Halong vs Merbok

Now that ERA5 data is in from last week, we can compare ex-Typhoon Halong with other historical storms on a level playing field, i.e. with the same model reanalysis.  The most obvious comparison to begin is with ex-Typhoon Merbok from September 2022.  First, here are the ERA5 tracks and minimum MSLP of the two storms, at 3-hour intervals:

Merbok tracked much farther west but was much bigger, with more total energy, and it moved much more slowly as it reached the northern Bering Sea, prolonging the period of high wind and flooding for western Alaska.  Here's a comparison of ERA5 wind gusts at a grid point near Hooper Bay, on the far western end of the Y-K Delta, where the peak wind was almost the same in both storms (according to the model).  Notice how quickly the winds dropped off in Halong, versus the prolonged period of high winds during Merbok.

A map comparison of peak wind gust shows that Halong was worse in terms of wind across the Y-K Delta and up to Norton Sound and the western interior; but Merbok was worse for most of the southern Seward Peninsula, including of course Nome.

The small size but ferocious wind of Halong made its impacts much more akin to those of a landfalling hurricane (or typhoon), although both storms were thoroughly extratropical by the time they impacted western mainland Alaska.

Halong was so intense that ERA5 indicates all-time record high wind gusts along a fairly lengthy swath extending northeastward from Nunivak Island.

Here are storm tracks and ERA5 peak wind gusts for a couple of other historic autumn storms in the Bering Sea (but in these cases having no relation to pre-existing typhoons, as far as I know).  First, the 2011 Bering Sea "Superstorm", a really massive storm:

And the infamous storm of 1974, which was catastrophic for Nome:

It is very interesting that all four of these storms occurred during La Niña conditions, and not only that, during second-year or third-year La Niña events (i.e. the second or third consecutive La Niña winter after an El Niño).  More on this "coincidence" another time.

More On Ex-Merbok

Following up on the meteorology of the Bering Sea storm (formerly Typhoon Merbok), Rick Thoman penned some comments that are well worth reading:

https://theconversation.com/typhoon-merbok-fueled-by-unusually-warm-pacific-ocean-pounded-alaskas-vulnerable-coastal-communities-at-a-critical-time-190898

To Rick's point about unusually warm SSTs, the map below demonstrates that the storm did indeed pass over very unusually warm water in the northwestern Pacific as it was undergoing its transition from tropical to extratropical cyclone.  An SST anomaly of +4°C is extremely large, but it's been like this on and off since mid-summer in various parts of the northern and northwestern Pacific.

 

I don't think we can say with absolute certainty without doing modeling experiments, but I agree with Rick that the unusual warmth and moisture were very likely significant factors in the unseasonably extreme intensification of ex-Merbok as it moved into the Bering Sea.  As noted in yesterday's post, however, the mid-latitude jet features clearly also played an important role, with their strength and timing being "just right" to facilitate an extreme outcome.

In terms of impacts, flooding was the big story, and it was devastating for some communities.  This article documents many of the events in sad detail:

https://alaskapublic.org/2022/09/17/powerful-storm-slams-western-alaska/

Some of the most dramatic pictures were before and after shots from FAA webcams, such as these posted by NWS Fairbanks:

Images from the FAA airport webcam in Unalakleet. One image shows a normal day, the other shows water inundating the area around the airport as of 8am this morning. Water levels will continue to rise another foot by this afternoon. #akwx pic.twitter.com/0tVjXy3dLv

— NWS Fairbanks (@NWSFairbanks) September 17, 2022

 

The news from Golovin at first light on Saturday was overwhelming:

Major flooding in Golovin this morning. Water is still expected to rise 1-2 feet by this afternoon Our thoughts are with the community. (photos courtesy C. Lewis) #akwx pic.twitter.com/BO63uN8dGL

— NWS Fairbanks (@NWSFairbanks) September 17, 2022

 

A fact I saw frequently mentioned is that this was the worst storm in nearly 50 years, and that refers back to the benchmark November 1974 storm.  The late Ted Fathauer wrote about that event, but I don't have access to the article (perhaps someone can help):

https://www.tandfonline.com/doi/abs/10.1080/00431672.1975.9931740

More recently, November 2011 saw a tremendous Bering Sea storm, as mentioned on this blog at the time (beginning with the following post and several thereafter):

http://ak-wx.blogspot.com/2011/11/bering-sea-superstorm.html

As for this weekend's flooding, Rick Thoman showed that Nome's peak water level was higher than 2011 but not as bad as 1974:

Observed ocean water levels from the NOAA/National Ocean Service gauges at Nome and Unalakleet across the Bering Sea storm. Exact levels not available but at Unalakleet the Oct 1960 and Nov 1974 storm levels were likely higher than this event. #akwx #superstorm @knomradio pic.twitter.com/fe6fqFdD5p

— Rick Thoman (@AlaskaWx) September 19, 2022

Flooding in Nome on Saturday pic.twitter.com/CgXO2i2Hk9

— KNOM (@knomradio) September 18, 2022

 

Here are a couple of photos I grabbed from the Nome Visitor Center webcam on Saturday, showing an extremely threatening sea, and waves running up beside the building:

Let's hope many, many years pass before a storm like this returns to the long-suffering communities of western Alaska.

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.