[Tropical] Tropical Cyclone Discussion

Tue Nov 10 14:52:44 CST 2020

WTNT44 KNHC 102052
TCDAT4

Tropical Storm Eta Discussion Number  42
NWS National Hurricane Center Miami FL       AL292020
300 PM CST Tue Nov 10 2020

Eta's convective structure has changed little since the previous
advisory. A CDO-like feature with cloud tops colder than -70C has
persisted, with some overshooting tops of -80C to -85C located east
and southeast of the center. Recent passive microwave satellite
data indicates that Eta is still sheared from the northwest, with
an intermittent mid-level eye feature showing up. Satellite
classifications have essentially remained unchanged, with SAB
reporting 45-55 kt and TAFB reporting 55 kt. The initial intensity
remains at 50 kt based on a blend of these satellite
classifications and a UW-CIMSS SATCON estimates of 45-48 kt.

The initial motion estimate is now northward, or 360/06 kt. The
biggest surprise is the large eastward shift in all of the NHC model
guidance, which was possibly due at least in part to all of the
dropsondes that the NOAA G-IV jet aircraft dropped around Eta
earlier this morning, All of the guidance is now in good agreement
on a broad, deep-layer trough moving eastward across the
south-central and southeastern United States, which will erode the
subtropical ridge to the north of Eta that has been impeding
Eta's poleward progress he past couple of days. This generally
northward to northeastward steering pattern is expected to persist
through the entire 120-h forecast period, with only slight shifts
east or west of he current forecast track due to how vertically deep
Eta remains when it reaches the northeastern Gulf of Mexico and
Apalachee Bay in a few days. The current forecast track maintains
Eta as at least a moderate tropical cyclone through the period, with
only a slight bend back toward the northwest when the system is
expected to interact with an approaching frontal system. The new NHC
track forecast has been shifted about 150 n mi east of the previous
advisory track at 96 and 120 hours, and further eastward shifts in
the track may be required, closer to the consensus models TCVA/TVCN
and NOAA-HCCA.

Eta is forecast to remain in a low-to-moderate vertical wind shear
environment and over SSTs of at least 27 deg C for the next 36 hours
or so. Intermittent entrainment of dry mid-level air should prevent
any rapid strengthening from occurring, but Eta could still become a
hurricane between in 24 to 36 h before more significant shear begins
to affect the cyclone. By day 3 and beyond, increasing northwesterly
vertical wind shear combined with cooler SSTs should cause Eta to
weaken. 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.

Due to the expected northwesterly shear after 36 hours, the 34-kt
wind radii were expanded in the eastern semicircle, which is the
side of the cyclone where most of the deep convection and associated
stronger winds will be located. Given this and the eastward
adjustment to the track forecast, a Tropical Storm Watch has been
issued for portions of the Florida west coast, and a Tropical Storm
Warning is in effect for the Dry Tortugas.

Key Messages:

1. Tropical-storm-force winds are possible along portions of the
Florida Gulf Coast by Thursday afternoon, and a Tropical Storm Watch
has been issued. Interests elsewhere along the Florida Gulf Coast
should monitor the progress of Eta, as additional watches may be
needed tonight.

2. Heavy rainfall from Eta will continue across western Cuba and
South Florida today and tonight, then potentially spread up the west
coast of the Florida Peninsula Wednesday through Thursday.
Additional flash and urban flooding will be possible in South
Florida, especially across previously inundated areas, and
eventually along portions of West Florida and the Sun Coast. Flash
and urban flooding will also be possible for western Cuba.

FORECAST POSITIONS AND MAX WINDS

INIT  10/2100Z 23.2N  85.1W   50 KT  60 MPH
 12H  11/0600Z 24.1N  84.9W   55 KT  65 MPH
 24H  11/1800Z 25.6N  84.8W   60 KT  70 MPH
 36H  12/0600Z 26.9N  84.5W   60 KT  70 MPH
 48H  12/1800Z 27.9N  84.2W   55 KT  65 MPH
 60H  13/0600Z 28.7N  84.0W   45 KT  50 MPH
 72H  13/1800Z 29.1N  84.0W   40 KT  45 MPH
 96H  14/1800Z 29.7N  84.6W   35 KT  40 MPH
120H  15/1800Z 30.7N  84.9W   30 KT  35 MPH...INLAND

$$
Forecaster Stewart
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