Wet Weather Patterns

Unusually wet weather has shifted from western to interior Alaska in the past 10 days, with upwards of 2 inches falling from the vicinity of McGrath to the hills north of Fairbanks.  A 30-day estimate shows 6-8 inches over most of the western interior, and that's over 3 times the normal amount for the time of year across the mid-Yukon region in the vicinity of Galena.

Fairbanks saw over an inch of rain late last week, and the Friday-Saturday two-day total was the greatest in September since 2016.

The 500mb height map for Wednesday through Sunday last week shows the same westerly flow regime that I highlighted previously, driven by unusual ridging (high pressure) to the south of Alaska.

Above-normal pressure to the south of Alaska is a key ingredient for wet weather in Fairbanks; here is a composite of the 500mb height maps for the 5 wettest 7-day periods in September from 1981-2025.

The composite analysis also shows that unusually low pressure from the Beaufort Sea to Arctic Canada tends to be involved when Fairbanks is very wet at this time of year.

The figure below shows the same Fairbanks-focused analysis for all 12 months of the year (click to enlarge).  The wettest weather in Fairbanks is associated with a North Pacific ridge in all months of the year except June and July, which are distinctly different; the wettest weather in high summer is instead linked to an Arctic trough axis extending all the way down to the Gulf of Alaska.

How about unusually dry weather?  Measuring this on a monthly basis (rather than 7-day periods), we see more variety in the flow patterns, but in general low pressure over the North Pacific favors dry weather in Fairbanks from September through April, whereas warm-season dryness (May through August) is particularly associated with a ridge axis over interior Alaska.

Back to Cool

[Update September 3: this morning brought a deeper frost for the eastern interior, and Fairbanks dropped to 30°F - the first freeze, and earlier than normal.  The usual cold spots were colder: 22°F at Goldstream Creek, 20°F at the Salcha RAWS, and the Chicken co-op observer reported a remarkable 11°F.  Data in Chicken only goes back to 1997, but this is the earliest on record for such cold; last year reached 14°F on August 30, but 11°F had not previously been measured before September 11 (2012).]

After a late summer climax of warmth in mid-August, temperatures have been on a steep downward slide for most of Alaska in the past two weeks.  The seasonal temperature change always accelerates into autumn for the interior in particular, but the transition from "summer" to "autumn" was especially sudden this year.

For now, temperatures are back into the "significantly below normal" category, and at this time of year that means widespread frost: many locations across the state dropped into the 20s Fahrenheit within the past few nights.  The coldest so far at Fairbanks airport is 34°F; the first freeze typically occurs in the second or third week of September.

Courtesy of NWS Fairbanks, here's a lovely pair of webcam photos from Eielson Visitor Center in Denali NP:

The chilly air mass of the last few days has reached the region from the northwest.  My previous post highlighted the west-southwesterly flow around the north side of the Aleutian ridge, but the flow orientation has shifted subtly to add a cooler northerly component over Alaska:

The forecast expects the ridge to shift east in the coming days, bringing a much warmer southwesterly flow pattern:

 

It will be interesting to see if high-pressure ridging remains a recurring feature near the Aleutians in the coming weeks and months.  The ongoing extremely unusual warmth in the northwestern Pacific may tend to favor a ridge near the Aleutians (or more precisely, weaker low pressure than normal), and that would normally be expected to keep Alaska on the cool side.

However, as winter approaches the influence of the very strong El Niño will probably have the opposite effect, favoring deeper low pressure near the Aleutians and bringing more warmth to Alaska, especially in the east.  Which will win out?  To my mind, this is more or less a tug-of-war between El Niño and the negative PDO phase; it's going to be very interesting to watch.

Very Wet

The past several days have been very wet across western Alaska, as strong westerly winds aloft have transported moisture-laden air into the state.  Estimated 7-day rainfall totals have exceeded 2.5 inches over a wide area, and just yesterday Kaltag reported a daily total of 1.53", taking the August to-date total to nearly 8" there.  Nome is at 6.3" for the month, which puts this month in the top 10 for highest August rain totals.

The NWS precipitation return-frequency viewer indicates a return period of 10-25+ years from the Nulato Hills towards McGrath, highlighting the historically unusual rainfall amounts.

A 30-day percent-of-normal analysis shows that August will go into the books with a big west-east contrast in precipitation:

The westerly flow regime of the past week has developed in response to relatively high pressure over the Aleutians and unusually low pressure in the Arctic, and the source origin of the moist air has been over the (unusually warm) northwestern Pacific.  Here's a forecast map from last Sunday evening showing the mid-atmosphere flow pattern for Monday-Friday of this week:

Tuesday evening saw strong west-southwest winds in the Fairbanks area as a strong front moved through:

🛰️ Satellite imagery today showed an impressive front tracking across Alaska, bringing rain and gusty winds. Here in the Fairbanks area, we saw wind gusts of 30–40 mph this evening, which resulted in several thousand power outages. Rain/high elevation snow will continue Wed.#akwx pic.twitter.com/7tbXUpd3xL

— NWS Fairbanks (@NWSFairbanks) August 26, 2026

There's a lot more rain on the way, and likely extending farther east in the coming days.  The NWS emphasizes the northwestern interior with particularly unusual conditions in the next 10 days, but the overall wet signal is widespread.

The CPC confirms the broad wet signal through the first week of September.  Not great for fall foliage viewing.

El Niño and Bering Sea Storms

With memories of last year's ex-typhoon Halong still fresh in many minds, it's worth asking the question, "What about this year?"  Does the major El Niño that is now occurring affect the risk of a similar outcome during this autumn's storm season on Alaska's Bering Sea coast?

It's well-known that El Niño tends to strengthen the "Aleutian Low" in winter as a response to major alterations in tropical patterns of convection/rainfall.  Looking at the strongest El Niño events of the past, this effect is evident from November on, but not in September and October.  The following maps show the average 500mb height (pressure) departure from normal during the strongest El Niño conditions for each month in autumn and early winter:

Looking at historical storm track data from ERA5 reanalysis (processed data courtesy of the University of Manitoba), there are some modest differences between El Niño and La Niña.  The maps below show the number of autumn storm tracks passing within each grid cell, for storms with a central pressure of 980mb or lower (a modest threshold - many of these are run-of-the-mill storms, not intense storms).  Note that here I'm defining El Niño and La Niña based on tercile thresholds from the Multivariate ENSO Index.  Click to enlarge:

The main feature that stands out to me is an indication that the Seward Peninsula and Norton Sound region tends to see more autumn storms during El Niño than during La Niña.  Farther south, the storm density is generally similar for the southern Y-K Delta and around Bristol Bay, and the Gulf of Alaska clearly sees more autumn storms during La Niña.

The classic El Niño enhancement of storm activity emerges in December through February, but by then the typical storm track is south of the Y-K Delta region, so the winter storm frequency is not notably enhanced for Alaska's west coast.

Looking more closely at the autumn results from the Seward Peninsula to the northern Y-K Delta, the increase in storm frequency during El Niño is mostly found in November and then continues into December, as we would expect from the 500mb maps.  Here's a table showing the number of 980mb or lower storms that passed through a box over the region:

Here's the box I used for the analysis:

Another way of looking at the ENSO sensitivity is that this box has seen 11 storms below 970mb during the El Niño tercile, but only 3 such storms during La Niña (since 1950).  The three strongest autumn storms in the El Niño set dropped below 960mb in this box and occurred in the Novembers of 1979, 2009, and 2015 (the latter being a very strong El Niño), whereas the lowest MSLP in a La Niña autumn (963mb) was last year's ex-typhoon Halong.  Ex-typhoon Merbok (September 2022) is in 3rd place (964mb) for a La Niña autumn storm.

It's interesting to observe that 3 of the 4 strongest storms in the Norton Sound box during La Niña have occurred in the last few years: Halong, Merbok, and another strong storm in mid-November last year.  In this context, it's worth bringing up again the remarkable warmth that has prevailed in the northwestern North Pacific in recent years, as discussed post-Halong here.  It's not unusual to see a warm northwest Pacific during La Niña, and especially after a multi-year La Niña, but the warmth of recent years has been exceptionally pronounced.

Checking in on current North Pacific SSTs, extreme warmth is still persisting from northern Japan to the waters south of the Aleutians.  This is uncommon during El Niño, when the circulation pattern tends to drive cooling to the east of Japan, i.e. a more positive PDO phase.  The combination of a strong El Niño with a significantly negative PDO phase is highly unusual.

I don't want to draw any strong conclusions from this brief analysis, but it does seem there are a couple of reasons to suggest the odds of west coast storm activity may be higher than usual this autumn.  In the first place, if extremely warm ocean temperatures persist to the south of the Aleutians, then decaying typhoons would be expected to be more vigorous as they enter the Bering Sea in general.  September and October are the peak months for decaying typhoon potential.

And then looking ahead to November and December, historical data suggests that El Niño does favor an increased risk of strong early winter storms for the central west coast of Alaska, i.e. around the Norton Sound region.  However, I haven't attempted to assess the statistical significance of this result, and certainly the last few years demonstrate amply that you don't need an El Niño to get a damaging west coast storm.

Heat Wave Ends

This summer's only significant heat wave has now ended in Alaska, and autumn is most definitely just around the corner.  But what a heat wave it was in the past week - a very unusual spell of warm weather that completely defied the character of the rest of summer and indeed the rest of the year so far.

My last post mentioned temperature records at Juneau and Uqiaġvik, but there's more to talk about.  The heat situation escalated significantly in Juneau on Friday, when the high temperature reached a remarkable 85°F - a new record for August and the latest date with such heat by more than a month.  Previous dates with 85°F or higher at Juneau airport have all occurred between mid-June and early July.

I also overlooked a similar record in Haines on Friday: 87°F, again easily the latest date with such a high temperature.  Only once, in 2017, has Haines reached 87°F in August, and that was on the 5th of the month.

Having said all this, mid-August 2004 was a close second for late heat in both locations: Haines reached 86°F then, and Juneau saw 84°F, just one degree lower in both places.

Here's a map view of high temperatures reported on Friday (August 14) across the northern Southeast (click to enlarge):

It was also very hot in the far southeast: Klawock airport measured 88°F (marked as 87°F on the map below - a data rounding issue).  This is a new August record by a margin of 4°F at Klawock, but data only goes back to 1998.  Nearby Ketchikan saw 89°F in mid-August 1977 and 87°F in mid-August 1950, so this kind of late summer heat isn't unprecedented in this area.

Emphasizing again the nearly statewide nature of the heat wave, Kotzebue broke a high temperature record on Sunday: 75°F, compared to a previous record of 69°F on that date.  Kotzebue has rarely seen such warmth after the middle of August, with the 2004 heat wave being the most notable such event in the past.

Referring back to UAF's statewide temperature index again, the value peaked at about 9.8 on Friday and Saturday, implying a statewide anomaly higher than about 99% of all days.  The last time it was this unusually warm (compared to normal for the time of year) was January 2025.

Here's a map of the standardized 850mb temperature anomaly in the last 7 days, illustrating the Alaska-centric nature of the feature.

Contrast the 30-day anomaly from late June through late July:

Serious Warmth and Ice Decline

As advertised in recent posts, warmth has become pronounced and widespread in Alaska this week, and yesterday saw the temperature rise all the way to 70°F in Utqiaġvik - a fairly rare event, especially this late in the season.  The last time 70°F was reached in Utqiaġvik was more than two years ago (August 6, 2024), and this is only the third time it has happened in August this century (2023 and 2024 being the other years).  The latest on record for such warmth is August 24, 1979.

Far to the southeast, Juneau also set a daily record high yesterday, at 80°F.  This isn't quite as unusual, although again it is pretty late in the season; it's only been this warm this late in the season in 5 other years out of the last ~75 years of climate data.  The latest on record is - not coincidentally - August 23, 1979.

On a statewide basis, UAF's aggregated and standardized temperature index shows yesterday being the warmest day relative to normal since October last year - see below.  The latest daily index value of +9.4 indicates that yesterday was warmer than about 97% of all days in terms of standardized departure from normal - again, a fairly rare extreme.  Time will tell as to whether this month's sudden reversal from unusually cool to unusually warm will be sustained, but this is certainly a change from the persistent character of the last 8 months.

The map below shows the sea-level pressure anomaly that has prevailed in the last two weeks.  With high pressure over Alaska and low pressure near the Arctic coast of Siberia, warm southerly flow has been pulled up across the Bering Sea and into the Arctic.

The result?  Sea ice extent has been dropping very rapidly in the Chukchi Sea this month.  At the end of July it was well above normal - a significant anomaly compared to recent years - but it has now dropped to near the 1991-2020 normal.

Beaufort Sea ice has seen much less decrease and remains well above normal and close to last year's extent.  It will be very interesting to see whether or not the next few weeks see a rapid decrease like last year.

The Northern Sea Route appears to be on the verge of opening up; the NSIDC sea ice index has shown an open passage in the last few days, but the NWS ice analysis suggests there are still patches of heavy ice on both sides of Wrangel Island.  Compare the two perspectives below:

The comparison illustrates some of the uncertainty in ice cover analysis.  For NSIDC, ice extent is defined as the area with at least 15% ice coverage, and the NSIDC data shows this boundary being well north of Wrangel Island all the way across to well north of Point Barrow.  In contrast, the NWS analysis shows 4-6 tenths or higher at lower latitudes around Wrangel Island, in the mid-Chukchi Sea, and to the west of Uqiaġvik.  The two products serve different purposes and differences are to be expected, but this illustrates the challenge of characterizing the "true" state of affairs.

Peak Summer Warmth

A significant warming trend in the next several days is going to push temperatures well above normal for the time of year across much of Alaska, and especially across eastern, interior, and northern Alaska as next weekend approaches.  Here are predicted 850mb temperature anomaly maps for Friday evening:

It looks likely that this will be the warmest episode of the summer - not just relative to normal, but in absolute terms.  This is a low bar to clear, of course; Fairbanks has so far reached only 81°F this summer, and the NWS is currently indicating 82°F for Thursday.  A typical summer in Fairbanks reaches the upper 80s at some point.

It's interesting to note the absence of very warm weather in Fairbanks in mid-late August in recent years.  The chart below shows that temperatures haven't exceeded 80°F after August 10 since 2010 - a spell longer than any other in the period of record.  Mid-late August has tended to feel more autumnal in the past 15 years, but the warm spell later this week will be reminiscent of earlier times.

It is certainly unusual for the highest temperatures of summer to occur this late in the season.  In Fairbanks, the brief but intense mid-August heatwave of 2010 (91°F on the 15th - easily the latest 90°F on record) was one of the latest dates for seeing the highest temperature of the summer.  The record latest was 83°F on August 30, 1974.

It's not too late to be concerned about fire risk in areas that have been drier recently, i.e. the southeastern interior: