[Tropical] Tropical Cyclone Discussion

[EMWIN Server]

WTNT44 KNHC 062059
TCDAT4

Tropical Depression Eta Discussion Number  25
NWS National Hurricane Center Miami FL       AL292020
300 PM CST Fri Nov 06 2020

Reports from an Air Force Reserve Hurricane Hunter aircraft
indicate that the center of Eta is elongated east-to-west with
multiple vorticity centers present.  The central pressure is near
1002 mb, and the maximum flight-level winds were 30-35 kt at 1500
ft.  There were a few spot reports of 35-45 kt winds from the
Stepped Frequency Microwave Radiometer, but these were in areas of
heavy rain and their reliability is suspect.  Based on these data,
the initial intensity is kept at 30 kt.

The aircraft data shows that the center is farther south than
previously thought, and the initial motion is an uncertain 040/6 kt.
There is no change to the previous track forecast philosophy.  A
developing mid- to upper-level trough over the Gulf of Mexico is
expected to steer Eta northeastward for the next 48 h or so with an
increase in forward speed.  Between 48-72 h, the trough is forecast
to become a closed low near south Florida and western Cuba, causing
Eta to turn northwestward around and into the low.  After 72 h, the
merged system is likely to move slowly westward to northwestward.
While the guidance is in good agreement on the synoptic evolution,
there remain differences in the track forecast details in terms of
both the heading and the forward speed, with the GFS-based guidance
tending to be to the east and north of the UKMET/ECMWF/Canadian.  In
addition, the HWRF/HMON/Canadian are faster than the other guidance.
The new forecast track overall is a little slower than the previous
track, and due to the initial position the first 36 h have been
shifted southward.

Eta will move over the warm waters of the northwestern Caribbean
during the next 48 h or so, and the upper-level divergence is
forecast to be quite strong.  This should allow some strengthening,
although this is likely to be slowed by 20-30 kt of southwesterly
vertical wind shear.  The intensity forecast for this part of the
cyclone's life is again similar to the previous forecast and lies
near the bulk of the intensity guidance.  Between 48-72 h , Eta may
take on at least some subtropical cyclone characteristics as it
merges with the mid- to upper-level low.  The HWRF and HMON models
still suggest the possibility that a tight inner wind core may
develop, however, the guidance has trended weaker since this
morning, and the new intensity forecast is above the bulk of the
guidance.  After 72 h, dry air entrainment is expected to cause the
cyclone to weaken.

The wind field of Eta is expected to increase in size during the
next few days, and the cyclone will likely produce a large area of
tropical-storm-force winds on its north side when it is near Cuba,
the Florida Keys, and southern Florida.

The new forecast track requires a Tropical Storm Warning for
portions of Cuba at this time.  Watches may be required for
portions of south Florida and the Florida Keys tonight.


Key Messages:

1. Heavy rainfall is diminishing across portions of Central America,
although the threat of life-threatening flooding may continue, along
with landslides in areas of higher terrain. Heavy rainfall from Eta
will move into the Cayman islands and portions of Cuba, resulting in
significant, life-threatening flash flooding and river flooding.
Flash flooding and urban flooding is also possible for Jamaica and
southeast Mexico.

2. Tropical storm conditions are expected this weekend in portions
of the Cayman Islands and Cuba, where a Tropical Storm Warning is in
effect.

3. There is an increasing risk of impacts from wind and flash and
urban flooding due to heavy rainfall in portions of southern
Florida, the Florida Keys, and portions of the Bahamas this weekend
and early next week. Tropical Storm or Hurricane Watches will likely
be issued for a portion of this area tonight.


FORECAST POSITIONS AND MAX WINDS

INIT  06/2100Z 17.3N  86.5W   30 KT  35 MPH
 12H  07/0600Z 18.2N  84.8W   35 KT  40 MPH
 24H  07/1800Z 19.4N  82.5W   40 KT  45 MPH
 36H  08/0600Z 20.8N  80.7W   45 KT  50 MPH
 48H  08/1800Z 22.5N  80.0W   50 KT  60 MPH...INLAND
 60H  09/0600Z 24.2N  80.6W   55 KT  65 MPH...OVER WATER
 72H  09/1800Z 24.9N  81.8W   55 KT  65 MPH
 96H  10/1800Z 25.0N  84.0W   50 KT  60 MPH
120H  11/1800Z 26.5N  85.5W   50 KT  60 MPH

$$
Forecaster Beven
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