Big Temperature Contrast

Stormy and active weather patterns usually involve strong temperature contrasts (except in the tropics), so it's been no surprise to observe larger than normal temperature differences across Alaska in the past couple of days.  A strong cold front swept east across the state Thursday night and yesterday, so that by yesterday afternoon most of the west and interior was much cooler than normal, while extremely warm conditions persisted in Southeast Alaska.  Here's a map of the 850mb temperature departure from normal at 4pm yesterday:

The warmth in the Southeast has been remarkable, and I think it's safe to say that locations from Juneau southward are experiencing their warmest last-ten-days of August since 1963 (longer in some locations), at least in terms of daily maximum temperatures.  Including today, Ketchikan has reached or exceeded 70°F for 9 consecutive days, and that would be an unusually lengthy warm spell even in the height of summer.  Juneau has seen 4 days in the mid-70s (74° or 75°) since the 23rd, and the only other time that happened so late in the year was back in 1941.

Perhaps most remarkably, Sitka airport appears to have reached 70°F for the 5th consecutive day today, and that has only happened once at any time of year: August 8-12, 2005.  [Update August 31: the high was actually 69°F, so four consecutive days of 70°+.  Still a rare occurrence even in high summer, and not previously observed this late in the season.]

Of course the magnitude and persistence of the warmth in Southeast is being boosted by the very warm water temperatures across the North Pacific.  Even though the northeastern Pacific isn't as absurdly warm as the northwestern Pacific, sea surface temperatures are still well above normal near the Gulf Coast.

On the flip side, the influx of cold air along with cloud and rain yesterday brought very chilly temperatures to the central interior by early afternoon; here are the observations from 2pm yesterday (click to enlarge).  Munson Ridge (3100' elevation) had dropped to the freezing point by noon, and with nearly half an inch of precipitation falling in the afternoon, there was presumably some snow up there.  Notice the warmth hanging on in the east: it was still 70°F in Chicken at that time.

The temperature then proceeded to plummet overnight in Chicken, apparently reaching an astonishing 14°F at the co-op site this morning.  Given that the RAWS thermometer reached only 25°F, the co-op observation seems slightly difficult to believe, although the RAWS site is up on the hillside, so naturally warmer.  If correct, it's the earliest on record for such cold in Chicken by more than a week (13°F on September 7, 2004), although the period of record only goes back to 1997.

Finally, I'll just note the hydrological situation: all the rain (including recently in South-Central) has produced minor to moderate flooding on several rivers, and there's a record flood on the Yentna River, a lower tributary of the Susitna.  The Tanana River near Fairbanks and Nenana is high, but not expected to reach flood stage in the immediate future.

Stormy Weather

As expected, the active weather pattern and strong west/southwest flow is producing a stormy end to climatological summer, with lots of rain and wind, for much of western and northern Alaska.  Here are rainfall totals reported from automated sites in the last 72 hours (click to enlarge, and ignore zero values which are mostly false):

The 3.39" of rain at Coldfoot is consistent with a substantial response in the local river gauge, which reached a record high level:

This gauge apparently has a decades-long period of record, so it's quite remarkable to see an all-time record crest.  The list of historic crests includes both spring meltout and late summer flood events:

River gauges in Fairbanks-land are high but don't yet indicate flooding, despite some impressive rain totals in the vicinity.  Here's a list of totals as of yesterday evening, courtesy of the NWS.

The Fairbanks airport's rainfall occurred mostly on Sunday, when the daily total of 1.30" was the highest in August since 1990.  It's also worth noting that the vigorous flow aloft produced strong winds in Fairbanks on Saturday afternoon, reaching 49mph at the airport and causing power outages.

Looking ahead, it's a bit disconcerting to see more of the same: the 15-day ECMWF precipitation forecasts are wetter now than they were a week ago for the same areas of the west and north.  Both models show another 6+ inches of rain across the southern slopes of the Brooks Range, as a powerful ridge redevelops near the northern Panhandle and keeps the flow locked in from the southwest across mainland Alaska.

Wet Forecast

The upcoming wet pattern is coming into better focus in the latest model forecasts, and indeed it looks very wet from southwestern Alaska across the western interior to the central Brooks Range.  Both the traditional and AI-based ECMWF models are showing widespread amounts above 3-4 inches in the next 10-15 days, with higher amounts locally.

The strong west-east gradient across the interior reflects the fact that the flow will turn more southerly by the middle of next week, shutting off big rain potential for locations downwind of the Alaska Range; but lots of moisture will continue to flow northward across western Alaska.  Here's a sequence of 500mb maps at 48-hour intervals beginning Saturday afternoon.

I picked out Kobuk as a location to illustrate the potential for very persistent rain that may eventually cause flooding issues - see below (note especially the two ECMWF models in the top panels).  However, the NWS notes that most rivers in the Koyukuk and Kobuk basins currently have low flow, so it won't be a problem immediately.

Looking Wet

Higher elevations of the Brooks Range had a taste of wintry weather this morning, with snow falling and accumulating for some hours.  Here's a webcam view from Anaktuvuk Pass, which saw about 8 hours of snowfall with temperatures near or just below freezing.

Snow isn't on the horizon just yet farther south, but rain most definitely is - and probably a lot of it.  The figure below shows one perspective based on ensemble forecasts from 4 models, including the new AIFS ensemble, which I mentioned the other day.  The black line shows the median predicted accumulation of precipitation in Fairbanks over the next 15 days, and the red and blue lines indicate the 75th and 25th percentile of each model's ensemble (i.e. a range of scenarios).  All 4 models show median expectations above 2 inches, with most of it falling around Sunday through Tuesday.

Ensemble-mean maps from the ECMWF (IFS) and AIFS systems show widespread amounts of 2-3" or more across much of the interior (although not in the east).  For reference, Fairbanks averages 2" for the whole month of August.

Here's the upper-level pattern responsible: as is typical in these events, strong west-southwesterly flow caused by a strong north-south pressure gradient.  Let's hope the moisture influx isn't too extreme.

August Changes

It's been a very chilly few days in the central and eastern interior, with an upper-level trough creating cloud and rain for many.  The high temperature in Fairbanks on Friday was only 52°F, a very low value for this point in August, and yesterday Delta Junction and Northway reached only 46°F and 47°F respectively.  This is the most anomalously cool weather since mid-June.

August is an interesting month for climatologists, because in the far north it is, in some ways, more like an autumn month than a summer month.  This contrasts sharply with its character farther south in the mid-latitude and sub-tropics, where summer drags on or even reaches its zenith of heat and humidity in August (just ask the poor residents of Houston, Texas).

The accelerated timetable of seasonal change in the high latitudes is of course directly tied to the much greater loss of solar heating by this time of year.  At 30°N the sun loses only a small fraction of its power from June to August, and the difference can be imperceptible to many; but the percentage decline is very much larger in the Arctic.

Here's a figure showing the loss of solar radiation from June to August.  Note that this includes the effect of increased cloud cover for many northern areas in late summer, so it's not all attributable to the sinking sun; but the north-south difference is clear.

With solar heating rapidly being lost, the effect on temperatures is inevitable; August is much cooler than July across nearly all Arctic and sub-Arctic land areas.  But the map below immediately reveals another very interesting aspect of August: the northern oceans (except for the ice-covered Arctic Ocean) continue to warm up despite the loss of solar heating.  This is because the heat capacity and vertical transport/mixing of heat are so much greater at the ocean surface than at the land surface: the August sun, weaker as it is, still outweighs the loss of heat from the ocean surface for a time (the air temperature over the northern extratropical oceans typically peaks in the second half of August).

With the distribution of land and ocean being what it is in the Northern Hemisphere, the effect of these seasonal changes is to substantially increase the overall north-south gradient of temperature at about 60°N in August.  And this in turn provides the driving force for a significant increase in westerly winds at that latitude (except farther south in the northwestern Pacific, and a bit farther north in Europe).  Here's a map of the July-August change in westerly winds at 250 mb, which is approximately the level of the jet stream.

The increased energy in the westerly flow causes more disturbed and cloudy weather around the high latitudes, and there's a substantial increase in precipitation compared to July across the North Pacific and North Atlantic - see below.  Interestingly the increase in precipitation doesn't extend to northern land areas in general, and some regions become significantly drier in August; this is at least partly because of the decrease in temperature over land, i.e. cooler air holds less moisture, but over the oceans the warmer water provides more moisture and more rainfall for maritime areas.

Below are zoomed-in versions of the July-August temperature and precipitation change maps for Alaska.  The drop-off in temperature is most pronounced across northern Alaska, and it's minimal along the Gulf Coast, with its heavy maritime influence.

Most of western and southern Alaska are distinctly wetter in August than in July, and it's a large difference for the northeastern Gulf Coast.  Yakutat is a classic example: August produces about 80% more rainfall than July on average.  However, the eastern interior and (according to ERA5 data) parts of the Brooks Range are drier in August.  Northway, for example, sees about 30% less rain in August than in July.

I've used data from the most recent 30-year period for the maps above, so it's interesting to see if the same trends were evident in the previous 30-year window.  The answer is, "essentially yes" - see below.

I also find it interesting to note that August is also considerably cooler than June over much of northern Alaska.  Unlike the comparison to July, this is not universally true across northern land areas, and in fact northern Alaska shows the largest temperature drop from June to August of any high-latitude location.

Finally, the nClimDiv data from NOAA/NCEI gives a good sense of how August temperature and precipitation have changed in the past several decades in relation to June and July.  The following charts show 30-year running mean values on a statewide basis. 

Interestingly, August has warmed up a lot less than either June or July on a statewide basis, and apparently it's only in recent decades that August has become cooler than June.  This is supported by ERA5 data (see below): the August-minus-June analysis for 1965-1994 shows much less pronounced cooling in the northern interior, and southern Alaska apparently used to be more significantly warmer than June.

A more detailed analysis of ground-truth station data and atmospheric circulation patterns would be required to confirm and understand this subtle change in early versus late summer temperatures.

Cooling Off, and July Climate Data

It's beginning to feel a lot like autumn already for much of Alaska, as temperatures have dipped below the declining seasonal normal in the past few days.  Sub-freezing temperatures occurred in many of the colder spots of the central and eastern interior this morning, including in the Fairbanks area: 27°F at Goldstream Creek and 31°F at the Creamers Field SNOTEL site.  The early chill will hasten the arrival of autumn colors in the next couple of weeks.

I've been too busy to post my usual monthly climate update in recent days, but better late than never - here's a look back at climate anomalies in the month of July.  It was a fairly warm month overall, and significantly drier than normal on a statewide basis, which marks a big contrast with last year's record wet July.  Warmth was widespread (except for Southeast), but the dryness was most significant along and north of the Gulf Coast from Cook Inlet to the far northern Panhandle.  Here are percentile rank maps from NCEI (top) and ERA5 reanalysis (bottom):

As usual, the NCEI and ERA5 maps differ significantly for precipitation, with ERA5 showing much wetter conditions from the western interior to the Alaska Peninsula and Kodiak Island; and ERA5 also highlights the unusually sunny and dry conditions across the North Slope.

One of the most significant events during the month was the heatwave focused on northwestern Alaska early in the month.  Rick Thoman comments a bit more on this in his blog:

https://alaskaclimate.substack.com/p/july-2025-arctic-and-alaska-climate

Wind was above normal in the zone of wet and cloudy weather across the southwest.  This region of poor weather seems to have been caused by a frontal zone along the eastern side of the (quasi-permanent) Bering Sea trough; as the second figure below illustrates, there was a subtle north-south ridge axis over Alaska, blocking the westerly flow.  The more significant upper-level feature was a ridge over the East Siberian and Alaskan Arctic Ocean, and that was a key player in the northern heat wave:

https://ak-wx.blogspot.com/2025/07/northern-heat.html

Regional lightning activity was not far from normal for the month overall, based on lightning strike counts that include northwestern Canada; but it was more active in parts of the Alaskan interior, and especially north-central areas.  As is typical, most of this activity occurred early in the month, including well over 10,000 strikes on the 7th.  It appears this is the first year (2012-present) with over 10,000 cloud-to-ground lightning strikes on 5 different days during summer, based on data collected by the Alaska Lightning Detection Network.  I'll have more to say about this summer's lightning and wildfire activity in a subsequent post.

AI Weather Forecasting

Back in March of 2024, I mentioned the new AI weather model developed by the world-leading European forecast center (ECMWF), and it's exciting now to be able to comment on a major recent upgrade to the ECMWF's AI forecasting technology.  In short, ECMWF has released a new version of their AI system (AIFS) that produces an ensemble of outcomes, similar to the ensemble output from traditional physics-based models.

https://www.ecmwf.int/en/about/media-centre/news/2025/ecmwfs-ensemble-ai-forecasts-become-operational

In many respects, this latest AI system is now significantly better than the ECMWF's own IFS (Integrated Forecasting System), the traditional physics-based model that has been developed over decades, and that grinds out its forecasts on large supercomputers.  In contrast, the AIFS is "data-driven", meaning it learns from the historical data with no computational constraint from the laws of physics; and its realtime forecasts run extremely quickly with a tiny fraction of the computing resources.

Here's a chart to illustrate the realtime performance of AIFS forecasts initialized from December 2024 through June 2025:

The AIFS-ENS model (black line) has the highest correlation of 30-90°N 500mb height anomalies, and for lead times beyond about a week it's dramatically better than the first AIFS version that had only a single ensemble member.

It's interesting to have a look at where on the globe the superior performance is to be found.  The maps below show the improvement of AIFS over the traditional IFS ensemble, measured in terms of the percentage change in variance explained for 500mb height at various lead times.  For instance, if the correlation coefficient improves from 0.8 to 0.85, that's a 13% improvement in variance explained.  Interestingly, the improvement seems to emerge first in the tropics at only 2-3 days into the forecasts, and then the higher latitude forecasts start to benefit - especially over the North Pacific and North America - after 4-5 days.

The spatial signals become very noisy at longer lead times because of the rather small sample size, but averaging the results across all longitudes reveals that the high latitudes see the most improvement beyond 7 days:

The significant improvement in the Arctic will be very encouraging for Alaska forecasters, because weather predictability is lowest in the high latitudes to begin with.  Redoing the first figure above for 60-90°N, we see a nice boost beyond 10 days (see below).  To be precise, the anomaly correlation at day 15 goes from +0.21 (IFS) to +0.27 (AIFS-ENS), and while that's still too low to be useful, it represents a 57% gain in variance explained.

It can't be overstated how remarkable it is to see performance gains like this from such young technology; presumably there is room for considerably more improvement in the years ahead.

Finally, readers may ask where the AIFS-ENS forecasts can be viewed?  Unfortunately, I'm not aware of any free websites that provide AIFS-ENS maps that include Alaska, with the exception of ECMWF's own site, and that's not particularly user-friendly:

https://charts.ecmwf.int/?facets=%7B%22Range%22%3A%5B%22Medium%20%2815%20days%29%22%5D%2C%22Type%22%3A%5B%22Forecasts%22%5D%2C%22Component%22%3A%5B%22Surface%22%2C%22Atmosphere%22%5D%2C%22Product%20type%22%3A%5B%22AIFS%20Ensemble%20forecast%22%5D%7D

I expect in due course the AIFS-ENS data will be added to sites like Tropical Tidbits, so check back there (under "Ensemble") occasionally.