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Hurricane Isaac Discussion Number  13
NWS National Hurricane Center Miami FL       AL092018
500 PM AST Mon Sep 10 2018

Isaac has not become better organized today.  The system continues
to display a ragged CDO with limited banding features.  Dvorak
T-numbers from both TAFB and SAB continue to support an intensity of
65 kt.  It is not clear why Isaac has failed to strengthen.  Cirrus
motions and microwave imagery suggest some shear over the tropical
cyclone, which may have had some influence.  Isaac should not
experience a significant increase in shear until about 48 hours, so
some strengthening is called for up to that time.  Thereafter, the
dynamical guidance shows a significant increase in shear, which
should reverse the intensity trend.  The HWRF model continues to be
an outlier in predicting significant intensification over the
Caribbean, whereas the other models do not.  The official intensity
forecast is above the model consensus over the first half of the
period, and below the consensus during the latter part of the
period, considering the HWRF could be making the consensus too high
at days 3-5.

Isaac continues moving westward or 270/12 kt.  There continue to be
a few differences among the track guidance, in particular the U.K.
Met, which shows the system turning northwestward to northward well
to the northeast of the Leeward Islands.   The GFS takes the
system into the Caribbean but shows a more west-northwestward
motion.  The official forecast stays close to the ECMWF and the
corrected consensus predictions, which is very similar to the
previous NHC track.  Given the spread in the guidance, the
confidence in the details of the track forecast beyond the first
couple of days is larger than usual.

Key Messages:

1. Isaac is a small hurricane and uncertainty in the intensity
forecast is greater than normal.  Although Isaac is expected to
begin weakening when it approaches the Lesser Antilles, it is still
likely to be at or near hurricane intensity when it reaches the
islands.

2. Interests in the Lesser  Antilles should continue to monitor
Isaac during the next few days.  Watches will likely be required for
portions of the Lesser Antilles tomorrow.

FORECAST POSITIONS AND MAX WINDS

INIT  10/2100Z 14.4N  45.0W   65 KT  75 MPH
 12H  11/0600Z 14.5N  47.1W   70 KT  80 MPH
 24H  11/1800Z 14.6N  49.8W   75 KT  85 MPH
 36H  12/0600Z 14.6N  52.3W   75 KT  85 MPH
 48H  12/1800Z 14.8N  54.7W   75 KT  85 MPH
 72H  13/1800Z 15.3N  60.2W   70 KT  80 MPH
 96H  14/1800Z 15.5N  65.5W   60 KT  70 MPH
120H  15/1800Z 15.5N  70.5W   50 KT  60 MPH

$$
Forecaster Pasch

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