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Tropical Storm Isaac Discussion Number  18
NWS National Hurricane Center Miami FL       AL092018
1100 PM AST Tue Sep 11 2018

A slew of recent microwave passes have revealed that Isaac's center
is outrunning the deep convection by about a degree due to
strengthening westerly shear.  With the degradation in structure,
Dvorak estimates have fallen to T3.0/3.5 from both TAFB and SAB.
The initial intensity is being set at 55 kt to match the CI number
(3.5), but I wouldn't be surprised if the actual intensity is
closer to the final-T number.

Isaac is moving quickly westward, or 270/14 kt.  Ridging to the
north should keep the cyclone on a similar speed and path for much
of the forecast period.  The only main difference among the track
models is Isaac's future speed, and the NHC official forecast
continues to favor the faster models--the GFS, ECMWF, and HCCA
models.  Since the forecast thinking is unchanged, the updated NHC
forecast is not too different from the previous one.  Isaac's
center is expected to move across the Lesser Antilles near
Martinique and Dominica on Thursday.

Vertical shear is expected to increase and turn more northwesterly
during the next 36 hours as Isaac moves closer to the base of an
upper-level trough which extends northeast of the Leeward Islands.
Therefore, deep convection is expected to remain decoupled from the
low-level center, and Isaac's structure could degrade further over
the next day or two.  The cyclone's maximum winds are also expected
to decrease, and the NHC official forecast is close to the HCCA,
Florida State Superensemble, and ICON intensity consensus aids.
Even though the statistical-dynamical models and the HWRF
restrengthen Isaac by days 4 and 5 when the shear decreases, there
may not be much left of the cyclone for any strengthening to occur.
Both the GFS and ECMWF global models show Isaac weakening to a
remnant low or opening up into a trough by days 4 or 5, which is
the scenario favored by the NHC official forecast.  As such,
dissipation is now shown in the forecast at day 5.


Key Message:

1. Isaac is expected to remain at tropical storm intensity when it
moves across the Lesser Antilles on Thursday, and tropical storm
warnings have been issued for Martinique, Dominica, and Guadeloupe.
Tropical storm watches are in effect for Montserrat, St. Kitts
and Nevis, Antigua, and Saba and St. Eustatius.  Interests on those
islands should follow any advice given by their local officials.


FORECAST POSITIONS AND MAX WINDS

INIT  12/0300Z 14.5N  52.3W   55 KT  65 MPH
 12H  12/1200Z 14.6N  54.7W   50 KT  60 MPH
 24H  13/0000Z 14.7N  57.4W   50 KT  60 MPH
 36H  13/1200Z 14.9N  60.4W   45 KT  50 MPH
 48H  14/0000Z 15.1N  63.7W   45 KT  50 MPH
 72H  15/0000Z 15.2N  69.7W   40 KT  45 MPH
 96H  16/0000Z 15.0N  74.5W   35 KT  40 MPH
120H  17/0000Z...DISSIPATED

$$
Forecaster Berg

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