01 September 2019

Category 5 Dorian lashing Bahamas, southeast US in for prolonged impacts

On Sunday morning, Dorian was upgraded to a rare Category 5 hurricane with 160 mph sustained winds. Unfortunately, this is also marked by its encounter with the northern Bahamas: Great Abaco and Grand Bahama. To make matters worse, it is moving due west at just 8 mph.

This marks the fourth consecutive year with a Category 5 hurricane in the Atlantic, which has never happened before during the reliable record (Matthew '16, Irma '17, Maria '17, Michael '18, Dorian '19).  Only in 2004-2007 were there more Category 5s in such a short span; there were seven during those four years.

The track forecast continues to indicate a nail-biting stall near the Florida coast on Monday into Tuesday... and "near" can mean just offshore, or making landfall. The subtropical ridge everyone has been talking about is the key player, and the longer that remains the dominant steering mechanism, the longer Dorian moves west. The sooner that begins to weaken/erode, the sooner Dorian will begin a northward turn. The feature that will help weaken/erode that ridge is a mid-latitude trough moving through the eastern US, and the timing and strength of that trough is really just as important as the ridge itself. But storms in weak steering environments are notoriously challenging; subtle nudges are all it takes to affect the outcome.

Mid-level steering features from the 20-member GFS ensemble. (College of DuPage)
For now, the northwest Bahamas are under a hurricane warning, and the Florida east coast has a tropical storm warning from Sebastian Inlet to Deerfield Beach and a tropical storm watch from Deerfield Beach to Golden Beach. Do not focus on the exact forecast positions, and do not focus on the boundary of the cone. Nothing magic happens at any of those places. The cone simply shows the extent of the track forecast with 2/3 probability -- it's designed such that 1/3 of storms historically track outside the storm.  If it were drawn using 60% probability rather than 67%, it would be smaller. If it were drawn using 75% probability, it would be bigger.
People on Florida's east coast have been preparing for Dorian for several days already, and the slowing forward motion could still have it sitting nearby on Tuesday (notice the times on that map above).  Even as far south as Miami, the likelihood of a direct impact is quite low, and tropical storm force winds might not ever reach that far south, but the risk of even a low-probability event is worth preparing for. Speaking for Miamians, it's too soon to relax. If you get excited about the insanely-low probability of winning the lottery, you should be extremely concerned about a much more likely destructive event. And as we saw with Irma in 2017, Miami experienced strong tropical storm conditions and it still created quite a mess.

Later in the week, Dorian is forecast to accelerate and turn toward the north then northeast, delivering significant impacts the entire southeast U.S. coast. Storm surge will be the biggest problem, as the storm pushes water onshore over many hundreds of miles.  As of now, the bulk of the heavy rain is forecast to remain offshore, but the closer the storm tracks to the coast, the more of that rain will fall on land.

Some hurricane history: Of the 15 major hurricanes that hit Florida's east coast (north of Marathon), three of them lost significant latitude over or near the Bahamas. It's that history that keeps us on our toes in southeast Florida: unlikely but not impossible.

The tropical wave near Cabo Verde continues to improve in organization, and NHC is now giving it a 70% chance of becoming at least a tropical depression in the next five days.  The next name on deck is Fernand.

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