Tropical Storm Isaias Forecast Discussion Number 20

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Issued at 1100 PM EDT Sat Aug 01 2020


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WTNT44 KNHC 020256
TCDAT4

Tropical Storm Isaias Discussion Number 20
NWS National Hurricane Center Miami FL AL092020
1100 PM EDT Sat Aug 01 2020

Deep convection has increased this evening in a cluster to the east
and northeast of the aircraft-observed center position. Radar data
suggests a mid-level center or vorticity maximum in that area,
which is between Grand Bahama and Andros Island. Observations from
the Air Force Hurricane Hunters indicate that Isaias has not
restrengthened thus far, but there is still a small window of
opportunity for it to regain hurricane intensity Sunday while
passing over the Gulf Stream waters. Synoptic surveillance data
from the NOAA Gulfstream-IV aircraft indicate that the upper-level
west-southwesterly flow over Isais will begin to increase by late
Sunday. The resulting increase in shear and interaction with land
should cause weakening beyond 24 hours, and the official intensity
forecast is near or above the intensity model consensus.

Assuming that the aforementioned mid-level feature is not the
dominant center of circulation, the aircraft fixes give a motion
estimate of about 310/8 kt. Isais has been moving northwestward
along the southwestern periphery of a mid-level high pressure area.
An approaching trough near the Ohio Valley is expected to cause the
tropical cyclone to turn northward within 48 hours. After that,
Isaias should accelerate north-northeastward to northeastward ahead
of the trough, and move near or over the east coast of the United
States. The official track forecast remains close to both the
simple and corrected dynamic model consensus tracks, TVCN and HCCA
respectively.


Key Messages:

1. Hurricane conditions and dangerous storm surge will continue in
portions of the northwest Bahamas through Sunday morning.

2. Hurricane conditions are expected along portions of the Florida
east coast on Sunday with tropical storm conditions expected to
begin late tonight. Preparations to protect life and property
should be rushed to completion.

3. Dangerous storm surge is possible along the Florida east coast
from Jupiter Inlet to Ponte Vedra Beach where water rises of 2 to 4
feet above ground level are possible along the immediate coastline
and adjacent waterways. Residents there should follow advice given
by local emergency officials.

4. Isaias will produce heavy rains and potentially life-threatening
flash flooding in the Bahamas, and flash and urban flooding,
especially in low-lying and poorly drained areas, along the east
coast of the United States. Minor river flooding and isolated
moderate river flooding is possible across portions of the Carolinas
and mid Atlantic early next week.

5. Tropical storm watches and warnings are in effect for portions of
the United States East Coast from northeast Florida to South
Carolina. Additional watches and warnings will likely be issued
on Sunday as Isaias is expected to move northward near or over the
southeast and mid-Atlantic coasts early next week.


FORECAST POSITIONS AND MAX WINDS

INIT 02/0300Z 25.5N 79.1W 60 KT 70 MPH
12H 02/1200Z 26.6N 79.9W 65 KT 75 MPH
24H 03/0000Z 28.0N 80.5W 65 KT 75 MPH...ON COAST
36H 03/1200Z 29.7N 80.8W 60 KT 70 MPH
48H 04/0000Z 32.0N 80.0W 55 KT 65 MPH
60H 04/1200Z 35.3N 78.1W 50 KT 60 MPH...INLAND
72H 05/0000Z 39.7N 74.8W 40 KT 45 MPH...INLAND
96H 06/0000Z 47.0N 66.0W 35 KT 40 MPH...POST-TROP/INLAND
120H 07/0000Z 51.5N 57.5W 30 KT 35 MPH...POST-TROP/INLAND

$$
Forecaster Pasch

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