[Tropical] Tropical Cyclone Discussion

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WTNT44 KNHC 111452
TCDAT4

Hurricane Eta Discussion Number  45
NWS National Hurricane Center Miami FL       AL292020
1000 AM EST Wed Nov 11 2020

Eta became much better organized between 0900-1000 UTC this morning,
including the formation of a well-defined, closed circular eye about
20-25 nmi wide. However, since then the satellite and radar
signature have become more ragged as dry air has entrained into the
western semicircle of the cyclone and has also penetrated into the
inner-core region, resulting in a significant degradation of the
convection in that portion of Eta's circulation. A couple of hours
ago, a NOAA Hurricane Hunter aircraft penetrated the remnant eye
feature and measured a central pressure of 984-983 mb, and also
measured 850-mb maximum flight-level winds of 83-85 kt east of the
center just outside of a deep convective band. WSR-88D Doppler
radar data from Tampa Bay (KTBW) indicated a long fetch of Doppler
velocities of 80-88 kt at 13,500 ft directly above and east of the
aircraft report, and this was within a band of strong convection
characterized by radar reflectivities of 45-49 dBZ. Based on the
combination of the wind data and the relatively low central
pressure, Eta was upgraded to hurricane status at 1200 UTC.

The initial motion estimate is north-northeastward, or 015/09 kt.
The latest NHC model guidance is excellent agreement on Eta moving
north-northeastward for the next 24 hours around the the western
periphery of a deep-layer subtropical ridge that extends westward
across the western Atlantic to just off the Florida east coast.
Thereafter, the cyclone will move north of the ridge axis and come
under the influence of southwesterly to westerly mid- to upper-level
flow associated with an approaching cold front, which should result
in a faster northeastward motion. By 72 hours, if not sooner, Eta is
forecast to merge with the aforementioned frontal system off of the
southeastern United States. The new official track forecast is
similar to the previous advisory track, and lies just a tad east or
to the right of the consensus models TVCN, NOAA-HCCA, and FSSE.

It is quite possible that Eta has peaked in intensity based on the
rapid erosion of the convective pattern and an eye feature no longer
evident in radar or passive microwave satellite imagery. However,
there still remains a band of strong convection in the northeastern
quadrant that contains Doppler radar velocity values of 80-88 kt
between 6000-9000 ft, which corresponds to equivalent surface winds
of at least 65 kt. As long as that feature persists, hurricane-force
winds are possible along immediate coastal areas within the
hurricane watch area. The latest GFS-and ECMWF-based SHIPS intensity
guidance shows significantly drier air wrapping into the center by
24 hours, along with the vertical wind shear increasing to more than
20 kt from the west at that time. That combination of unfavorable
environmental parameters is expected to lead to gradual weakening
until landfall occurs in about 24 hours, followed by rapid weakening
after landfall. Eta is forecast to degenerate into a remnant low by
60 hours due to even stronger wind shear, and dissipate by 96 hours
due to frontal interaction.

Key Messages:

1. There is a danger of life-threatening storm surge along portions
of the Florida Gulf Coast from Bonita Beach to Suwannee River,
including Tampa Bay and Charlotte Harbor. Residents in this area
should follow any advice given by local officials.

2. Hurricane-force winds are possible along portions of the west
coast of Florida from Anna Maria Island to Yankeetown this evening
and early Thursday. Tropical-storm-force winds are expected later
today along portions of the Florida Gulf Coast from Bonita Beach to
Suwanee River, and are possible early Thursday from Suwannee River
to Aucilla River. Interests elsewhere along the Florida Gulf Coast
should monitor the progress of Eta.

3. Heavy rainfall from Eta will continue across western Cuba and
south Florida and spread northward across portions of west and north
Florida today through Friday. Additional flash and urban flooding
will be possible in South Florida today, especially across
previously inundated areas, and across portions of west and central
Florida today through Friday.

FORECAST POSITIONS AND MAX WINDS

INIT  11/1500Z 26.2N  83.7W   65 KT  75 MPH
 12H  12/0000Z 27.5N  83.4W   60 KT  70 MPH
 24H  12/1200Z 28.9N  82.7W   55 KT  65 MPH...NEAR FLORIDA COAST
 36H  13/0000Z 30.5N  81.3W   30 KT  35 MPH...OVER WATER
 48H  13/1200Z 31.6N  79.1W   30 KT  35 MPH
 60H  14/0000Z 32.5N  76.9W   25 KT  30 MPH...POST-TROP/REMNT LOW
 72H  14/1200Z 34.1N  73.0W   25 KT  30 MPH...POST-TROP/EXTRATROP
 96H  15/1200Z...DISSIPATED

$$
Forecaster Stewart
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