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Hurricane Isaac Discussion Number  12
NWS National Hurricane Center Miami FL       AL092018
1100 AM AST Mon Sep 10 2018

Isaac's cloud pattern is currently a bit ragged-looking, with an
irregular CDO and limited convective banding.  Dvorak intensity
estimates from both TAFB and SAB are 65 kt, and that value will be
used for the advisory.  There is significant uncertainty in the
intensity forecast for this system.  Vertical wind shear over the
hurricane is forecast to be weak to moderate during the next day or
so, which should allow Isaac to strengthen somewhat.  By 48 hours,
however, the global models depict significantly stronger
northwesterly shear, associated with an upper-level trough over the
central Atlantic, affecting the tropical cyclone.  Most of the
guidance does not show Isaac strengthening over the Caribbean.  The
HWRF model is an outlier, strengthening Isaac into a major hurricane
later in the period, and it is the main contributor to high
consensus forecasts at days 3-5.  The NHC intensity forecast is
above the model consensus through 72 hours and below it by the end
of the period.

Isaac continues to move westward, or 275/12 kt.  A subtropical
ridge should be maintained to the north of the tropical cyclone for
the next several days, so a continued westward track seems likely.
The U. K. Met. Office model is an outlier and shows Isaac turning
northwestward and northward to the northeast of the Leeward Islands.
The official track forecast, like the previous one, follows the
other reliable global models and is also close to the HFIP corrected
consensus guidance.

Key Messages:

1. Isaac is a small hurricane and uncertainty in the forecast is
greater than normal.  Although Isaac is forecast to begin weakening
when it approaches the Lesser Antilles, it is still expected 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.

FORECAST POSITIONS AND MAX WINDS

INIT  10/1500Z 14.7N  43.9W   65 KT  75 MPH
 12H  11/0000Z 14.9N  45.9W   70 KT  80 MPH
 24H  11/1200Z 15.0N  48.7W   75 KT  85 MPH
 36H  12/0000Z 15.0N  51.4W   75 KT  85 MPH
 48H  12/1200Z 15.1N  54.0W   75 KT  85 MPH
 72H  13/1200Z 15.4N  59.4W   70 KT  80 MPH
 96H  14/1200Z 15.5N  64.5W   60 KT  70 MPH
120H  15/1200Z 15.7N  70.0W   50 KT  60 MPH

$$
Forecaster Pasch

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