[Tropical] Tropical Cyclone Discussion

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WTNT41 KNHC 182032
TCDAT1

Tropical Storm Nestor Discussion Number   6
NWS National Hurricane Center Miami FL       AL162019
400 PM CDT Fri Oct 18 2019

Nestor's structure has evolved quite a bit today. Since the release
of the previous advisory, the center has become better defined based
on data from aircraft, satellite, and surface observations, and on
that basis, the system was classified as a tropical cyclone around
1800 UTC. Since that time, the center has become more separated from
the area of deep convection to the east, consistent with 30-40 kt of
deep-layer westerly shear analyzed by the SHIPS model and UW-CIMSS.
The initial intensity remains 50 kt based on aircraft data and
earlier ASCAT data.

The mid/upper-level trough seen in water vapor imagery over
southeastern Louisiana is beginning to impinge on Nestor, and the
cloud pattern has become more lopsided. It would not be surprising
to see Nestor take on a more subtropical appearance overnight, as
the GFS and ECMWF show the upper trough becoming superimposed on the
low-level circulation, with the pressure continuing to deepen and
some increase in the peak winds noted in both those models. This is
reflected in the 12-h intensity forecast of 55 kt. After that time,
the upper-level pattern becomes less favorable and Nestor should
weaken while it moves inland over the southeastern United States
and becomes post-tropical. Gale-force winds are shown over the
western Atlantic waters, but are not expected over land areas from
24 h onward. The global models show Nestor's circulation dissipating
by 96 hours, and that is reflected in the NHC forecast.

The initial motion estimate is 050/19, but remains somewhat
uncertain given the recent formation of a better defined center.
The track forecast reasoning remains unchanged, with Nestor
expected to move quickly northeastward ahead of the approaching
upper trough and moving inland over the Florida Panhandle early
Saturday. The circulation center will move across the southeastern
United States before moving back offshore after 48 hours with a
slower east-northeastward to eastward motion shown late in the
period before dissipation. The new NHC track forecast lies a little
to the right of the previous and is close to the middle of the
guidance envelope. It should be noted that dangerous storm surge
and tropical storm force winds will occur along a large portion of
the Gulf Coast well east of the track of Nestor's center.

Key Messages:

1. There is a danger of life-threatening storm surge inundation of
up to 5 feet above ground level beginning this evening along the
Florida Gulf Coast from Indian Pass to Clearwater Beach, where a
Storm Surge Warning is in effect. Residents in these areas should
follow advice given by local officials.

2. Tropical storm force winds are likely later today and tonight
along portions of the Florida Gulf Coast, where tropical storm
warnings are in effect. Regardless of the exact track and intensity
of the system, these winds will cover a large area, especially east
of the center.

3. Isolated flash flooding is possible along the central and eastern
Gulf Coast and Southeast United States coast from late tonight
through Sunday.

4. Wind and coastal flooding hazards along the U.S. East Coast will
be covered by non-tropical watches and warnings issued by local NWS
offices, since the system is expected to lose its tropical
characteristics after it moves inland along the Gulf Coast.

FORECAST POSITIONS AND MAX WINDS

INIT  18/2100Z 27.0N  88.5W   50 KT  60 MPH
 12H  19/0600Z 28.9N  86.2W   55 KT  65 MPH
 24H  19/1800Z 31.1N  83.7W   35 KT  40 MPH...POST-TROPICAL
 36H  20/0600Z 33.4N  80.3W   35 KT  40 MPH...POST-TROP/EXTRATROP
 48H  20/1800Z 35.4N  75.9W   35 KT  40 MPH...POST-TROP/EXTRATROP
 72H  21/1800Z 36.6N  68.6W   35 KT  40 MPH...POST-TROP/EXTRATROP
 96H  22/1800Z...DISSIPATED

$$
Forecaster Brennan
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