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Tropical Storm Isaac Discussion Number  22
NWS National Hurricane Center Miami FL       AL092018
1100 PM AST Wed Sep 12 2018

A NOAA P-3 Hurricane Hunter plane has been investigating Isaac this
evening, and the data show that the cyclone has weakened a bit.
The maximum 850-mb flight-level wind measured by the plane was 47
kt, and the highest SFMR winds not coincident with a rain spike were
around 45 kt.  Based on these wind data, the initial intensity is
lowered to 45 kt, and the minimum pressure is up to 1006 mb based on
dropsonde data.  Additionally, a superposition of the flight-level
wind and dropsonde surface wind data suggest that Isaac may not have
a closed circulation at 850 mb but is still hanging on to one at the
surface.

All of Isaac's deep convection is displaced 60-120 n mi to the
northeast and southeast of the low-level center due to around 30 kt
of westerly shear.  This magnitude of the shear is not expected to
decrease during the next 12-24 hours, so at the very least, gradual
weakening is anticipated.  With the circulation so fragile and
limited to below 850 mb, however, it's entirely possible that Isaac
could open up into a wave at any time.  Even if degeneration into a
wave occurs, the system would likely carry tropical-storm-force
winds across the Leeward Islands on Thursday.  After Isaac moves
into the eastern Caribbean Sea, there is a lot of uncertainty
regarding its future.  The 18Z GFS has come back in line with the
ECMWF, showing Isaac opening up into a trough over the central and
western Caribbean Sea, but the environmental conditions (lower
shear, warm sea surface temperatures, etc.) would suggest that the
system would have an opportunity to restrengthen.  For now, the new
NHC intensity forecast maintains continuity from the previous
advisory, showing gradual weakening through 48 hours and then
holding the system at 35 kt through day 5.  This remains a low
confidence forecast until we know if Isaac survives the next couple
of days.

Isaac continues to move quickly westward with an initial motion of
270/17 kt.  The track guidance remains tightly clustered on a
nearly due westward motion for much of the forecast period, with
some of the same speed differences noted in previous forecasts.
Especially since Isaac's speed has been faster than forecast, the
NHC track forecast continues to favor the faster guidance, in
particular the ECMWF, GFS, and HCCA models.

Reconnaissance and scatterometer data suggest that there are no
tropical-storm-force winds within the southern semicircle, but the
radii we've been carrying within the northern semicircle appear
reasonable.  The wind radii forecast have been adjusted to account
for the new initial radii.

Key Messages:

1. Isaac is expected to still be producing tropical-storm-force
winds 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, Saba and St. Eustatius, St. Maarten,
and St. Martin.  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  13/0300Z 15.3N  58.0W   45 KT  50 MPH
 12H  13/1200Z 15.5N  60.4W   45 KT  50 MPH
 24H  14/0000Z 15.6N  63.4W   40 KT  45 MPH
 36H  14/1200Z 15.7N  66.4W   40 KT  45 MPH
 48H  15/0000Z 15.7N  69.2W   35 KT  40 MPH
 72H  16/0000Z 15.6N  73.9W   35 KT  40 MPH
 96H  17/0000Z 16.5N  77.5W   35 KT  40 MPH
120H  18/0000Z 17.5N  82.0W   35 KT  40 MPH

$$
Forecaster Berg

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