[Tropical] Tropical Cyclone Discussion

[EMWIN Server]

WTNT44 KNHC 312050
TCDAT4

Hurricane Isaias Discussion Number  15
NWS National Hurricane Center Miami FL       AL092020
500 PM EDT Fri Jul 31 2020

Deep convection, with occasional overshooting cloud tops of -85C to
-90C just north of the center, has continued to develop during the
normal diurnal convective minimum period, which is quite impressive.
The most recent Air Force Reserve recon flight-level wind data,
along with ASCAT surface wind data, indicate that the inner-core and
outer wind field have both contracted in size. Furthermore, radar
data from the Bahamas and an 1810Z AMSR-2 microwave pass also
indicate that a small 10-nmi-wide mid-level eye is forming. The last
recon central pressure was 991 mb and the 700 mb height had
decreased by 30 meters since the earlier maximum height around
1230Z. These data indicate that Isaias is getting better organized.
The initial intensity remains 65 kt based on an earlier 700-mb
flight-level wind speed of 72 kt, which reduces to a 65-kt surface
wind speed using a 90-percent adjustment factor.

The initial motion remains northwestward or 305/12 kt. The 12Z
global models have once again made a westward shift due to the ridge
to the north of Isaias not weakening as quickly as expected. This is
partly due to the ridge being stronger than expected and a shortwave
trough over the central United States moving a little slower into
the southeastern U.S. than previously indicated. The UKMET and ECMWF
explicitly show Isaias making landfall in 36-48 hours along the
southeast Florida coast, but appear to weaken the system below
hurricane strength. The GFS similarly brings the cyclone close to
the southeast and east-central Florida coasts, but also as a
somewhat weaker system. In the 48 to 60-hour period, the cyclone is
forecast to move slowly north-northwestward and northward through a
break in the subtropical ridge extending westward from the Atlantic
across Florida and into the northern Gulf of Mexico. By that time,
however, Isaias is expected to weaken below hurricane strength due
to the combination of strong southwesterly vertical wind shear and
interaction the Florida  peninsula. Around 72 hours, the cyclone
should accelerate northeastward and possibly strengthen some before
passing over eastern North Carolina on day 4, and across eastern New
England on day 5. The NHC track forecast lies close to a blend of
the consensus models TVCA and NOAA-HCCA and is east of the UKMET
and ECMWF with the system forecast to be stronger than those
models indicate. Due to the westward shift in the NHC forecast
track, a Hurricane Warning and Storm Surge Watch have been issued
for portions of the Florida east coast.

The center of Isaias is now located in the center of an expanding
CDO feature. The improved inner-core wind field and aforementioned
convective structure, along with very warm SSTs near 30C, should
support some strengthening overnight and early Saturday morning.
However, increasing southwesterly vertical wind shear is expected to
cause a gradual decrease in intensity by Sunday and continue into
early next week. The new official intensity forecast is a little
lower than the previous advisory and is near the higher end of the
intensity guidance.

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
has been issued. 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. Isolated minor river flooding is possible
across the eastern 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  31/2100Z 22.6N  75.7W   65 KT  75 MPH
 12H  01/0600Z 23.9N  77.2W   70 KT  80 MPH
 24H  01/1800Z 25.4N  78.8W   75 KT  85 MPH
 36H  02/0600Z 26.8N  79.9W   70 KT  80 MPH
 48H  02/1800Z 28.3N  80.4W   65 KT  75 MPH
 60H  03/0600Z 30.0N  80.4W   60 KT  70 MPH
 72H  03/1800Z 32.5N  79.4W   60 KT  70 MPH
 96H  04/1800Z 39.0N  74.5W   55 KT  65 MPH
120H  05/1800Z 45.4N  65.9W   50 KT  60 MPH...POST-TROP/EXTRATROP

$$
Forecaster Stewart
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