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Tropical Storm Isaac Discussion Number   5
NWS National Hurricane Center Miami FL       AL092018
500 PM AST Sat Sep 08 2018

The structure of the cyclone, while still a shear pattern, is
improving, with the low-level center now near the convective
overcast and improved outflow in the eastern semicircle.  Satellite
intensity estimates have increased to the 30-45 kt range, with the
CIMSS satellite consensus technique now suggesting winds near 35
kt.  Based on these estimates, the initial intensity is increased to
35 kt and the depression is upgraded to Tropical Storm Isaac.

Isaac is now moving slowly westward, with the initial motion 275/6.
The subtropical ridge to the north of Isaac is forecast to steer the
system westward with an increasing forward speed for at least the
next 2-3 days.  After that time, there is some uncertainty in the
track forecast due to the formation of a mid- to upper-level trough
over the central Atlantic, with some of the track guidance showing a
decrease in forward speed and a more northerly motion near the end
of the forecast period.  The GFS and ECMWF models show a continued
westerly motion through that time, and the official forecast follows
that scenario, although with a slower forward speed than in the
previous forecast.  The new forecast is again similar to the
earlier forecast and lies near the various consensus models.

Conditions appear favorable for gradual strengthening during the
next 72 h as Isaac moves westward across the warm waters of the
tropical Atlantic.  In best agreement with the SHIPS model for the
next three days, the new intensity forecast is raised from the
previous forecast during this time.  From 72-120 h, the system is
expected to encounter westerly wind shear, which is expected to
cause some weakening.


FORECAST POSITIONS AND MAX WINDS

INIT  08/2100Z 14.5N  36.6W   35 KT  40 MPH
 12H  09/0600Z 14.6N  37.7W   40 KT  45 MPH
 24H  09/1800Z 14.7N  39.5W   50 KT  60 MPH
 36H  10/0600Z 14.7N  42.0W   60 KT  70 MPH
 48H  10/1800Z 14.7N  44.6W   65 KT  75 MPH
 72H  11/1800Z 14.5N  50.0W   75 KT  85 MPH
 96H  12/1800Z 14.5N  55.5W   75 KT  85 MPH
120H  13/1800Z 15.0N  61.0W   70 KT  80 MPH

$$
Forecaster Beven

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