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Tropical Storm ISAAC


Tropical Storm Isaac Discussion Number   8
NWS National Hurricane Center Miami FL       AL092018
1100 AM AST Sun Sep 09 2018

WindSat microwave data from several hours ago show that Isaac has
developed a well-defined low-level ring in the 37-GHz channel, and
the center is embedded in the middle of a small central dense
overcast.  TAFB and SAB Dvorak estimates have increased to T3.5
given Isaac's improved structure, so the initial intensity is raised
to 55 kt.

Isaac's future intensity is the most challenging part of the
forecast.  Since sea surface temperatures will be warm (between
27-28 degrees Celsius) and vertical shear will be almost
non-existent (less than 10 kt) for the next 36-48 hours, the tiny
storm will be prone to a possibly significant increase in intensity
during the next couple of days.  For that period, the NHC intensity
forecast is above the guidance, and above the previous forecast, due
to relatively high numbers from the various rapid intensification
indices.  However, in 2-3 days, the global models are showing
northwesterly shear steadily increasing and potentially reaching a
maximum of around 30 kt by Thursday.  The big question is the
timing of this shear and how much it will affect Isaac's intensity
before the cyclone reaches the Lesser Antilles.  The HWRF, which
brings Isaac to major hurricane intensity despite the forecast
shear, appears to be an outlier, and is therefore pulling up the
ICON intensity consensus and HCCA.  As a result, the NHC intensity
forecast is below ICON on days 4 and 5.  This new forecast shows a
more peaked maximum intensity, with Isaac possibly increasing and
then decreasing in strength at faster rates than shown here.

While the microwave data indicate that Isaac's center is slightly
south of previous estimates, the current motion estimate is 270/8
kt.  A low- to mid-level ridge is expected to move westward in
tandem with Isaac for much of the forecast period, causing the
cyclone to accelerate toward the west, reaching a forward speed of
around 15 kt by 36 hours, and then holding that speed through day
5. With the exception of the UKMET and HWRF models, the guidance
envelope is tightly packed from north to south, and the speed
differences noted over the past few days have decreased.  The
updated NHC track forecast is just a little south of the previous
one to account for the adjusted initial position, and lies slightly
south of the multi-model consensus in the direction of HCCA and the
Florida State Superensemble.


INIT  09/1500Z 14.3N  39.1W   55 KT  65 MPH
 12H  10/0000Z 14.4N  40.8W   65 KT  75 MPH
 24H  10/1200Z 14.4N  43.3W   75 KT  85 MPH
 36H  11/0000Z 14.4N  46.3W   85 KT 100 MPH
 48H  11/1200Z 14.3N  49.1W   85 KT 100 MPH
 72H  12/1200Z 14.2N  54.8W   80 KT  90 MPH
 96H  13/1200Z 14.5N  60.5W   70 KT  80 MPH
120H  14/1200Z 15.0N  66.5W   60 KT  70 MPH

Forecaster Berg