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Tropical Storm ISAAC


Tropical Storm Isaac Discussion Number  24
NWS National Hurricane Center Miami FL       AL092018
1100 AM AST Thu Sep 13 2018

Satellite imagery indicate that Isaac remains a poorly organized
tropical cyclone, with the low-level center mostly exposed during
the past several hours.  The center of Isaac passed between Dominica
and Martinique around 1300 UTC, with westerly winds reported on both
St. Lucia and Martinique, indicating that Isaac is still a tropical
storm. Data from a NOAA P-3 Hurricane Hunter aircraft indicate that
maximum winds remain near 40 kt, although the area of tropical-
storm-force winds has shrunk since yesterday.

1200 UTC radiosonde data from the eastern Caribbean and Hurricane
Hunter aircraft data indicate that the circulation of Isaac is
fairly shallow and small.  It is entirely possible that Isaac will
degenerate into a tropical wave during the next day or two due to
the continued effects of strong shear.  However, the environment
becomes much more conducive for increased organization in a few days
when the cyclone, or its remnants, moves across the west-central
Caribbean Sea.  Model guidance is not consistent at all on whether
Isaac will survive as a tropical cyclone, which frankly might not be
a very predictable event given the initial uncertainties.  The best
course of action is to let the full 1200 UTC guidance suite ingest
all of the latest radiosonde and other data before making any
changes, so this advisory holds the status quo for now.

Isaac is moving a little south-of-west and a bit faster, since it
was devoid of convection most of night, at about 265/17. A mid-level
ridge is forecast to persist over the southwestern Atlantic,
steering Isaac westward for the next few days.  The track forecast
is fairly dependent on the intensity forecast, since a deeper
tropical cyclone will move slower due to slightly weaker winds in
the middle part of the atmosphere.  The forecast has been too slow
so far, and with the current forecast showing little strengthening,
the official track prediction is shifted west of the previous one,
close to the NOAA corrected consensus aid.  At the end of the
forecast, if Isaac remains a tropical cyclone, it could start moving
more toward the west-northwest, but I would place little emphasis on
the 96-hour point until we get a better handle on the future
structure of Isaac.

Key Messages:

1. Isaac is expected to produce tropical-storm-force winds across
portions of the Lesser Antilles into this afternoon, and tropical
storm warnings are in effect for Martinique, Dominica, and

2. Tropical storm watches are in effect for Montserrat, St. Kitts
and Nevis, Antigua, Barbuda, Saba and St. Eustatius, St. Maarten,
and St. Martin.  Tropical storm conditions are possible on these
islands today and interests on those islands should follow any
advice given by their local officials.


INIT  13/1500Z 14.9N  61.8W   40 KT  45 MPH
 12H  14/0000Z 14.9N  64.2W   35 KT  40 MPH
 24H  14/1200Z 14.9N  67.0W   35 KT  40 MPH
 36H  15/0000Z 14.9N  69.8W   35 KT  40 MPH
 48H  15/1200Z 15.0N  72.3W   35 KT  40 MPH
 72H  16/1200Z 15.5N  76.5W   35 KT  40 MPH
 96H  17/1200Z 17.0N  80.5W   30 KT  35 MPH
120H  18/1200Z...DISSIPATED

Forecaster Blake