Official Discussion issued by the National Hurricane Center

Isaias (AL092020) DATA RELEASED: 7/31/2020 9:00:00 AM UTC

Copy of official data

Hurricane Isaias Discussion Number  13
NWS National Hurricane Center Miami FL       AL092020
500 AM EDT Fri Jul 31 2020

Satellite images indicate that Isaias is maintaining a small
central dense overcast, with a new burst of convection recently
forming near the center. Earlier aircraft data indicated that the
pressure had fallen to 990 mb, although there was little from the
aircraft that supported more than the previous advisory's intensity
of 70 kt.

The hurricane has been moving between west-northwest and northwest
during the last several hours, estimated at 305/15 now.  Isaias
should gradually turn to the north-northwest and north over the
weekend as it moves around the western periphery of a weakening
western Atlantic subtropical ridge.  Guidance is in fairly good
agreement over the first couple of days of the forecast, so little 
change is made at that time, other than a small westward adjustment 
near the Bahamas.  After that time, there is a considerable spread, 
growing to over 700 miles, in the reliable models on how quickly 
Isaias moves northeastward near the east coast of the United States. 
The large spread appears to be related to both how strong the 
cyclone is by early next week and how much mid-level ridging 
remains near the Carolinas.  There are no obvious reasons to choose 
either the fast GFS or slow ECMWF right now, so the official 
forecast will remain close to the previous one and the model 
consensus.  It should be noted that given this large spread, the 
extended forecast could be subject to large speed/timing changes if 
either of those solutions become more likely.

Strengthening is expected during the next day or so while the 
hurricane remains over the very warm waters near the Bahamas with 
reasonably low vertical shear.  The forecast for the first 24 hours 
has been raised from earlier, at the top of the guidance spread.  
Increasingly southwesterly flow aloft should cause an increase in 
shear over the weekend, which is forecast by most of the guidance 
to lead to a gradual decrease in intensity.  The details are not 
very clear at all, especially considering the track uncertainty, 
but there is a notable chance of a hurricane moving close to the 
U.S. East coast, so the forecast continues to show that scenario.

Key Messages:

1.  Isaias will produce heavy rains and potentially life-threatening 
flash flooding and mudslides across the Dominican Republic, northern 
Haiti, Turks and Caicos, and the Bahamas. Heavy rains associated 
with Isaias may begin to affect South and east-Central Florida 
beginning late Friday night, and the eastern Carolinas by early next 
week, potentially resulting in isolated flash and urban flooding, 
especially in low-lying and poorly drained areas. Isolated minor 
river flooding is possible in the Carolinas early next week.

2. Hurricane conditions and dangerous storm surge are expected in 
portions of the Bahamas today and Saturday, and Hurricane Warnings 
are in effect for these areas. Preparations to protect life and 
property should be rushed to completion. 

3. Tropical storm conditions are possible along portions of the
Florida east coast beginning Saturday, and a Tropical Storm Watch
remains in effect.  While storm surge watches are not currently 
needed for this area, they may be required later today if the 
forecast track shifts closer to the coast.  Heavy rains associated 
with Isaias may begin to affect South Florida and east-Central 
Florida beginning late tonight, potentially resulting in isolated 
flash and urban flooding, especially in low-lying and poorly 
drained areas.

4. There is a risk of impacts from winds, heavy rainfall, and storm
surge late this weekend from the northeastern Florida coast and
spreading northward along the remainder of the U.S. east coast
through early next week.  The details of the track and intensity
forecast remain uncertain, and it is too soon to determine the
magnitude and location of these potential impacts, but interests
along the entire U.S. east coast should monitor the progress of
Isaias and updates to the forecast.


INIT  31/0900Z 20.9N  73.3W   70 KT  80 MPH
12H  31/1800Z 22.3N  75.0W   80 KT  90 MPH
24H  01/0600Z 24.0N  77.0W   85 KT 100 MPH
36H  01/1800Z 25.7N  78.5W   80 KT  90 MPH
48H  02/0600Z 27.3N  79.4W   75 KT  85 MPH
60H  02/1800Z 28.8N  79.6W   75 KT  85 MPH
72H  03/0600Z 30.6N  79.3W   70 KT  80 MPH
96H  04/0600Z 36.5N  75.5W   65 KT  75 MPH
120H  05/0600Z 42.5N  69.0W   55 KT  65 MPH

Forecaster Blake