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Tropical Storm Isaac Discussion Number   9
NWS National Hurricane Center Miami FL       AL092018
500 PM AST Sun Sep 09 2018

Isaac is a tiny tropical cyclone, but even still, recent SSMIS
data showed that it has developed a mid-level microwave eye
feature.  At the time of the pass (1800 UTC), the convection was a
little thin on the southern side, but geostationary satellite images
suggest that it has filled in since that time.  Dvorak intensity
estimates are T4.0 from TAFB and T3.5 from SAB, so the maximum
winds are estimated to be 60 kt.

The intensity forecast philosophy has not changed since this
morning.  Isaac is strengthening in an environment of low shear and
over warm sea surface temperatures, and for the next 36-48 hours,
the NHC intensity forecast is above the bulk of the intensity
models.  The main reason for this is that Isaac's tiny size could
allow the intensity to increase quickly within the favorable
environment.  However, the cyclone's tiny size will likely also be
to its detriment after 48 hours when northwesterly vertical shear is
expected to develop and increase to 20-30 kt.  The shear will
probably easily decouple the small system, causing the intensity to
decrease much faster than suggested by most of the intensity models.
For that reason, the official NHC intensity forecast is below the
intensity consensus on days 3 through 5.  It should be noted that
both the GFS and ECMWF models show a weakening cyclone moving into
the eastern Caribbean Sea by days 4 and 5, with the GFS even making
the system an open wave by the end of the forecast period.  These
global model solutions lend credence to the belief that it may be
difficult for Isaac to maintain hurricane status while it approaches
the Lesser Antilles.

Isaac is accelerating toward the west with an initial motion of
275/10 kt.  Ridging to the north is expected to keep the cyclone on
a westward trajectory for the entire forecast period, with
acceleration continuing for the next 36 hours.  The UKMET remains
the biggest outlier, showing Isaac turning northwestward and
northward into the central Atlantic after 48 hours.  That still
appears to be unlikely at this time, and the NHC track forecast is
still close to the center of the guidance envelope.  Only a slight
northward shift was made to the new forecast based on the latest
model solutions.


Key Messages:

1. Isaac is expected to become a hurricane tonight or on Monday
while it moves westward across the tropical Atlantic Ocean.

2. While Isaac is forecast to begin weakening by Tuesday while it
approaches the Lesser Antilles, it is still expected to be at or
near hurricane intensity when it reaches the islands, and the
uncertainty in the intensity forecast is higher than usual.
Interests in the Lesser Antilles should continue to monitor Isaac
during the next few days.


FORECAST POSITIONS AND MAX WINDS

INIT  09/2100Z 14.5N  40.3W   60 KT  70 MPH
 12H  10/0600Z 14.6N  42.2W   70 KT  80 MPH
 24H  10/1800Z 14.8N  45.0W   80 KT  90 MPH
 36H  11/0600Z 14.9N  48.0W   85 KT 100 MPH
 48H  11/1800Z 14.9N  50.9W   85 KT 100 MPH
 72H  12/1800Z 15.0N  56.5W   70 KT  80 MPH
 96H  13/1800Z 15.5N  63.0W   60 KT  70 MPH
120H  14/1800Z 15.5N  69.5W   50 KT  60 MPH

$$
Forecaster Berg

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