142 
WTNT44 KNHC 130859
TCDAT4

Tropical Storm Isaac Discussion Number  23
NWS National Hurricane Center Miami FL       AL092018
500 AM AST Thu Sep 13 2018

Data from a NOAA P-3 Hurricane Hunter aircraft that departed the
storm shortly before 0600 UTC indicated that the tropical storm has
weakened a little more.  Maximum flight-level winds were around 51
kt, while maximum SFMR winds were only 41 kt.  Both of these
support an initial intensity of 40 kt.  Wind measurements from the
plane also suggested that Isaac is close to opening up into a
tropical wave and its circulation is likely barely closed at the
surface.  Another P-3 mission is scheduled for this morning.

Isaac is nearly completely devoid of deep convection at the moment,
and is not trackable in radar data from Guadeloupe and Martinique.
Given the lack of deep convection to sustain the cyclone, continued
gradual weakening is expected.  However, given the extent of
40 mph winds observed by the NOAA plane, Isaac is still likely to
bring gusty conditions to many of the Leeward Islands through
this afternoon.  The dynamical guidance is now in fairly good
agreement on the intensity of Isaac for the next few days, and
nearly all of the models depict Isaac opening into a trough between
48 and 96 h.  The NHC forecast conservatively maintains Isaac
through day 4 in an attempt to maintain some continuity, but it
is very possible that Isaac will open into a trough sooner than
indicated.  Some of the dynamical models suggest that regeneration
could occur in the western Caribbean, however the predictability of
such an event is too low to explicitly show in the forecast at this
point.

Isaac is still moving quickly westward, with an initial motion of
275/15 kt.  Confidence in the track forecast remains high, and the
track guidance is tightly clustered through the forecast period.
Very little change was made to the track forecast, which continues
to closely follow the HCCA model.  As long as Isaac remains a
shallow cyclone, it should continue to be steered westward by the
low-level tradewind flow, even if, or when, it opens up into a
trough.


Key Messages:

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

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.


FORECAST POSITIONS AND MAX WINDS

INIT  13/0900Z 15.4N  59.7W   40 KT  45 MPH
 12H  13/1800Z 15.5N  61.8W   35 KT  40 MPH
 24H  14/0600Z 15.5N  64.8W   35 KT  40 MPH
 36H  14/1800Z 15.5N  67.8W   35 KT  40 MPH
 48H  15/0600Z 15.4N  70.4W   35 KT  40 MPH
 72H  16/0600Z 15.6N  74.8W   35 KT  40 MPH
 96H  17/0600Z 16.9N  78.5W   30 KT  35 MPH
120H  18/0600Z...DISSIPATED

$$
Forecaster Zelinsky