Official Discussion issued by the National Hurricane Center

Isaias (AL092020) DATA RELEASED: 7/30/2020 9:00:00 AM UTC

Copy of official data

Tropical Storm Isaias Discussion Number   8
NWS National Hurricane Center Miami FL       AL092020
500 AM AST Thu Jul 30 2020

Isaias is sending some mixed signals tonight.   The 1-min rapid
scan data from GOES-16 indicates that the low-level center is likely
displaced west of a very intense burst of deep convection on the 
northeastern side of the circulation.  However, the satellite data 
also shows increased banding features overnight and a more 
organized central cloud pattern, with recent hints that perhaps a 
low-level center is trying to re-form closer to the convection.  
Radar observations from San Juan show 60-65 kt Doppler wind 
velocities during the past few hours near 5000 ft, so the initial 
wind speed is raised to 50 kt.

Model forecasts are showing a complex evolution of the tropical 
cyclone during the next day or two.  There is good agreement that 
Isaias will move across Hispaniola later today, and its low-level 
center will likely become disorganized over the high terrain. 
However, the strong burst of convection currently near Puerto Rico 
is associated with a mid-level circulation, which should pass 
along the north coast of Hispaniola later today. Most of the model 
guidance suggest that this feature will cause the re-development of 
a surface center over the northern part of the broader system while 
the mid-level circulation moves close to the southeastern Bahamas. 
Afterward, the cyclone would then move northwestward until the 
weekend, and gradually turn northward and northeastward close to the 
U.S. East Coast into early next week ahead of a mid-latitude trough. 
The official track forecast is a little to the east of the previous 
one and close to the NOAA corrected dynamical model consensus.  It 
should be noted that further adjustments to the forecast tracks are 
indeed possible, especially after Isaias moves north of Hispaniola.

The intensity forecast is quite tricky.  In the short term, Isaias 
is expected to move across Hispaniola, as the storm's interaction 
with the mountainous island should cause some weakening and 
disruption to the circulation.  However, as mentioned before, the 
models suggest that a new center could form, and the 
environmental conditions would support gradual intensification.  
The intensity models have been trending higher, and the official 
forecast is nudged upward accordingly, now showing a peak 
intensity of 60 kt when the storm is near the coast of Florida and 
the Southeast U.S. Coast.  It should be noted that there are models 
that show hurricane strength near the U.S. but, given the large 
amount of uncertainty, it is preferred to stay on the 
conservative side for now.  We should have a better idea of how 
strong Isaias will become near the U.S. after reconnaissance 
aircraft sample the storm and after it passes Hispaniola later 

Key Messages:

1. Isaias will produce heavy rains and potentially life-threatening 
flash flooding and mudslides across the Virgin Islands, Puerto Rico, 
the Dominican Republic, northern Haiti, and over the Bahamas.

2. Tropical storm conditions are likely across portions of the
the Virgin Islands, and Puerto Rico through this morning and will 
spread westward to portions of the Dominican Republic, Haiti, and 
the Turks and Caicos and the Bahamas later today and Friday.  
Tropical Storm Warnings are in effect for these areas. Do not focus 
on the details of the track forecast, as rainfall and wind hazards 
will extend far from the center of the system.

3. While this system could bring some rainfall and wind impacts to
portions of Cuba and Florida later this week and this weekend, it 
is too soon to determine the location or magnitude of those impacts. 
Interests there should monitor the progress of this system and 
updates to the forecast over the next couple of days.


INIT  30/0900Z 17.2N  67.9W   50 KT  60 MPH
12H  30/1800Z 18.8N  70.2W   45 KT  50 MPH...INLAND
24H  31/0600Z 20.6N  72.9W   45 KT  50 MPH...OVER WATER
36H  31/1800Z 22.3N  75.5W   50 KT  60 MPH
48H  01/0600Z 24.0N  77.9W   55 KT  65 MPH
60H  01/1800Z 25.5N  79.2W   60 KT  70 MPH
72H  02/0600Z 27.3N  80.0W   60 KT  70 MPH
96H  03/0600Z 31.0N  80.0W   60 KT  70 MPH
120H  04/0600Z 37.0N  74.5W   60 KT  70 MPH

Forecaster Blake/Cangialosi