Still Raining

I'm traveling this week and most of next, so posts will be light, but it's worth noting the exceptional persistence of wet weather in Fairbanks.  With well over an inch of rain in the last two days (and it's still falling), this month has brought 3.77" through the 16th, making this already the wettest September since 1925.

With incessant cloud cover, the average daily temperature range so far this month in Fairbanks is only 12.4°F, far lower than the normal range of 20°F, and easily the lowest on record.  No crisp frosty mornings and dry sunny days this year.

Here's a chart (click to enlarge) of the number of hours with rain, snow, or drizzle reported in Fairbanks in the first half of September since 1950.  The 2026 total would rise quite a bit if we included the 16th and 17th, as it's been raining almost non-stop in the last two days.

Notice the consecutive high number of hours since 2022 - this is the fifth straight year with unusually frequent precipitation in the first half of September.  It seems quite likely that this is related to the persistence of unusual warmth in the northwestern Pacific Ocean since 2022.  Some may recall that the emergence of that sea surface temperature anomaly was implicated in the intensity of ex-Typhoon Merbok in September of that year:

https://ak-wx.blogspot.com/2022/09/more-on-ex-merbok.html

Last month's SST anomaly map shows the similarity of the ocean temperature pattern this autumn:

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.

Record Rain and Snow

Fairbanks schools and borough offices are closed for the second day today, as record-breaking rain and wet snow has produced difficult travel conditions in the area.  The airport's two-day liquid-equivalent precipitation total of 2.28" is within the top 10 for any time of year (1930-present), and of course most of the heaviest precipitation events occur in the warm season in Fairbanks.  Only two cold season events have seen higher two-day totals: the Christmas storm of 2021 (2.45", blog post here) and January 19-20, 1937 (which was actually a three-day event totaling 3.21"). 

More superlatives: yesterday's calendar day precipitation total of 1.99" is the third largest of any day since 1930 in Fairbanks, the only wetter days being July 27, 2003, and August 12, 1967 (the great Fairbanks flood).  And according to Rick Thoman, the peak 24-hour total (Sunday evening through Monday evening) of 2.07" is the highest on record for the cold season, and the first to exceed 2".  The previous cold season record for 24 hours was 1.93" in the 2021 storm, and before that 1.84" in 1937.

Higher amounts were reported elsewhere: a CoCoRaHS site near the airport measured 2.96" in the two days, and the Keystone Ridge site came in at 3.27".

At least half of the precipitation fell as plain rain, according to the airport ASOS data, and owing to temperatures persistently below 20°F late last week, the cold ground led to lots of ice formation.  There's a window for some thaw today, but temperatures will drop back below freezing tonight and stay there, so some of that ice is going to be around all winter, unfortunately.  It's a theme we've seen repeatedly from winter rain events in Fairbanks in recent years.

Here's a snapshot of the atmosphere's vertical profile at 4pm AKDT yesterday; it was still raining at the airport, but rain had switched back to snow on Keystone Ridge:

Check out the wind speeds aloft: over 150 mph at 34,000 feet.  The southwesterly flow direction through much of the lower and middle atmosphere is what brought the moisture into the central interior without it being squeezed out by the Alaska Range to the south.

As for storm impacts elsewhere in Alaska, the west coast has been very windy, strong winds affected south-central areas yesterday, and Bering Sea coastal flooding has undoubtedly been significant, but I haven't seen detailed reports yet.

Far to the west over Chukotka's Arctic coast, the storm's central pressure bottomed out at around 956 mb according to Environment Canada.  As noted in Friday's post, this is very likely an October record for that location, but we'll have to wait a few days for the ERA5 data to be sure.  Here's the surface analysis from 4am AKDT yesterday.

A World of Difference

The last 3-4 weeks have seen a remarkable turnaround in weather fortunes for much of Alaska, with dramatically wetter - and considerably cooler - conditions bringing a very welcome end to the wildfire season.  Compare the two 30-day precipitation anomaly maps below: the top one showing the situation as of a month ago, and the latest analysis on the bottom.

 

On July 5, 18% of Alaska's land area was classified as having moderate or severe drought, according to the U.S. Drought Monitor, but now drought remains in only a small area (less than 1%) just to the south of Fort Yukon.

Reader Gary commented that "it seems like August in July for Interior Alaska", and indeed the change to notably wetter and cooler weather is very typical for August.  I've pondered whether it would be possible to identify a sudden regime shift that marks this transition in many years; it would be nice to measure the change in an objective way, and look at how the transition varies from year to year and from decade to decade.  For example, has it become earlier over time?  Any suggestions from readers would be welcomed.

In the absence of objective criteria to define an August-like "regime shift", I thought it would be interesting to look at the timing of the warmest week of the year in Fairbanks: see below.  Early July is the most common time for the warmest week to occur, like this year, but there is a lot of variability, with some years seeing the warmest week in early August or before the middle of June.  There's a hint of a trend towards earlier peak warmth, although it's certainly not statistically significant.

A much more significant trend is evident if we look at the wettest week of the year in Fairbanks, based on rain amounts only (I've excluded weeks with any measurable snowfall).  The timing of peak rains really has changed a lot: prior to about 1970, the wettest week was more often in August or September than in July, but in the past 20 years it has become rather unusual to see the peak in August, let alone in September.

From this perspective, then, it does look like the transition from dry spring weather to wet "late summer" weather has become earlier over time.  But we should also recognize that changes in July rainfall have been dominated by an increase of infrequent but heavy events, as discussed elsewhere on this blog (e.g. here), and this is not necessarily the same thing as the August wet regime arriving earlier (i.e. July rains are more convective, whereas August is typically more stratiform).

Another intriguing angle on the timing of the regime shift is found in the midpoint date for the wildfire season: see below for one of Rick Thoman's excellent Twitter graphics.  Only 30 years of data are available, but there's a hint of a trend towards earlier fire season midpoints.  Interestingly this season was a big one (over 3 million acres burned), and yet its midpoint was on June 30 (assuming acreage does not increase much from here).  No other year in the series had both a large acreage total and a relatively early midpoint.

That's enough speculation for one post.  But for the sake of completion, here's the timing of the coldest week in Fairbanks.  Given the profound warming throughout the Arctic in autumn and early winter, it's no surprise to see that it has become less common for Fairbanks to have the winter's coldest weather before the turn of the year.

Weather Relief

I'm traveling at present and can't comment in detail, but major weather relief has arrived at last when it comes to Alaska's nasty fire and smoke season.  A change of pattern was advertised well in advance by the Climate Prediction Center, and widespread rainfall has been a big help for fire-fighting efforts.  Here's a temperature anomaly plot for Fairbanks and the latest 7-day precipitation analysis from the NWS:

The 30-day departure from normal analysis (below) now shows some green across southern Alaska, but notice how much brown there still is on the 90-day anomaly map (also below).  The drought isn't over yet.

Returning to the topic of lightning, a few days ago I witnessed an impressive night-time display of lightning in Florida, and I got to thinking about the contrast in lightning density between the two states: Florida having the most lightning per square mile, and Alaska the least.  Based on the BLM/ALDN data, Alaska's peak lightning density has been about 5 strikes (cloud or ground) per square km over about 11 years, i.e. approximately 0.5 strikes per km² per year; this lightning hotspot is just to the north of the Alaska Range, roughly between Minchumina and Healy.

In contrast, the entire state of Florida averages about 7 strikes per km² per year, according to a 10-year study from Vaisala, and some areas have considerably more.  So even the most lightning-prone parts of Alaska don't come close to what's considered normal in the so-called Sunshine State.  As far as I'm concerned, less lightning is always better, although it can be a visual spectacle: here's a video taken by my daughter last Wednesday evening as we were driving in Florida (apologies for inferior quality).  It would be interesting to dig through the Alaska lightning data to find the single most prolific lightning storm and see how it compares to these flash rates.

https://youtu.be/M63plvvaVfw

August Chill

Ten days ago I was writing about the hottest weather of the summer for interior and western Alaska, but a dramatic reversal has suddenly produced unseasonably chilly conditions instead.  Widespread significant rain occurred again from the west coast to the White Mountains, and the scene has turned wintry in the Brooks Range: Atigun Pass is now reporting about 6" of snow on the ground.

Here's the FAA webcam view from Chandalar Shelf (elevation 3285') this morning:

With cloud and rain holding daytime temperatures down, Fairbanks has seen the chilliest couple of days for this early in the season since August 2002.

Interestingly the late summer of 2002 evolved quite similarly, with some distinctly warm weather in the second half of July and the beginning of August, followed by below-normal temperatures.  Both years had a significant rain in Fairbanks around August 7-9, then heavy rain around the 16th.

With more than an inch of rain in Fairbanks since Sunday morning (and nearly continuous too), the summer's rainfall total is now above 5.5" and above the long-term normal, for the 8th consecutive year.  I wrote about this remarkable trend in a few posts last year, for example:

https://ak-wx.blogspot.com/2020/08/back-to-rain.html

https://ak-wx.blogspot.com/2020/07/rainfall-trends.html

For a broader look at why August tends to be wet, see the 2018 post below.  The short answer is that the rapid loss of solar heating over the Arctic is already causing substantial strengthening of the jet stream, leading to more vigorous large-scale weather disturbances, and there's still a lot of moisture available in the atmosphere to produce copious rain across much of the high latitudes.

https://ak-wx.blogspot.com/2018/08/why-is-august-so-wet.html

Rain on Permafrost

Reader Gary posted a comment about possible ocean impacts of excess water runoff from this summer's heavy rainfall over western Alaska, and while I don't have any insight on that question, it got me thinking about effects on permafrost.  Back in March I commented on a National Park Service study showing that permafrost warming has been extreme in northwestern Alaska, and it seems likely that excessive rainfall can only worsen the situation.  Here's a different study, also published this year, that reaches the striking conclusion that "precipitation is as important an environmental control on permafrost degradation as surface air temperature":

Changes in precipitation and air temperature contribute comparably to permafrost degradation in a warmer climate

I'll be the first to admit I don't know much about permafrost physics, but a back-of-the-envelope estimate seems to confirm the significance of the issue.  For example, if 10 inches of rain falls at an air temperature of 10°C, and all the water percolates into the subsurface, this makes available about 10 MJ of energy per square meter for warming or thawing near-freezing soils (based on the heat capacity of rain water: 250 kg x 10°C x 4200 J/kg/°C).

If we're talking about simply melting ice, this amount of heat could melt 30 kg of ice per square meter, which is only about 3cm depth of pure ice.  However, if the heat goes only into warming the soil, then 1°C of warming could occur through as much as 5m or more of the ground - based on the heat capacity and density of "icy peat" that I found here.  I'll refrain from any further guesswork, but it seems the effect could be significant.

Here's a paper - apparently under review - that argues the same thing:

https://www.researchsquare.com/article/rs-512675/v1

On the other hand, another study focused on Tibet finds the opposite, with enhanced rainfall being associated with a decrease in heat conduction (presumably because rainy weather is also cooler and cloudier).  Looks like there is scope for further research.

https://www.sciencedirect.com/science/article/abs/pii/S0048969721040420

Extreme July Rainfall

Following up on last week's post, recent rainfall has been prodigious in western and northwestern Alaska, leading to the wettest calendar month of record in Kotzebue (5.32" versus 5.26" in August 1998) and easily the wettest July on record (old record 4.16" in 1931).  Nome surpassed the very wet July of 2012, with the wettest July since 1920 (6.41" vs 8.43").

Note, however, that the 30-day rainfall record in Kotzebue is 6.54", which occurred mostly in August 1946.  Remarkably, Nome has twice seen over 10" of rain in 30 days: in 1922 and 1954.

The rainfall since late June has been most excessive in the far western Brooks Range; check out this accumulated precipitation chart from the Red Dog CRN site, at about 1000' elevation and 80 miles north of Kotzebue.  The site has seen over 12" of rain since late June, and the June-July total was an astonishing 14.5".

The nearby Kelly Station SNOTEL instrument (~20 miles SE of the CRN site) has measured over 10" since late June:

Much farther to the south, the Rocky Point SNOTEL on the southern coast of the Seward Peninsula (not far from White Mountain) has seen 9" in the same period:

For the record, and following on from the last post, here are the NWS rainfall estimates from the past week:

Looking more broadly afield, the west-east contrast in July rainfall across Alaska was very striking; here is Rick Thoman's summary graphic, via Twitter:

The monthly-mean circulation pattern involved an unusually strong trough of low pressure from the Chukchi Sea down to the western Bering Sea, but a ridge over southern Alaska kept jet stream disturbances away from the eastern and southeastern parts of the state.

Not surprisingly, temperatures were quite cool in the west, but were generally warmer than normal in the Arctic, eastern interior, southeast, and Alaska Peninsula.

Big Rain in the Northwest

A lot of rain has fallen above the Arctic Circle in northwestern Alaska this week, courtesy of strong westerly flow circulating around a deep upper-level low to the north of Alaska.  The high-quality CRN site near the Red Dog mine (940' elevation) reported a remarkable 4.0 inches of rain in the last 3 days of June:

NWS estimates (see below) suggest that more than 6 inches may have fallen in some spots, but I'm not sure if any ground-truth measurements came in that high.  Here's one list that the NWS provided:

Regardless of the highest amounts, this was a very significant rain event over a large area, including the interior North Slope.  Click to enlarge:

The 500mb analysis from Tuesday afternoon (see below) shows the strong Arctic low, and notice the contrast with the extremely strong ridge over southwestern Canada.  All-time heat records were shattered in Oregon, Washington, and British Columbia, including an outrageous 121°F in southern BC on Tuesday (by far Canada's highest observed temperature of all time).  There was certainly a dynamical connection between the two features - the ridge and the Arctic trough - and the large amplitude of the overall flow anomaly helps explain the extreme rain outcome in northwestern Alaska.

Warm Season Precip Normals

I mentioned in the last post that July retains the top spot for monthly mean precipitation in Fairbanks, according to the new 1991-2020 climate normals, and that historically this was not true for most of Fairbanks' modern climate history; August used to be wetter than July.  A July rainfall peak also differs from most of the state, as August is considerably wetter than July in all but the eastern interior and the North Slope.

To look at the shifting climate normal in more detail, here's a chart showing the overlapping 30-year normals since 1931-1960 for the warm season months.  A spate of wet July's from 2001-2010 brought a significant increase to the July average in the 1981-2010 normal, but the other months were largely unchanged until more recent years.  However, the 1991-2020 normal is wetter in each of June through September (but interestingly there's a slight downward trend in May).

 

Here's a time series view of the June-September totals (blue columns) and the fraction that is attributable to July (gray columns).

 

The change in July is largely attributable to a handful of very wet months, but the consistency of the high seasonal totals since 2014 is quite remarkable.  Here are a few blog posts from last summer about this:

https://ak-wx.blogspot.com/2020/07/wet-summers.html

https://ak-wx.blogspot.com/2020/07/rainfall-trends.html

https://ak-wx.blogspot.com/2020/08/back-to-rain.html

Another interesting aspect of the Fairbanks warm season climate history is that the early decades were also relatively wet, but the excess was seen mostly in August back then; August used to be distinctly wetter than July.  But this change isn't just about a shift in rainfall timing from one month to another; it also reflects a major change in the character of summer rain events.

Consider the chart below, showing the total precipitation in categories of daily totals, i.e. how much of the long-term normal is attributable to light versus heavy amounts.  The first two decades in this analysis (1931-1950) saw a higher fraction of precipitation from relatively light daily amounts, but over time there's been an upward trend in the contribution of high daily amounts.

The change in the distribution of daily amounts is connected to the July-August shift, because July rainfall is typically much more convective (brief, heavier events), whereas August rains tend to be stratiform in nature (lighter, more extended rains).  In recent years Fairbanks has seen a decreased frequency of August rains, but very heavy rains in July - while still quite rare - have begun to occur frequently enough to make a significant difference in the long-term rainfall totals.

For more on this topic, see this post from a few years ago:

https://ak-wx.blogspot.com/2017/07/wetter-in-july.html