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Tropical Storm ISAAC


Tropical Storm Isaac Discussion Number  16
NWS National Hurricane Center Miami FL       AL092018
1100 AM AST Tue Sep 11 2018

One-minute visible satellite data from GOES-16 indicate that
the center of Isaac remains on the westward edge of the central
dense overcast. The initial wind speed is kept at 60 kt, which is a
compromise between the lower subjective estimates and higher ADTs
and microwave estimates.  This is a more uncertain estimate than
normal due to the spread of the intensity fixes.  The initial wind
and 12 ft seas radii have been expanded out to the north based on
TAFB estimates and microwave data.

The intensity forecast continues to be tricky.  Westerly shear
should increase over the next couple of days, which would normally
cause some weakening.  However this effect could be tempered by
increasing SSTs, mid-level moisture and instability in the path of
the storm.  Given our limited skill in forecasting intensity changes
in marginal environments, plus the 5 or 10 kt of initial wind speed
uncertainty, the official forecast will show no significant change
in intensity for the next couple of days. Because of these
uncertainties, a Hurricane Watch has been issued for many of the
Leeward Islands, even though Isaac is not explicitly forecast to be
a hurricane.  Some weakening is indicated at long range after
considering the global model trends in intensity, which have done
fairly well for Isaac.

The latest initial motion estimate is 270/14.  Isaac is forecast to
move westward at about the same speed for the next few days due to
a mid-level ridge over the central Atlantic Ocean.  Other than the
UKMET model, there is not much spread in the guidance, except with
the forward speed.  Given the strength of the ridge, the cyclone
isn't likely to slow down much, so the official forecast is
basically along the previous forecast track but faster.  The
uncertainty increases at long range, and is somewhat dependent on
the intensity of Isaac, but faster seems to be way to go given the
weakening trend anticipated at that time.

Key Messages:

1. Isaac is expected to be at or near hurricane intensity when it
approaches the Lesser Antilles on Thursday, and hurricane and
tropical storm watches have been issued for several islands.
Interests in those areas should follow any advice given by their
local officials.

2. Interests elsewhere in the Lesser Antilles should continue to
monitor Isaac during the next couple of days, as additional watches
could be needed for other islands.


INIT  11/1500Z 14.6N  49.7W   60 KT  70 MPH
 12H  12/0000Z 14.6N  51.8W   60 KT  70 MPH
 24H  12/1200Z 14.7N  54.4W   65 KT  75 MPH
 36H  13/0000Z 14.9N  57.1W   60 KT  70 MPH
 48H  13/1200Z 15.1N  59.9W   60 KT  70 MPH
 72H  14/1200Z 15.5N  65.5W   55 KT  65 MPH
 96H  15/1200Z 15.5N  71.0W   50 KT  60 MPH
120H  16/1200Z 15.5N  75.0W   50 KT  60 MPH

Forecaster Blake