Flood Follow-Up

A few different items of comment today.  First, a significant snowfall is under way across the south-central region today, signaling the start of winter proper for many valley locations in that area.  If Anchorage's snow cover hangs around in the coming weeks, it will mark an earlier than normal onset of permanent snow pack; the normal date is about November 9th.  Of course Anchorage occasionally lacks snow cover even in the depths of winter, but it's unusual, occurring most recently in 2015-16 and for a few days in late December 2019.

As for Fairbanks, the normal date for establishing permanent snow cover is October 18, and it was right on time this year.  However, the most recently reported snow depth of 10" is the highest for the date since 1992, and more fell today.  (But 11" was on the ground in September 2015.)

Looking back at last week's historic west coast storm, it's worth highlighting blog comments by Rick and Gary.  Rick Thoman confirmed that long-time Kotzebue residents reported the highest water in living memory, which is why it was such a devastating event for some in the community:

https://www.adn.com/alaska-news/rural-alaska/2024/10/26/after-flood-kotzebue-residents-take-stock-of-damaged-homes-and-property-and-take-care-of-one-another/

Although the meteorology was different, we might say this was Kotzebue's version of ex-Typhoon Merbok, which caused such havoc for coastal locations farther south about two years ago:

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

The article highlighted by Gary indicates that flooding was not quite as bad this time around from Nome down to Unalakleet:

http://www.nomenugget.net/news/high-winds-cause-erosion-and-flooding-region

Rick also commented that storms in November 1973 and 1974 may be the most analogous events in the modern climate record.  Certainly the storm of November 10-11, 1973, shows a rather similar sequence of events: low pressure over the Chukchi Sea created strong winds out of the south early on the 10th, and then the winds went around to the west and blew fiercely with a strong north-south pressure gradient, pushing water into Kotzebue Sound.  Here are MSLP maps:

Compare to the recent event:

Interestingly, the wind speed itself was not particularly unusual with last week's storm, and there have been many past events with equally strong, or stronger, winds from a westerly direction in Kotzebue.  I think therefore it was the very low pressure, the large size of the storm, and the long duration of southerly fetch that allowed such a large surge to develop before the winds turned westerly and pushed the water onto the coast.

Kotzebue Flood

In yesterday's post I remarked that I hadn't yet seen specific reports of storm impacts for the west coast, but it turned out that the worst was yet to occur in the northwest, and specifically in Kotzebue:

https://www.adn.com/alaska-news/rural-alaska/2024/10/23/after-severe-flood-displaces-80-people-kotzebue-begins-recovery/

Stormy weather began way back on Saturday in Kotzebue, and the highest winds (gusts to 55 mph) occurred on Monday morning, but it wasn't until yesterday that flooding became severe as the winds went around to the west.

I think what happened here is that a prolonged and very strong southwesterly flow built up a considerable surge of water in the southern Chukchi Sea while the storm's center was at its most intense over Russia's northeastern coast on Monday.  When the circulation finally pushed east towards the North Slope on Tuesday, and the winds went around to the west in Kotzebue Sound, that elevated water rushed into the Sound and inundated the city rapidly.  The "shockingly fast" rise of the water sounds very much like a classic storm surge event that is more commonly seen in landfalling tropical cyclones.

Below is a sequence of model analysis maps showing the pressure and winds at 6 hour intervals from 10am on Monday through 10pm last night.  Judging from the severe impacts, it seems this was essentially a worse-case scenario for Kotzebue flooding.

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.

Big Storm Coming

Weather warnings have been hoisted for Alaska's west coast as well as the central and northwestern interior, as a major storm is set to develop over Russia's Kamchatka peninsula tomorrow and then move northeastward, bringing impacts to Alaska starting on Sunday.  According to weather models, it's going to be a big one - potentially one of the strongest on record for the time of year over far eastern Russia.

The storm is developing along a frontal boundary to the north of Japan, as cold air pushes east from Siberia and encounters very warm conditions over the northwestern Pacific.  It's been notably cold in recent days in eastern Siberia, with the notoriously cold town of Verkhoyansk dropping below -30°C (-22°F) the last three nights.

Here's a look at projected MSLP and precipitation from Sunday afternoon through Monday afternoon, according to the leading ECMWF model.

The expected minimum MSLP of 957mb over far eastern Russia on Sunday afternoon would be a record low for October in that inland location, although sub-950mb MSLP has occurred before over the Gulf of Anadyr (based on ERA5 data):

The NWS discussion says it well: "It doesn`t matter how you slice it, this storm will have some extreme values associated with it from a climatological standpoint, as the upper jet streak of 180 knots, the precipitable water values near record levels for this time of year, and total precip amounts (for this time of year) are pretty much off the charts in terms of anomaly/return intervals."

Snowfall is expected to be "extreme" and/or "epic" (to quote the NWS) for western and south-central Brooks Range locations.

The upper-level forecast charts show the very powerful jet stream that will move across western and northern Alaska:

And the 500mb charts highlight the extreme contrast between warmth and ridging to the south, and the cold trough to the northwest.

Batten down the hatches!  It's going to be a wild few days for western and northern Alaska.

In the meantime, here are a couple of webcam photos from Koyuk today, showing freeze-up on the river as it flows into Norton Bay:

 

Overnight temperatures were well below 0°F last night in the hills to the north of Fairbanks, as well as the Yukon Flats and other northern interior spots.  The Beaver RAWS reached -16°F, and Bettles saw -8°F.  This kind of cold isn't usually seen until late October or early November, although of course it varies; in the past Bettles has been this cold as early as October 5th (1958) or, more recently, October 15th (2008).

Fire Season Stats

Winter is fast descending on Alaska, and freeze-up is under way for the northern interior.  Snow is lying in northern interior communities from the Kobuk River valley across to the Yukon Flats, and the forecast shows a deepening freeze in the next week that will bring ice formation to lakes and rivers.

Looking back at the 2024 fire season, the AK Fire Service has released statistics for 2024 activity.  Total statewide acreage burned was 667,000 acres, which is just a little higher then the 30-year median of 600,000 acres.  Of course, it's far lower than the hyper-active seasons of the past; here's a figure from the AICC report:

The most significant fire activity was heavily concentrated at the end of June and the very beginning of July - really only about a week.  This is somewhat unusual: it normally takes more than two weeks to accumulate the central 50% of the annual acreage.  The most analogous year in recent decades may be 2000, when statewide acreage jumped from 149,000 on June 29 to 569,000 only 8 days later; and the annual total was 756,000.

The fire preparedness level was elevated (level 3 or higher) for 23 days, and 12 days were spent in the two highest levels:

Alaska is a very big place, of course, and Fairbanks-area residents suffered through more smoke than might be expected with near-normal statewide fire acreage.  There were 9 days with smoke and visibility of 2 miles or less, and that's not much less than the big fire seasons of 2019 and 2015.  Last year was pretty smoky too, despite less than 300,000 acres of Alaska fire; it all depends on lightning and fire locations, and local wind patterns.

September Climate Data

Looking back at September climate anomalies in Alaska, I think the most unusual aspect statewide was the excess cloud cover that was produced by low pressure aloft extending east from Russia.  The abundant cloudiness prevented temperatures from dropping off at night and therefore held frost at bay until the very end of the month in some places.

Here's the estimated percentile of September solar radiation compared to the prior 30 years: below normal nearly everywhere, and among the lowest (cloudiest) in the SE interior, the western North Slope, and the central Aleutians.

Here's the mid-atmosphere pressure anomaly pattern:

Overall temperatures (day and night combined) were not a whole lot different from normal in much of the state, but the North Slope was an exception, with very unusual warmth.  It was the second warmest September on record in Utqiaġvik (formerly Barrow).  In contrast, a lot of unusual coolness persists in the Bering Sea, associated with a very strongly negative PDO phase.

Precipitation was below-normal in parts of northwestern Alaska, a welcome change after an extremely wet summer; but the SE interior was wetter than normal, which also reflects a change from summer.

As for wind, September was much calmer - relative to normal - than summer, and was much less windy than normal in the north and east of Alaska.  This change is closely linked to a major reversal in the Arctic pressure pattern: the Arctic Oscillation was strongly positive in August (Arctic low pressure), but it flipped to significantly negative in September (Arctic high pressure).  This phase reversal appears to be somewhat typical of La Niña.