[Tropical] Tropical Cyclone Discussion

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WTNT44 KNHC 310235
TCDAT4

Tropical Storm Isaias Discussion Number  11
NWS National Hurricane Center Miami FL       AL092020
1100 PM EDT Thu Jul 30 2020

Isaias is beginning to form a small Central Dense Overcast, but
much of the deep convection is situated over the eastern portion of
the circulation.  Water vapor imagery suggests that upper-level
outflow is modest to the northwest of the cyclone.  The southern
portion of the system is still interacting with Hispaniola at this
time.  Dvorak intensity estimates are 45 kt from SAB and 55 kt from
TAFB so the current maximum wind speed is 50 kt for this advisory.
An Air Force Hurricane Hunter aircraft is headed for Isaias and
should soon provide a more precise intensity estimate.

Although southwesterly shear may limit strengthening of the system
during the next couple of days, the environment should still be
conducive enough for Isaias to become a hurricane in 24 to 36 hours.
The official intensity forecast is similar to the previous one and
a little above the model consensus.

The initial motion is a somewhat uncertain 305/16 kt.  There are no
basic changes to the track forecast or reasoning from the previous
advisory.  Isaias should continue to move generally northwestward on
the southern and southwestern side of a subtropical high pressure
area for the next day or so.  Then, as an approaching
mid-tropospheric trough erodes the high, a gradual turn toward the
north-northwest and north should occur.  In 3 to 5 days, the trough
should cause Isaias to turn toward the north-northeast and northeast
and accelerate.  The official track forecast is close to the
previous one and follows the latest simple and corrected dynamical
model consensus tracks.


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.

2. Hurricane or tropical storm conditions and dangerous storm surge
are expected in portions of the northwestern and central Bahamas
late Friday and Saturday, and Hurricane and Tropical Storm
Warnings, respectively, are in effect for these areas. Preparations
to protect life and property should be rushed to completion.
Tropical storm conditions will continue to spread across portions
of the Dominican Republic, Haiti, the Turks and Caicos and the
southeastern Bahamas tonight through Friday, and Tropical Storm
Warnings are in effect for these areas.

3. Tropical storm conditions are possible along portions of the
Florida east coast beginning Saturday, and a Tropical Storm Watch
is in effect.  While storm surge watches are not currently needed
for this area, they may be required on Friday 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 Friday night, 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  31/0300Z 20.5N  71.9W   50 KT  60 MPH
 12H  31/1200Z 21.6N  73.9W   55 KT  65 MPH
 24H  01/0000Z 23.5N  76.1W   60 KT  70 MPH
 36H  01/1200Z 25.2N  77.8W   65 KT  75 MPH
 48H  02/0000Z 26.7N  79.0W   65 KT  75 MPH
 60H  02/1200Z 28.3N  79.6W   65 KT  75 MPH
 72H  03/0000Z 30.0N  79.5W   65 KT  75 MPH
 96H  04/0000Z 34.6N  76.8W   65 KT  75 MPH
120H  05/0000Z 42.0N  69.0W   60 KT  70 MPH

$$
Forecaster Pasch
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