Ingredients of fall storms

By Rick Thoman
Alaska Climate Specialist
Alaska Center for Climate Assessment and Preparedness
International Arctic Research Center/University of Alaska
Fairbanks

With the fall sea storm season now well underway, let’s review the key ingredients that go into producing storms that bring the threat of coastal flooding to the southern Seward Peninsula and eastern Norton Sound coasts.
Most importantly, there must be a long (hundreds of miles) fetch of south to southwest winds across the central and eastern Bering Sea. The most common scenario that produces this long fetch is a storm track that runs from the central or western Aleutians northeast into the eastern Gulf of Anadyr or right through the Bering Strait.
Because winds in the northern hemisphere rotate counter-clockwise around low-pressure systems, this puts our region on the south wind side of the storm. Also important is the environment into which the storm moves. As we saw this past week, a precursor storm that raises eastern Bering Sea water levels just prior to a stronger storm can set the stage for more impacts than might have otherwise occurred. It is also important that the storm center moves at least as far north as the Bering Strait if not into the Chukchi Sea. Storms that stall out south of St. Lawrence Island, even storms that are quite deep (have low central pressure), rarely produce significant flooding in the Bering Strait and Norton Sound region because winds are more southeast or even easterly, though significant flooding is possible in these situations from Emmonak southward.
Another important factor is the speed the storm system is moving. Fast-moving storms have less time to push ocean water northward.
A third factor is how the depth (lowest pressure) of the storm is changing. Storms that are weakening as they move north of St. Matthew Island tend to produce less flooding problems than storms that are still deepening as they move north of the Bering Strait.
Of course, the presence or absence of sea ice at any particular location will modulate flooding, and even a little ice can be pushed onto the beach and form something of a berm against rising ocean levels.
Finally, the timing of peak storm surge in relation to the tides can play a critical role in the maximum water levels, especially if it’s a short but sharp peak, as happened at many communities during Merbok.