The entire Eastern Seaboard is on high alert for the first time during this hyperactive hurricane season as Tropical Storm Isaias bears down on the Caribbean, with increasing potential for a Southeast US landfall. Isaias is expected to approach the Florida peninsula this weekend and could bring tropical impacts from Miami to Cape Cod.
Isaias was drenching Puerto Rico, Hispaniola (the Dominican Republic and Haiti), and the Virgin Islands with wind-whipped rain bands early Thursday afternoon. The system had maximum sustained winds of 60 mph and was producing tropical storm-force winds as far as 310 mi north from its center just southeast of the Dominican Republic coast. The storm was moving west-northwestward at roughly 20 mph toward Hispaniola’s mountainous terrain.
Before Isaias approaches the US coastline it must confront two significant impediments to both its intensity and its track. The mountains of Hispaniola will quite literally be the most colossal hurdle on Isaias’s journey before confronting a region of wind shear – a change in wind speed and direction with height – between the Bahamas and the Turks and Caicos.
The mountains of the Hispaniola will serve to slow Isaias’s modest forward momentum Friday while simultaneously tearing apart its circulation. Land interaction alone is enough to make the track and intensity forecast of Isaias highly uncertain as it barrels toward the US. Doubling down on this uncertainty is the fact that Isaias has always been a disorganized storm. A blossoming region of showers and thunderstorms located over the northeast coast of the Dominican Republic as of Thursday afternoon could foster the development of a new center of circulation.
If Isaias develops a new center north of Hispaniola, it could redevelop faster, follow a further northeast track, and become a more menacing threat for the mainland US as a possible hurricane. If a new circulation does not develop then Isaias would remain a weaker storm as it approaches the Bahamas and Florida, and its impacts would likely be lighter.
Regardless of whether or not Isaias redevelops a new center, it will have to combat a region of hostile wind shear between the Bahamas and the Turks and Caicos Islands Friday. Wind shear is so destructive to tropical cyclones because it can rip apart their circulation.
Upon emerging from the wind shear, Isaias will approach South Florida in some form Saturday. Some rain and wind should therefore be expected along Florida’s East Coast, but more specific impacts are uncertain at this time due to the two earlier described hurdles.
Isaias will be steered to the northeast Saturday night by a trough of low pressure sweeping towards the Eastern US and a weakening ridge of high pressure over the southwest Atlantic. The strength and positioning of these two steering features will determine if and where Isaias makes landfall. The most likely scenario as of Thursday afternoon is for Isaias to track just off Florida’s East Coast, brush North Carolina’s Outer Banks with a direct landfall or near-miss, and then continue off the coast towards the Northeast. Other, less likely scenarios include a track through Florida or a track that stays well off shore and heads out to sea off the Mid-Atlantic coast.
Warm sea surface temperatures (SSTs) on the order of 84-86°F in the Caribbean and off the Southeast Coast will greet Isaias after clearing the wind shear. In conjunction with light upper-atmospheric winds, these will serve as the ideal environment for tropical cyclone intensification. It isn’t just warm in the Southeast either — the 79°F SST benchmark to sustain a tropical system exists as far north as the Jersey Shore.
Given the warm sea-surface temperatures, if Isaias follows a track that just barely avoids landfall, it could spread tropical storm conditions – if not brief hurricane conditions – to coastal areas as far north as New York City, Long Island, and Cape Cod early next week. Alternatively, if a redeveloping Isaias makes landfall over South Florida as a growing Tropical Storm, impacts to the mainland US would be limited outside of a period of heavy, breezy downpours over Florida and nearby states. These are all details that will be hashed out soon, once the storm passes through Hispaniola.
Ultimately, Isaias is expected to impact the mainland US in some form between Saturday and early next week. Heavy rain and wind could impact the Eastern US from Florida to coastal New England during this period. However, it is still too early to more clearly determine Isaias’s future track and impacts.