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Tropical Depression Isaac Discussion Number  27
NWS National Hurricane Center Miami FL       AL092018
500 AM AST Fri Sep 14 2018

Northwesterly shear continues to take a toll on Isaac.  Although the
system is still producing areas of deep convection, an ASCAT
overpass that arrived shortly after the release of the previous
advisory, indicated that the circulation had become even less
defined.  The ASCAT data also showed that the winds are below
tropical storm strength, therefore the initial intensity has been
reduced to 30 kt.

Moderate to strong northwesterly shear and dry mid-level air are
likely to cause additional weakening, and Isaac is forecast to
degenerate into an open wave within the next several days, but this
could occur much sooner if the current trends continue.
When the system reaches the western Caribbean Sea the shear may
relax somewhat and moisture levels are expected to be higher.  These
conditions could allow for regeneration of the system, but the ECMWF
and GFS both show Issac weakening and moving over the western
Caribbean as an open wave, and the NHC forecast follows this
scenario.

A low- to mid-level ridge over the southwestern Atlantic continues
to steer Isaac westward, and this general motion should continue
until dissipation occur.  The new track forecast is once again
essentially an update of the previous advisory.

FORECAST POSITIONS AND MAX WINDS

INIT  14/0900Z 15.0N  65.5W   30 KT  35 MPH
 12H  14/1800Z 15.1N  67.3W   30 KT  35 MPH
 24H  15/0600Z 15.2N  69.7W   30 KT  35 MPH
 36H  15/1800Z 15.2N  72.0W   25 KT  30 MPH
 48H  16/0600Z 15.4N  73.8W   25 KT  30 MPH
 72H  17/0600Z 16.4N  77.7W   25 KT  30 MPH
 96H  18/0600Z...DISSIPATED

$$
Forecaster Brown

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