[Tropical] Tropical Cyclone Discussion

[EMWIN Server]

WTNT44 KNHC 101455
TCDAT4

Tropical Storm Eta Discussion Number  41
NWS National Hurricane Center Miami FL       AL292020
900 AM CST Tue Nov 10 2020

A significant increase in deep convection has occurred since the
previous advisory this morning, including a CDO-like feature with
cloud tops of -85C to -87C very near the low-level center. However,
recent passive microwave satellite images indicate that the center
is displaced to the northwest of the coldest cloud tops due to
modest northwesterly mid- and upper-level vertical wind shear. The
initial intensity has been increased to 50 kt, which is based on an
average of Dvorak satellite intensity estimates of T3.0/45 kt from
TAFB and T3.5/55 kt from SAB.

Although the initial motion estimate is 360/02 kt, Eta has
essentially been stationary for the past 9 hours or so. Radar data
from Casablanca, Cuba, and satellite fixes suggest that Eta has been
making a small cyclonic/counter-clockwise loop within the larger
gyre in which the small center is embedded. Until the gyre breaks
down or moves northward, there will be little poleward motion by Eta
today. By tonight, however, the global and regional models are in
good agreement on a broad mid-tropospheric trough moving eastward
across the central and eastern United States, which is expected to
erode a subtropical ridge to the north of the Eta, allowing both the
larger gyre and Eta to move slowly northward. This steering pattern
is expected to continue through about 72 hours. Thereafter, the
model guidance diverges significantly between motions ranging from
westward (GFS/GFS-ensemble) to northward (ECMWF) to northeastward
(COAMPS-TC). The 96-120 hour motions are directly related to the
strength of the cyclone, with a much weaker Eta forecast to move
westward and a stronger hurricane solution moving northeastward.
The latter scenario seems unlikely given that the vertical shear is
forecast to increase from the northwest and west at more than 25 kt,
which acts to weaken Eta and also impart a slight eastward tug on
the system. As a result, the official forecast track calls for Eta
to basically move slowly northward through the 120-h forecast
period and gradually weaken into a shallow cyclone that drifts
northward. The new NHC track forecast is similar to the previous
advisory track, and lies close to a blend of the consensus models
TVCA, NOAA-HCCA, and the Florida Superensemble (FSSE).

Eta is expected to remain in a low-to-moderate vertical wind shear
environment and over warm SSTs of 27-28 deg C for the next couple of
days. Although the surrounding environment is expected to be
somewhat dry, the other two favorable environmental factors should
allow for some strengthening into Thursday, Thereafter, increasing
shear from the northwest and west, along with drier mid-level air
and cooler SSTs are likely to cause Eta to weaken. The rate of this
weakening remains uncertain, and depends heavily on how much Eta
re-intensifies over the next couple of days. The new intensity
forecast is essentially the same as the previous advisory, and is a
little below the consensus models IVCN, HCCA, and FSSE, all of which
make Eta a hurricane again by 36 hours.

Key Messages:

1. Heavy rainfall from Eta will continue across western Cuba and
South Florida today and tonight.  Additional flash and urban
flooding, especially across previously inundated areas, will be
possible in South Florida. Flash and urban flooding will also be
possible for western Cuba.

2. Eta could approach the northeastern or north-central U.S. Gulf
Coast later this week as a tropical storm, and possibly bring
impacts from rain, wind, and storm surge. Interests in this area
should continue to monitor the progress of Eta and updates to the
forecast this week.

FORECAST POSITIONS AND MAX WINDS

INIT  10/1500Z 22.7N  85.3W   50 KT  60 MPH
 12H  11/0000Z 23.1N  85.4W   55 KT  65 MPH
 24H  11/1200Z 24.0N  85.3W   60 KT  70 MPH
 36H  12/0000Z 25.0N  85.2W   60 KT  70 MPH
 48H  12/1200Z 25.9N  85.1W   55 KT  65 MPH
 60H  13/0000Z 26.6N  85.2W   45 KT  50 MPH
 72H  13/1200Z 27.2N  85.5W   40 KT  45 MPH
 96H  14/1200Z 28.6N  86.4W   35 KT  40 MPH
120H  15/1200Z 30.1N  86.4W   30 KT  35 MPH

$$
Forecaster Stewart
This weather bulletin was brought to you by:
www.nashvilleweather.net