[Tropical] Tropical Cyclone Discussion

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WTNT44 KNHC 310855
TCDAT4

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.


FORECAST POSITIONS AND MAX WINDS

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
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