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Tropical Storm Isaac Discussion Number  20
NWS National Hurricane Center Miami FL       AL092018
1100 AM AST Wed Sep 12 2018

GOES-16 1-minute satellite imagery along with NOAA Hurricane Hunter
data show that the center of Isaac has become fully exposed on the
northwestern side of a shrinking area of deep convection.  There is
still a lot of wind there, however, with the NOAA plane observing
SFMR and adjusted flight-level winds to support 50 kt as an initial
intensity.

An upper-level trough is dropping southward over the central
Atlantic, which has increased the shear this morning over Isaac.
This high shear is forecast to remain for the next day or two, and
it is possible that Isaac will degrade into a tropical wave during
that time, although the area of tropical-storm-force winds will
likely not go away for a while.  In a couple of days, while the
wind shear could decrease over the east-central Caribbean Sea, there
might not be much of a system left to take advantage of the more
conducive conditions.  As a compromise, the official forecast
shows a slow weakening, in the direction of the model consensus,
and continues to show dissipation after 96 hours in line with the
GFS/ECMWF guidance.  I wouldn't pay much attention to the long
range forecast for now, given the uncertainties in Isaac surviving
during the next day or two.

The low-level center of Isaac has recently accelerated, moving
westward at 15 kt, as it decoupled from the slower mid-level flow.
Assuming the system stays vertically coherent, a large
mid-tropospheric high is forecast to persist for the next several
days.  This should keep the system moving westward at a somewhat
slower pace throughout the forecast period.  There isn't much spread
in the guidance, and the official forecast is close to the previous
one after accounting for the initial position.  The long-range track
forecast highly depends on the intensity forecast, so it should be
considered low confidence.

Key Messages:

1. Isaac is expected to remain at tropical storm intensity when it
moves across the Lesser Antilles on Thursday, and tropical storm
warnings are in effect 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.

2. Life-threatening flash flooding is possible with Isaac.  The
storm is expected to produce total rainfall accumulations of 2 to 4
inches with isolated amounts up 8 inches across Martinique,
Dominica and Guadeloupe. Rainfall amounts of 1 to 2 inches with
isolated amounts to 4 inches are expected across Puerto Rico and the
southern United States Virgin Islands.


FORECAST POSITIONS AND MAX WINDS

INIT  12/1500Z 15.0N  54.7W   50 KT  60 MPH
 12H  13/0000Z 15.1N  56.7W   50 KT  60 MPH
 24H  13/1200Z 15.3N  59.4W   45 KT  50 MPH
 36H  14/0000Z 15.3N  62.6W   45 KT  50 MPH
 48H  14/1200Z 15.4N  65.6W   40 KT  45 MPH
 72H  15/1200Z 15.5N  71.0W   40 KT  45 MPH
 96H  16/1200Z 15.5N  75.0W   35 KT  40 MPH
120H  17/1200Z...DISSIPATED

$$
Forecaster Blake

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