02 August 2020

Isaías weakens along Florida coast, hazards continue up US east coast

During the day on Saturday, Isaías got completely overwhelmed by a combination of dry air and wind shear... suffocation and decapitation. While that doesn't immediately "kill" a tropical cyclone, it weakens it in a hurry. It was downgraded to a tropical storm on Saturday evening, and remains a tropical storm with 65 mph peak sustained winds on Sunday morning, though those winds are well offshore. It is not expected to regain hurricane intensity (but briefly could because it's so close to the 74 mph cutoff between a tropical storm and a hurricane).

Since passing over the warm Gulf Stream ocean current, vigorous thunderstorms have re-erupted and persisted over the center, breathing new life into the struggling storm. In the enhanced satellite image at the beginning of the post (from Sunday morning), the mid-level dry air is shown by the oranges to the west of the storm, and the cold cloud tops associated with the strong thunderstorms near the center are the bright blues/greens/reds.

There have been minimal impacts on the east coast of Florida because the thunderstorm activity and stronger winds have been confined to very close to the center or east of the center.  However, the northern Bahamas have received very heavy rain and fairly significant storm surge. A 1-day radar loop from Miami shows the (lack of) structure as it made its closest approach before heading north.  This and other long radar loops can be found at http://bmcnoldy.rsmas.miami.edu/tropics/radar/

Tropical Storm Isaías will continue to bring periods of rain and minor coastal flooding to the eastern Florida coast from West Palm Beach northward.  A 2-4-foot storm surge is possible up to Cape Fear, NC as it tracks along the coast over the next couple days. The greatest hazard will be heavy rain and flash flooding in the Carolinas, the mid-Atlantic states, and then the northeast U.S. through Wednesday.

The map below shows the full path that Isaías took, beginning when it left the African coast back on July 23rd. It battled dry air the entire time, but was able to briefly reach Category 1 hurricane status immediately after leaving Hispaniola.

Tropical Depression 10, which was out near Cabo Verde, has already dissipated and never became a tropical storm. The tropical wave near the northern Lesser Antilles, identified as Invest 94L, still has a good chance of forming this week, but model guidance is in agreement on it turning north into the open Atlantic.  The next name on this list is Josephine, and the record earliest formation of the "J" storm is August 22nd.

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