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Tropical Storm Isaac Discussion Number  26
NWS National Hurricane Center Miami FL       AL092018
1100 PM AST Thu Sep 13 2018

A burst of deep convection has developed and covered the previously
exposed center of Isaac this evening.  However, there is still
evidence of northwesterly shear over the cyclone as there is a
rather sharp edge to the convective mass and the cirrus is quickly
blowing off toward the southeast.  A recent ASCAT pass shows that
the circulation is not particularly well defined, but it did show
some light west or west-southwesterly winds, enough to keep the
system classified as a tropical cyclone.  The ASCAT also revealed
winds of 30-32 kt, so the initial wind speed is held at 35 kt, which
is also an average of the latest Dvorak current intensity numbers.

Moderate to strong northwesterly shear and dry mid-level air are
likely to cause Isaac to weaken to a tropical depression or even
become an open wave within the next day or so.  When Isaac reaches
the west-central Caribbean Sea in a couple of days, the shear may
relax somewhat and the global models suggest that there will be an
increase in mid-level moisture.  These conditions could allow for
regeneration of the system, but given the uncertainty in whether
Isaac will survive long enough to take advantage of these
conditions, the official forecast still calls for dissipation like
most of the guidance.

Isaac continues moving westward at about 14 kt.  The system should
move generally westward over the next couple of days as it is
steered by a low- to mid-level ridge to the north.  Later in the
period, the models show that Isaac could gain some latitude, but
the official forecast is along the southern side of the guidance
since a weaker system is likely to move more westward.  The new NHC
track forecast is essentially an update of the previous advisory.

FORECAST POSITIONS AND MAX WINDS

INIT  14/0300Z 14.9N  64.4W   35 KT  40 MPH
 12H  14/1200Z 15.0N  66.5W   35 KT  40 MPH
 24H  15/0000Z 15.0N  69.1W   30 KT  35 MPH
 36H  15/1200Z 15.0N  71.4W   30 KT  35 MPH
 48H  16/0000Z 15.2N  73.3W   30 KT  35 MPH
 72H  17/0000Z 16.3N  77.5W   30 KT  35 MPH
 96H  18/0000Z 17.7N  82.0W   30 KT  35 MPH
120H  19/0000Z...DISSIPATED

$$
Forecaster Brown

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