Tropical Storm ETA Advisory Mon Nov 09

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Tropical Storm Eta Discussion Number  37
NWS National Hurricane Center Miami FL       AL292020
1000 AM EST Mon Nov 09 2020

Eta's overall appearance in satellite radar imagery has become quite
fragmented, with inner-core convection having become vertically
shallow and broken due to mid-level dry air entrainment. Some of the
recent Air Force Reserve reconnaissance flight-level data indicate
that dry air has worked way down to the 850-mb level based dewpoint
spreads of more than 8 deg C. In the northeastern quadrant, a large
curved convective band persists that has produced widespread
rainfall amounts exceeding 10 inches along with strong gusty winds
across portions of the southern Florida peninsula. However, even
that band of thunderstorms has become less pronounced in both
satellite radar imagery over the past few hours. Doppler velocities
over land and over water have also come decreased significantly, and
the latest reconnaissance flight-level (58 kt) and SFMR surface wind
data (49 kt) supports lowering the intensity to 50 kt. The decrease
in intensity is also supported by the gradual rise in the central
pressure, which is now up to 994 mb.

Eta has finally made the expected southwestward turn, and the
initial motion is now 235/12 kt. A strong deep-layer ridge across
the Gulf of Mexico, Florida, and near the U.S. east coast is
expected to keep Eta moving southwestward for the next 24-36 hours,
accompanied by a steady decrease in forward speed. Steering currents
are forecast to collapse by 36-48 hours, causing Eta to possibly
stall and/or make a small loop just northwest of western Cuba. By 60
hours and beyond, a broad mid-latitude trough currently located over
the Rocky Mountains is forecast to move eastward and gradually erode
the portion of the ridge over the Gulf of Mexico, allowing Eta
slowly move northward to north-northeastward through 20 hours.
However, there remains considerable divergence between the global
models on days 4 and 5 with regards to how far north Eta will move,
with the ECMWF showing a more northward progression while the GFS
and UKMET models show the trough lifting out and not eroding the
ridge as much. For now, the official forecast track remains a
compromise of these two extremes, and shows a slow poleward
progression on days 3-5, similar to the consensus models TCVA and
NOAA-HCCA.

Eta could still re-strengthen some during the 24-48 hour period when
the cyclone will be moving over the warm waters of the Gulf of
Mexico Loop Current and the wind shear gradually decrease to less
than 10 kt. However, occasional intrusions of very dry air will
likely continue to plague the system, which would prevent any rapid
intensification from occurring and could keep Eta from regaining
hurricane status. By 48 hours and beyond, increasing vertical wind
shear and dry air should cause a gradual weakening of the cyclone
through end of the forecast period. The new NHC intensity forecast
is a little lower than the previous advisory, and closely follows
the intensity consensus models IVCN and HCCA.

Key Messages:

1. Heavy rainfall from Eta will continue across portions of Cuba,
the Bahamas, and southern and central Florida. Life-threatening
flash flooding will be possible across urban areas of southeast
Florida today.  Flash and urban flooding will also be possible for
Cuba, the Bahamas and the remainder of southern Florida, along with
potential minor river flooding in central Florida.

2. Eta could approach the Florida Gulf Coast later this week as a
tropical storm, and possibly bring impacts from rain, wind, and
storm surge. Interests in this area should monitor the progress of
Eta and updates to the forecast this week.

FORECAST POSITIONS AND MAX WINDS

INIT  09/1500Z 24.6N  83.4W   50 KT  60 MPH
 12H  10/0000Z 23.9N  84.7W   50 KT  60 MPH
 24H  10/1200Z 23.6N  85.3W   55 KT  65 MPH
 36H  11/0000Z 24.5N  85.4W   60 KT  70 MPH
 48H  11/1200Z 25.6N  85.5W   55 KT  65 MPH
 60H  12/0000Z 26.5N  85.3W   55 KT  65 MPH
 72H  12/1200Z 27.2N  85.1W   50 KT  60 MPH
 96H  13/1200Z 28.4N  84.4W   45 KT  50 MPH
120H  14/1200Z 29.6N  83.7W   35 KT  40 MPH...INLAND

$$
Forecaster Stewart