[Tropical] Tropical Cyclone Discussion

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WTNT44 KNHC 010855
TCDAT4

Hurricane Isaias Discussion Number  17
NWS National Hurricane Center Miami FL       AL092020
500 AM EDT Sat Aug 01 2020

Isaias has a somewhat ragged appearance in satellite imagery this
morning, likely due to the impact of westerly shear and dry air
entrainment.  The area of central convection has shrunk in size,
although radar data from the Bahamas shows some banding near the
center and occasional attempts to wrap up an eyewall.  Also, a dry
slot is now present in the eastern semicircle between the central
convection and the outer banding.  The last Hurricane Hunter
mission indicated that the maximum winds had increased to near 75
kt, so that is maintained for this advisory.  NOAA and Air Force
Reserve Hurricane Hunter aircraft are scheduled to investigate the
storm during the next several hours.

The initial motion is now northwestward or 315/10 kt.  The track
forecast philosophy remains unchanged, as Isais should continue to
move northwestward on the southwest side of a mid-level ridge today
and begin to turn north-northwestward through a weakness in the
ridge on Sunday.  After that, the storm should recurve into the
mid-latitude westerlies, with a turn toward the north-northeast and
northeast with an increase in forward speed likely.  The track
guidance envelope has shifted a little to the east near the Florida
coast and calls for a slower forward motion that the previous
guidance.  Thus, the new forecast track has been adjusted a little
east of, and slower than, the previous track.

The hurricane is currently undergoing about 25 kt of westerly
vertical shear, and some mid-level dry air is present west of the
center.  This combination should prevent any more intensification,
and, while Isaias is expected to remain a hurricane as it passes
near the Florida coast, at least slight weakening should occur
during this time.  Current indications from the global models are
that the storm will continue to experience strong shear as it
recurves, and thus the intensity forecast follows the previous
forecast in calling for weakening during this time.


Key Messages:

1. Hurricane conditions and dangerous storm surge are expected in
portions of the Bahamas through Saturday, and Hurricane Warnings
are in effect.

2. Hurricane conditions are expected along portions of the Florida
east coast late Saturday and Saturday night, and a Hurricane Warning
is in effect.  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 in these areas should follow
advice given by local emergency officials.

4. Isaias will produce heavy rains and potentially life-threatening
flash and urban flooding, especially in low-lying and poorly drained
areas, across South to east-Central Florida, and across the
Carolinas to the mid Atlantic. Minor river flooding is possible
across portions of the Carolinas and into Virginia early next week.

5. There is a risk of impacts from winds, heavy rainfall, and storm
surge spreading along much of the the U.S. east coast through early
next week, and interests there should monitor the progress of Isaias
and updates to the forecast.


FORECAST POSITIONS AND MAX WINDS

INIT  01/0900Z 23.9N  77.1W   75 KT  85 MPH
 12H  01/1800Z 25.0N  78.2W   75 KT  85 MPH
 24H  02/0600Z 26.3N  79.2W   70 KT  80 MPH
 36H  02/1800Z 27.6N  79.9W   70 KT  80 MPH
 48H  03/0600Z 29.1N  80.3W   65 KT  75 MPH
 60H  03/1800Z 31.2N  79.8W   60 KT  70 MPH
 72H  04/0600Z 34.0N  77.8W   60 KT  70 MPH
 96H  05/0600Z 41.5N  71.4W   50 KT  60 MPH...INLAND
120H  06/0600Z 47.5N  61.5W   40 KT  45 MPH...POST-TROP/EXTRATROP

$$
Forecaster Beven
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