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.