West Coast Storms

Northwestern Alaska was heavily affected by a strong storm earlier this week, bringing a sharp reminder of the severe flooding that occurred in Kotzebue last October:

https://ak-wx.blogspot.com/2024/10/kotzebue-flood.html

https://ak-wx.blogspot.com/2024/10/flood-follow-up.html

The tide gauge at Kotzebue reached well into major flood stage, and according to Rick Thoman the peak water level matched that of last October, when damage was severe.  Evacuations were carried out in Kotzebue and Kivalina this time, and the preparations seem to have prevented a repeat of last year's major impacts.

https://www.adn.com/alaska-news/rural-alaska/2025/10/09/incoming-coastal-storm-looms-as-northwest-alaska-evacuees-return-home-amid-receding-floodwaters/

The center of the storm tracked just to the west of the Bering Strait with minimum MSLP of about 970 mb, which is very low but not record-breaking.  Here's a sequence of maps showing estimated MSLP and 10m wind speed at 6-hourly intervals from early Tuesday morning through late Wednesday; the strongest winds developed in Kotzebue Sound on Wednesday after the storm center passed to the north and the wind direction went around to the southwest.

Unfortunately a second and potentially more serious threat has now emerged with a second storm that is originating from the remnants of Typhoon Halong, a tropical cyclone that passed to the south and east of Japan in the last couple of days.  Weather models show ex-Halong becoming powerfully re-energized as it gets caught up in the North Pacific jet stream tomorrow and into Sunday.  The forecasts for Sunday morning are frankly quite alarming, with minimum MSLP near 950 mb as the storm passes near the eastern end of St Lawrence Island:

If these MSLP forecasts are correct, ex-Halong may well break 1950-present records for lowest MSLP along its track in the northern Bering Sea; ERA5 data shows records of about 956 mb from St Lawrence Island to Nome.

Needless to say, impacts up and down the west coast will be severe even if the forecast details are a little off.  The latest NWS forecast shows water levels as high as Wednesday's event in Kotzebue, and higher in Nome.

The climate context for these storms will be worthy of comment in a future post.  For now, it's interesting to note that last year's storm reminded Rick Thoman of west coast storms in 1973 and 1974; and now we again have back-to-back autumns with similar severe storms.  It's likely no coincidence that the winters of 1973-74 and 1974-75 were back-to-back La Niña winters following a strong El Niño in 1972-73; and we are in the same situation today (El Niño in 2023-24, La Niña last winter, and another one on tap for this winter).

Strong Bering Sea Storm

Rick Thoman pointed out that an unusually strong storm made its way across the northwestern Bering Sea earlier this week.  Here's the surface analysis from 4pm AKDT Wednesday, courtesy of Environment Canada, with a red circle indicating the location of St Lawrence Island.  Click to enlarge.

The storm's minimum MSLP was estimated to be 974mb as it passed west of St Lawrence Island, and it "made landfall" in eastern Chukotka at 977mb.  This is unusually low for the time of year - in fact, it's the lowest MSLP on record (1950-present) anywhere in the northern Bering Sea for this late in the season.

To illustrate this, the map below shows the 1950-2023 minimum MSLP for May 11th through the 31st, based on ERA5 data.  The record low values from the northern Gulf of Anadyr across to the northeastern Bering Sea and Norton Sound are 980mb or above; so this was a record strong storm for this late in the season (the June record values are higher). 

Are strong storms becoming more common at this time of year in the Bering Sea?  Not at all.  Interestingly, the trend is the opposite: here's the annual series of May minimum MSLP in a box that approximates the northern Bering Sea.  Note that this is for all of May: there have been stronger storms than this week's event in the first week of the month.

The trend is statistically significant at a fairly high level (R=+0.28, 74 years), and it makes this week's event all the more notable.  But admittedly there is some uncertainty in the reanalysis data, especially for the earliest years (pre-1958, in particular).

Looking at trends in monthly minima through the year, spring stands out as a time with positive trends, i.e. less extreme storms over time, whereas February shows a trend for increasingly strong storms.

The time series for February again has a significant trend, and indeed the change around 1990 does look real.  I don't have an explanation for these trends.

Follow-Up

A quick follow-up post on last week's big pressure gradient and associated wind: with ERA5 data now available, the peak MSLP difference between two Alaska land points was 95mb, i.e. not an all-time record, but certainly in the upper tier of these events.

However, I redid the historical calculation for land points from 170°W eastward, i.e. excluding the central and western Aleutians.  The rationale is that last week's Aleutian cyclone was farther east than others that produced extreme pressure differences across the state, and I wanted to see if it might be an all-time record for the more restricted domain.

And the answer is - yes!  The pressure difference of 95mb within Alaska east of 170°W is indeed the greatest back to 1950, based on hourly ERA5 data.  Only two previous events exceeded 92mb across this domain: November 26, 1985, and December 5, 2009.

Here's a revised chart of the annual (cold season) maxima for the smaller domain:

Interestingly there seems to be a slight downward trend in the series, although it's not statistically significant.  Another interesting aspect is the tendency for large positive outliers once every 10-20 years.  Last week's set-up was a rare event.

Most regrettably, the high winds produced by this system led to treacherous conditions high on the Steese Highway last Wednesday, leading to the deaths of two motorists:

https://www.adn.com/alaska-news/2024/01/05/recovery-of-2-found-dead-in-interior-alaska-vehicle-complicated-by-poor-weather-troopers-say/

Conditions were indeed very poor: here's the hourly data from the Eagle Summit SNOTEL site (click to enlarge).  Columns are temperature and wind chill in °F, wind direction, wind speed in mph, and solar radiation (absolute and percent of possible).

I don't have historical data for comparison, and Eagle Summit is notoriously cold and windy, but it's clearly not the kind of weather you want to encounter unprepared.

Big Pressure Gradient

There's been a very large pressure gradient across Alaska in the past couple of days, reflecting the contrast between Arctic high pressure to the northeast of Alaska and a very strong storm in the eastern Aleutians.  As of yesterday morning, the difference in central pressure between these two features was 100mb, or about 10%.

Strong winds are the immediate result of big pressure differences, and so it's been very breezy in many places across Alaska; blizzard conditions have been occurring for the eastern Arctic coast.

The 100mb pressure difference led me to wonder about the largest pressure difference ever measured across the state of Alaska.  In the current event, the maximum pressure difference between two land points appears to have been around 90-95mb, judging from model MSLP maps - for example this one from Tuesday night (a range of 944mb to 1036mb).

Using hourly ERA5 reanalysis data back to 1950, I determined that the largest MSLP difference was 103mb, on February 4, 1989.  This was a case with strong high pressure over the interior and southern Alaska (at the tail end of the notorious January 1989 cold spell, with record high MSLP), and only a modest cyclone over the Aleutians.

In contrast, just three years ago the MSLP difference across Alaska exceeded 100mb for only the second time since 1950, with an exceptionally strong western Aleutian cyclone being the culprit rather than a very strong anticyclone over the interior.  This event also saw the lowest MSLP on record at an Alaskan land location, 923mb (according to ERA5 data since 1950).

The time series of annual (winter) peak pressure differences shows no significant long-term trend.  The peak pressure differences of each cold season have occurred in all months from November through April, with December being most common.

There's also no significant overall relationship with El Niño/La Niña, although it's slightly intriguing that the strongest El Niño winters have tended not to produce a high pressure difference.  The current event is therefore a bit unusual in this regard, because we certainly have a strong El Niño in play at the moment.

I'll provide an update with the ERA5 peak MSLP difference for the current event when the data is released in a few days' time.

Record Bering Sea Storm

The extreme Bering Sea storm of the past three days has now decayed to a weaker low pressure system near Point Lay, and west coast communities are beginning to clean up where they can.  Storm surge flooding was devastating in some villages and caused major damage in Nome.  Without question, this was one of the most intense and damaging storms in living memory for the coast from Hooper Bay up to Nome, and recovery will take a long time.

From a meteorological standpoint, the situation was remarkable and extreme.  The ECMWF model analysis indicates that the storm's central pressure was easily the lowest on record for September in the northern Bering Sea (data since 1950); but more remarkably it appears to have been the lowest for any time of year along part of its track leading up to St Lawrence Island.  The ASOS instrument at Savoonga reported a minimum MSLP of 953.2mb, which matches the ECMWF analysis very well.  Rather remarkably, the storm's center appears to have passed over the island, although extratropical cyclones don't have the calm, well-defined center that tropical storms do.  ERA5 has a 1950-2021 record low MSLP of about 956mb at Savoonga.

Here are maps of the ERA5 September record low MSLP and all-time record low MSLP, with the ECMWF 6-hourly minima annotated:

 

One word of caution here: the current ECMWF model isn't the same version as the ERA5 reanalysis model, so we'll need to wait a few days to confirm the records in the self-consistent ERA5 system.

Looking back at the formation of the storm, the main culprit was certainly not the size or strength of Typhoon Merbok.  It was only marginally a Category 1 storm in the tropics, with maximum sustained winds of 80mph and minimum pressure of 965mb.  This is almost a non-event as far as West Pacific typhoons are concerned.

The reason that the storm transitioned into such an intense extratropical cyclone was the fortuitous ("unlucky") interaction of not one but two upper-level disturbances with the decaying, northward-moving tropical storm.  Both upper-level disturbances originated in the mid-latitude jet stream over southern Russia, which was unusually strong because of a sharp gradient between a strong trough over the Russian Arctic and a strong ridge from Mongolia to the Sea of Okhotsk.

The first map below (courtesy of Tropical Tidbits) shows the situation a full week ago, with the three key features highlighted: the one to the left is Merbok, and the middle one is a subtle feature that intensified significantly as it moved off the coast of Russia.  Here's the progression at 24 hour intervals:

 

 

The first mid-latitude disturbance set up the initial round of extratropical transition and re-intensification for decaying Merbok, but this was quickly followed by another interaction with the third (rightmost) feature when the storm neared the Aleutians.  The one-two punch of mid-latitude jet stream "energy" is what took ex-Merbok to its extreme intensity over the central Bering Sea.  Here's the continued progression at 24 hour intervals up to Saturday afternoon.

More to come on the impacts of ex-Merbok in a subsequent post.