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Tropical Storm Isaac Discussion Number  21
NWS National Hurricane Center Miami FL       AL092018
500 PM AST Wed Sep 12 2018

The satellite presentation of Isaac hasn't changed much in the past
several hours, with the center exposed on the west side of a
re-developing band of convection.  On the last pass of the NOAA
Hurricane Hunter aircraft, peak flight-level winds were 61 kt and
SFMR values were about 50 kt.  These data support an initial wind
speed estimate of 50 kt.  It is interesting to note that the
surface center is estimated to be a tenth of a degree or two south
of the well-defined swirl on satellite--a function of the fast
forward speed and high shear.

The shear is forecast to continue for the next day or two, and
gradual weakening is anticipated, in line with the model
guidance.  Given the strong shear, it is possible that Isaac could
degenerate into a wave near the Lesser Antilles.  After that time,
Isaac could escape the upper-level trough over the central Atlantic
that has been shearing the cyclone, and find a more conducive
environment. More of the model guidance is suggesting some
reintensification of the cyclone in the western Caribbean Sea, with
the biggest hold-out being the ECMWF.  Continuity dictates that the
forecast not change as much as the guidance, most of which is
showing a hurricane at long range, so the compromise solution is no
longer dissipating the cyclone and flat-lining the intensity at
long range.  It almost goes without saying that this is a
low-confidence intensity forecast.

Isaac has accelerated to near 17 kt over the past 12 hours, and the
shorter-term motion is closer to 20 kt with the convection-less
center. A mid-tropospheric high is forecast to persist for the next
several days, which should keep the cyclone moving westward, albeit
at a slightly slower pace.  The model guidance has shifted a bit to
the north on this cycle, but is mostly in response to the initial
latitude.  The NHC track forecast is moved northward as well but
still lies on the south side of the consensus.  At long range, it
makes sense when forecasting a weaker system to stay on the left
and faster side of the consensus, so that's where the new NHC
prediction lies.


Key Messages:

1. Isaac is expected to remain at tropical storm intensity when it
moves across the Lesser Antilles on Thursday, and tropical storm
warnings are in effect for Martinique, Dominica, and Guadeloupe.
Tropical storm watches are in effect for Montserrat, St. Kitts and
Nevis, Antigua, and Saba and St. Eustatius.  Interests on those
islands should follow any advice given by their local officials.

2. Life-threatening flash flooding is possible with Isaac.  The
storm is expected to produce total rainfall accumulations of 2 to 4
inches with isolated amounts up 8 inches across Martinique,
Dominica and Guadeloupe. Rainfall amounts of 1 to 2 inches with
isolated amounts to 4 inches are expected across Puerto Rico and the
southern United States Virgin Islands.


FORECAST POSITIONS AND MAX WINDS

INIT  12/2100Z 15.4N  56.6W   50 KT  60 MPH
 12H  13/0600Z 15.6N  59.0W   50 KT  60 MPH
 24H  13/1800Z 15.8N  61.8W   45 KT  50 MPH
 36H  14/0600Z 16.0N  64.8W   45 KT  50 MPH
 48H  14/1800Z 16.0N  67.6W   40 KT  45 MPH
 72H  15/1800Z 16.0N  72.5W   40 KT  45 MPH
 96H  16/1800Z 16.5N  76.5W   40 KT  45 MPH
120H  17/1800Z 17.5N  80.5W   40 KT  45 MPH

$$
Forecaster Blake

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