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TROPICAL STORM MARTY DISCUSSION NUMBER   7
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP172015
400 AM CDT MON SEP 28 2015

The tropical cyclone is producing a large area of deep convection
and cloud tops colder than -80 degrees Celsius.  Although the
low-level center is embedded within the cold cloud canopy, earlier
microwave data showed that it was located slightly west of the
convection due to some westerly shear.  Subjective and objective
T-numbers are between 3.5 and 4.0 on the Dvorak scale, so the
initial wind speed has been increased to 60 kt.

Based on the recent satellite and microwave fixes the initial
motion is estimated to be 360/2 kt.  Marty remains in an area of
weak steering currents, as the tropical cyclone is embedded within
the base of a mid- to upper-level trough that extends southwestward
from the eastern Texas coast.  The tropical storm is forecast to
drift northward or northeastward during the next day or so.  The
model guidance is in disagreement on how close Marty will get to the
coast of Mexico during the next 36-48 hours.  The GFS continues to
take Marty inland, while the GFS ensemble mean and the ECMWF show
less of a northeastward motion and keep the tropical cyclone
offshore.  The latest runs of the GFDL and HWRF have trended
eastward and both models show landfall in a couple of days.  The NHC
track will remain between these various solutions, close to the GFS
ensemble mean and the Florida State Superensemble during the first
36 to 48 hours.  After that time, the upper-level trough is forecast
to lift northeastward, while a ridge builds over northwestern
Mexico. This should cause Marty to move westward or west-
northwestward between 72 and 120 hours.

The tropical storm is expected to remain in an environment of
moderate westerly shear during the next 12-24 hours, and little
change in strength is expected in the short term.  Increasing
vertical shear and potentially cooler SSTs due to upwelling are
expected to cause a gradual decrease in strength beginning on
Tuesday.  Weakening should then continue throughout the remainder of
the forecast period while Marty remains in an area of moderate to
strong southwesterly shear.  The NHC intensity forecast is a little
below the statistical guidance during the first 24 hours, but
follows a blend of the SHIPS/LGEM models during the remainder of
the period.


FORECAST POSITIONS AND MAX WINDS

INIT  28/0900Z 15.6N 103.0W   60 KT  70 MPH
 12H  28/1800Z 16.0N 102.7W   60 KT  70 MPH
 24H  29/0600Z 16.3N 102.3W   55 KT  65 MPH
 36H  29/1800Z 16.6N 102.1W   55 KT  65 MPH
 48H  30/0600Z 16.9N 102.2W   45 KT  50 MPH
 72H  01/0600Z 17.0N 103.5W   35 KT  40 MPH
 96H  02/0600Z 17.0N 105.0W   30 KT  35 MPH
120H  03/0600Z 17.5N 106.5W   25 KT  30 MPH...POST-TROP/REMNT LOW

$$
Forecaster Brown

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