[Tropical] Tropical Cyclone Discussion

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WTNT41 KNHC 191458
TCDAT1

Post-Tropical Cyclone Nestor Discussion Number   9
NWS National Hurricane Center Miami FL       AL162019
1000 AM CDT Sat Oct 19 2019

Nestor has made the transition to a post-tropical extratropical
cyclone. There has been no significant convection near the
low-level center for more than six hours, and satellite and surface
observations indicate that the cyclone's center has merged with a
nearby frontal system that lies along the coast of the Florida
panhandle. The overall cloud pattern more closely resembles that
of an occluded low pressure system now, including a pre-frontal
squall line or convergence zone a few hundred nmi east of the low.
Sustained tropical-storm-force winds with gusts to near 50 kt have
been reported by some of the buoys and coastal marine stations over
the northeastern Gulf of Mexico during the past few hours. The
initial intensity of 45 kt is based on Doppler velocity values
of 50-55 kt between 4000-6000 ft ASL located over Apalachee Bay,
which approximately equals surface winds of 40-44 kt.

The now well-defined low-level center made a brief jog toward the
northwest early this morning as an upper-level low passed over the
larger cyclonic gyre. However, the motion since that time has
been slowly eastward, and the initial motion estimate is now
east-northeastward or 075/08 kt. Despite the earlier erratic motion,
the latest NHC track model guidance remains in very good agreement
on post-tropical Nestor moving northeastward and accelerating over
the next 36 hours or so, followed by a turn toward the east once
the cyclone reaches the North Carolina Outer Banks. By 48 hours, the
parent upper-level low is expected to weaken and open up into a
shortwave trough, and leave Nestor behind as a weakening
extratropical cyclone that dissipates by 96 h east of the U.S. east
coast. The new official forecast track was only nudged a little to
the west of the previous advisory due to the more westward initial
position, and lies close to an average of the deterministic 0000 UTC
and 0600Z GFS, ECMWF, and UKMET model runs.

No strengthening is anticipated before Nestor moves inland later
this afternoon. Some slight weakening will occur after the center
moves inland, but gale-force winds are expected develop over the
Atlantic waters of northeastern Florida to the Carolinas this
afternoon and tonight, and across the mid-Atlantic U.S. coastal
waters on Sunday. The extratropical cyclone is expected to dissipate
or merge with a cold front in about 4 days or sooner.

Given the non-tropical structure of Nestor, dangerous storm surge
and tropical-storm-force and gale-force winds will occur along a
large portion of the Florida Gulf Coast well east of the track of
Nestor's center today.

Key Messages:

1. There is a danger of life-threatening storm surge inundation of
up to 4 feet above ground level 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 will continue across 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 across the southeastern
United States into Sunday morning.

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  19/1500Z 29.3N  86.3W   45 KT  50 MPH...POST-TROP/EXTRATROP
 12H  20/0000Z 31.0N  83.5W   35 KT  40 MPH...POST-TROP/EXTRATROP
 24H  20/1200Z 34.1N  79.5W   40 KT  45 MPH...POST-TROP/EXTRATROP
 36H  21/0000Z 36.0N  75.1W   40 KT  45 MPH...POST-TROP/EXTRATROP
 48H  21/1200Z 36.6N  71.5W   40 KT  45 MPH...POST-TROP/EXTRATROP
 72H  22/1200Z 36.4N  66.6W   35 KT  40 MPH...POST-TROP/EXTRATROP
 96H  23/1200Z...DISSIPATED

$$
Forecaster Stewart

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