Hurricane Dorian is the strongest hurricane to hit the northwestern Bahamas in modern records. But why didn’t it continue its westerly course to strike Florida? And why did the storm sit over the Bahamas for many hours on end, unleashing catastrophic damage? Here’s why (full details below):
-Bermuda High steers Dorian west
-Bermuda High weakens so no more winds are available to steer Dorian; Hurricane sits over Bahamas
-Trough finally arrives from the US mainland, starting to pull the system north, avoiding a landfall in Florida
BERMUDA HIGH FORCES DORIAN WEST
Hurricanes are massive and complex systems that can only be moved by steering upper-level winds. Many times Atlantic hurricanes and tropical storms are steered by the Bermuda High. The westerlies (south of highest pressure) generally help storms cruise west towards North America. The Bermuda High directed Hurricane Dorian west and northwest for several days. That is when the forecast cone aimed the system towards the Space Coast of Florida.
The forecast became tricky when the Bermuda High showed signs of weakening and dissipating. That means there was nothing to push Dorian farther west. It also opened up an opportunity for a trough, an area of upper-level low pressure, to arrive just in time for a northerly push.
For several hours over the weekend of Sept 1-2, 2019, Hurricane Dorian sat over the Abacos Islands and the Grand Bahama Island, teetering between 1 MPH speeds and no movement at all. With winds sustained over 180 MPH and gusts near 220 MPH, the islands were utterly destroyed by relentless winds, heavy rain, and storm surge. In the meantime, millions of Americans waited to see if and when the storm would inch closer to Florida.
Because of a weakened Bermuda High and no westerly winds to push the storm closer to Florida, an opening came for a weak trough to sweep across the Midwest and southeastern USA. (This trough was always on its journey towards the Atlantic, but if the Bermuda High’s west wind stayed strong, it would have been too late to do anything and Dorian could have likely ended up crossing Florida and into Gulf of Mexico.)
Hurricane Dorian, while being a CAT 5 storm, sat over the Bahamas long enough for the trough to arrive, helping to pull the system north.
TROUGH TURNS DORIAN NORTH
Light southwesterly winds from the trough are slowly moving Dorian north along the Florida coast. While it will not make landfall in Florida, gusty winds and downpours are likely to lash the peninsula for a couple more days. Once Hurricane Dorian moves a bit closer to the trough where the steering winds are stronger, it will accelerate in its northward progression.
NEW PROJECTED PATH FOR DORIAN
Overtime the tropical system will speed up towards the north and east thanks to the trough. It may never actually make landfall in the USA but its effects will be felt along the entire eastern seaboard, including the Northeast in cities like New York and Boston where the winds will pick up and extratropical showers move inland.