[Tropical] Tropical Cyclone Discussion

[EMWIN Server]

WTNT44 KNHC 311501
TCDAT4

Hurricane Isaias Discussion Number  14
NWS National Hurricane Center Miami FL       AL092020
1100 AM EDT Fri Jul 31 2020

After a brief convective hiatus earlier this morning, a small burst
of deep convection with cloud tops as cold as -80C to -85C have
developed over the previously exposed low-level center. Dry
downslope air coming off of Hispaniola wrapped into the center,
eroding the inner-core convection somewhat. However, a surge of
moist southwesterly inflow moving through the Windward Passage now
appears to be fueling the recent increase in convection. The
upper-level outflow has improved somewhat in the western
semicircle but still remains restricted due to some modest
southwesterly vertical wind shear. The initial intensity has been
lowered to 65 kt based on 700-mb flight-level winds of near 70 kt
and an increase in the central pressure of 992 mb, which is based on
a dropsonde report of 995 mb with 26-kt surface winds.

Isaias is continuing on a steady northwestward motion of 305/14 kt.
This motion is expected to continue for the next 36 hours or so as
the hurricane rounds the southwestern periphery of the
Bermuda-Azores High. A gradual turn to the north-northwest and
north is expected by 48-60 hours due to a break in the ridge
developing in response to a central U.S. shortwave trough digging
into the southeastern United States. The timing and strength of this
trough will determine how far west Isaias moves before the hurricane
turns northward. The 06Z UKMET and and GFS have shifted westward,
closer to the Florida coast, which is similar to the westward shift
seen in the 00Z ECMWF model fields. By 72 hours, the hurricane is
forecast to begin accelerating northeastward, possibly passing over
eastern North Carolina by day 4 and across eastern New England on
day 5. As a result of the westward shift in the latest model
guidance, the new NHC forecast track has also been shifted farther
west closer to the southeastern U.S. coast, and lies a little to the
west of the consensus models TVCA and NOAA-HCCA. The westward shift
in the track forecast has required the issuance of a Hurricane Watch
for portions of the Florida east coast.

Although some slight weakening has occurred, radar data from the
aircraft and the Bahamas radar indicate about a 60-percent
eyewall has formed in the northeastern semicircle, which is an
indication that the cyclone is trying to reorganize. As a result,
strengthening is still expected during the next day or so,
especially tonight and Saturday morning during the convective
maximum period when the hurricane will be moving over the Gulf
Stream where SSTs are 30C or warmer and while the vertical remains
reasonably low. Increasing southwesterly shear could cause a gradual
decrease in intensity over the weekend. The new official intensity
forecast is similar to the previous advisory, and is a little above
the available model guidance out of respect for continuity.

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 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. Preparations to protect life and property should be
rushed to completion.

3. Tropical storm conditions are expected with hurricane conditions
possible along portions of the Florida east coast beginning
Saturday, and a Tropical Storm Warning and Hurricane Watch are in
effect.  Storm surge watches or warnings could be needed for part of
this area this afternoon.

4. There is a risk of impacts from winds, heavy rainfall, and storm
surge beginning late this weekend along the northeastern Florida
coast and spreading northward along the remainder of the U.S. east
coast through early next week.  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  31/1500Z 21.7N  74.5W   65 KT  75 MPH
 12H  01/0000Z 23.0N  76.1W   70 KT  80 MPH
 24H  01/1200Z 24.8N  78.0W   80 KT  90 MPH
 36H  02/0000Z 26.3N  79.2W   80 KT  90 MPH
 48H  02/1200Z 27.7N  79.8W   75 KT  85 MPH
 60H  03/0000Z 29.2N  80.0W   70 KT  80 MPH
 72H  03/1200Z 31.6N  79.5W   65 KT  75 MPH
 96H  04/1200Z 36.4N  76.2W   55 KT  65 MPH
120H  05/1200Z 43.3N  69.0W   55 KT  65 MPH...POST-TROP/EXTRATROP

$$
Forecaster Stewart
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