequence of hot days.

Here's the mid-atmospheric pressure pattern that produced the hot weather: a ridge over eastern interior Alaska, and a strong Bering Sea trough; the southerly flow between the two features pumped warm air northward across the state.


The departure from normal of the 500mb height was concentrated over the eastern Arctic coast.


This pattern, involving high pressure centered over northern Alaska, is ripe for interior Alaska thunderstorms, and indeed there was a veritable onslaught of lightning.  The following figure compares this year's cumulative lightning strike count to previous years in the modern history of the detection network since 2012:


There were over 80,000 cloud-to-ground lightning strikes recorded in 10 days, and this is nearly a record: only 2015 (late June) and 2016 (early-mid July) had higher counts in a 10-day period.

Looking at the regional distribution of the lightning, it was concentrated in the central and northeastern interior as well as the North Slope, and this extends farther north than we would normally expect for the time of year.  Alaska lightning in the earlier part of the season tends to be most abundant in the Tanana, Kuskokwim, and middle and lower Yukon regions, rather than the northern interior.

Remarkably, three of Alaska's fire-monitoring Predictive Service Areas are already in third place for total annual lightning strikes: the Upper Yukon Valley, the northern Tanana Zone, and the North Slope.  The year-to-date activity is well above anything previously observed since 2012 in these areas:






Here's a map of the PSAs:



With such an outbreak of lightning, it was inevitable that many fires would be started, bringing smoke and evacuation orders, including in the Fairbanks area.  The total fire acreage quickly grew to nearly 250,000 acres earlier this week, but cooler weather and some fairly widespread rainfall has suppressed fire growth in recent days, and the year-to-date fire acreage is now only modestly above the median for the time of year.




Where to from here?  Warmer weather is returning, and the leading ECMWF ensemble model shows signs of a ridge setting up again over northern Alaska by the middle of next week (see below), so the reprieve may be short-lived.  There's a lot of summer left, although the likelihood of seriously problematic fire weather tends to diminish significantly after mid-July; the last year with really significant fire activity after July 15 was 2019.





Saturday, June 14, 2025

Heat Advisory

I'm at the outset of a 10-day spell of travel, so probably won't be posting in the near future - but I wanted to comment on the news that a NWS "heat advisory" has been issued for the first time in Alaska.  This has made headlines and stirred up interest quite widely.

First, the facts: there is indeed a heat advisory in effect for a large part of interior Alaska, from the Tanana region to the Fortymile Country (including Fairbanks-land), the Yukon-Tanana uplands, and the Yukon Flats.


Details:


This is indeed the first heat advisory issued in Alaska... but that's because the Alaska region NWS offices did not issue this product at all until this year.  Special weather statements were used instead to draw attention to excessive warmth.

As for whether this event is particularly extreme - no, it's not, although it is an unusually hot spell for the region.  Fairbanks is currently expected to see 85°F on three consecutive days, and that doesn't happen in most years - although it did in both 2023 and 2024 (in July).  It's particularly unusual for June, having happened in only 6 Junes since 1930 (most recently in the very hot June of 2013).

Rick Thoman has an excellent write-up today on the same topic, including an explanation of the rationale for the NWS procedural change:

https://alaskaclimate.substack.com/p/alaskas-first-heat-advisory


Thursday, June 12, 2025

May Climate Data

It finally happened - Fairbanks reached 70°F yesterday.  Based on NWS data since 1930, this is the latest on record by 4 days, although back in 1911 the university farm apparently didn't see 70°F until a day later, June 12.


It's interesting to take a look at global temperature patterns for the month of May in previous years with a "first 70°F" in June:


At least in the Northern Hemisphere, this has a lot of similarity with this year's global temperature patterns in May: note the negative PDO-like pattern in the North Pacific domain, the warmth over central Canada, warmth in western Europe and central Asia, and cold in eastern Europe.


How about the rest of summer?  It's interesting to observe that June is more likely to be warm than cold in central and northern Alaska, and that fits with the current major warm-up; Fairbanks is expected to reach the mid 80s this weekend.


But cooler than normal weather tends to return in these years in July and especially in August:



Here's the mid-atmosphere pressure pattern that produced the chilly May in Alaska this year: as noted in a previous post, a trough was locked in over the state.


The pattern shows considerable similarities to both May 2024 and May 2023, and it was the third cooler-than-normal May in a row for Alaska.  Here are my usual percentile rank maps based on (top) ground-truth observations and (bottom) ERA5 model reanalysis.



Interestingly, this was the coldest May since 2013 for the North Slope and Northeast Interior divisions, and the coldest since 2012 for the northern Panhandle division.  For the North Slope, it was the first colder-than-normal month since last July (with normal defined as the 1991-2020 average).

Perhaps a bigger story than the lack of warmth, however, was the very wet weather in much of southern and eastern Alaska, and especially in the panhandle.  For the second month in a row, the statewide average precipitation was very nearly the greatest on record for the month, and that makes four months in the last year with record or second-highest statewide monthly precipitation:

July 2024   Record wettest

January 2025   Tied record wettest

April 2025   Second wettest

May 2025   Second wettest



Rick Thoman reports that all the long-term climate sites in southern Southeast Alaska set records for May precipitation, and Juneau saw measurable precipitation on 30 of 31 days.  However, it was unusually dry in northwestern Alaska.

As is to be expected, sunshine mostly mirrored the precipitation contrasts across the state:


Finally, wind patterns showed a lot of spatial variability, according to the ERA5 data: