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Tropical Storm ISAAC


Tropical Storm Isaac Discussion Number  25
NWS National Hurricane Center Miami FL       AL092018
500 PM AST Thu Sep 13 2018

The center of Isaac has been mostly exposed during the past several
hours as temporary bursts of convection fire just east of the
center.  Assuming some spin down of the circulation after the plane
left, the initial wind speed is set to 35 kt.  Since there haven't
been any reports of sustained tropical-storm-force winds in the
Lesser Antilles and the center is moving away, all watches and
warning have been lowered. However, there remains a threat of flash
flooding due to a large area of rain that has yet to move through
the Lesser Antilles.

Strong shear and mid-level dry air is forecast to persist for the
next day or so, which should cause Isaac to drop back to tropical
depression status on Friday.  However, the shear is forecast to
weaken by the weekend, and moisture values are expected to rise as
the system gets deeper into the Caribbean Sea.  These conditions
would normally allow for some restregthening, but the initial
character of the system is pretty weak, and it is uncertain whether
there will be much left of Isaac to take advantage of the more
conducive conditions.  Model guidance has been inconsistent on the
future of Isaac, which speaks to the lack of predictability of this
situation.  The bulk of the guidance show Isaac staying weak or
degenerating into a tropical wave over the next few days, so the
official forecast follows that consensus.  At this point it is
just best to keep an eye on the long-range forecast on each
advisory to see if there are any changes.

Isaac is moving westward at about 15 kt.  A mid-level ridge is
forecast to persist over the southwestern Atlantic, steering Isaac
westward for the next few days, with a turn to the west-northwest
possible at long range.  The future track is very uncertain at long
range because it is dependent on the intensity.  If Isaac
degenerates into a wave, the remnants will likely move south and
west of the guidance mean, and if the cyclone is stronger, it will
probably move slower and more to the right.  The new NHC forecast
assumes that Isaac will stay weak, and therefore is on the left and
faster side of the guidance.


INIT  13/2100Z 14.9N  63.2W   35 KT  40 MPH
 12H  14/0600Z 14.9N  65.4W   35 KT  40 MPH
 24H  14/1800Z 14.9N  68.2W   30 KT  35 MPH
 36H  15/0600Z 14.9N  70.8W   30 KT  35 MPH
 48H  15/1800Z 15.0N  73.0W   30 KT  35 MPH
 72H  16/1800Z 16.2N  77.4W   30 KT  35 MPH
 96H  17/1800Z 17.5N  81.5W   30 KT  35 MPH
120H  18/1800Z...DISSIPATED

Forecaster Blake