Tropical Atlantic Update: Isaías finally forms near Puerto Rico

30 July 2020

Isaías finally forms near Puerto Rico

A week after departing the African coast, the tropical wave we've been patiently watching was finally upgraded to Tropical Storm Isaías on Wednesday night. This is the season's ninth named storm and the earliest ninth named storm on record -- the old record was set on August 7th, 2005 by Irene. So not only did it break the record by over a week, the *average* date of the ninth named storm formation (over the past fifty years) is September 26th, nearly two months from now! So, 2020 has now bumped 2005 off the record list for the earliest E, F, G, H, and I storms.

As of Thursday morning, Isaías is nearly on the shores of Hispaniola and has 60 mph peak sustained winds. Some strong rainbands have been impacting Puerto Rico resulting in flash flooding and power outages. The wind field is very large for a new tropical storm, but also very lopsided. It will undoubtedly weaken during the day as the center passes over the large mountainous island. Then it is expected to emerge over the southern Bahamas on Friday morning and have the opportunity to start re-organizing.

Tropical storm warnings cover the Virgin Islands, Puerto Rico, Dominican Republic, Haiti, and the southern 2/3 of the Bahamas... the northern 1/3 of the Bahamas is under a tropical storm watch. A chunk of southeast Florida will likely be added to that watch list shortly. Long radar loops from Puerto Rico and the Bahamas are available at http://bmcnoldy.rsmas.miami.edu/tropics/radar/

Models are gradually coming together in showing a track that recurves toward the north over the Bahamas and scrapes the east coast of the Florida peninsula. Like Matthew four years ago, just a little wiggle makes a difference in what is experienced on land. But unlike Matthew, this isn't a Category 4 hurricane! Based on the current NHC intensity and track forecast, which are admittedly highly uncertain because of the upcoming interaction with Hispaniola, here are the most likely arrival times and probabilities of tropical storm force winds along the track.

The intensity guidance from the dynamical models has been fairly consistent with showing this becoming a strong tropical storm or Category 1 hurricane, but it's a tough call not knowing what condition it will be in by Friday morning.

The ocean temperature along the forecast track is very warm (>29°C, 0.5-1.5°C above average), including the energy-rich Gulf Stream that follows the shape of the southeast US coastline. However, a limiting factor to keep Isaías in check is vertical wind shear which is expected to ramp up beginning this weekend as a trough approaches from the west.

Since the forecast is more uncertain than usual, we need to pay extra close attention to this storm for any changes and to be prepared if it comes your way. You can always find the latest at https://www.nhc.noaa.gov/graphics_at4.shtml?start#contents

Turning our eyes eastward, there's a weak tropical wave near 40°W that has a slight chance of developing in the coming days, but should also remain over the ocean... it's something to be aware of anyway. The next name on this year's list is Josephine.