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TROPICAL STORM MARTY DISCUSSION NUMBER   4
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP172015
1000 AM CDT SUN SEP 27 2015

First-light visible imagery shows that Marty has developed a
central dense overcast with a large outer convective band in the
eastern semicircle.  A recent SSM/IS overpass showed a well-defined
convective band, but did not show an eye or eyewall underneath the
overcast.  Various subjective and objective satellite intensity
estimates range from 35 kt to 65 kt, so the initial intensity is
raised to 50 kt as a compromise.  The cirrus outflow is good to the
east but non-existent to the west, likely due to the the effects of
20 kt of westerly vertical shear.

Marty remains in a moist airmass and over sea surface temperatures
of near 30C.  However, all of the dynamical guidance suggests the
cyclone should experience increasing shear during the next 72
hours. The guidance is in agreement that Marty should strengthen
for another 24 hours or so, followed by significant weakening.
Based on this, the new intensity forecast is an update of the
previous forecast.  However, it should be noted that this is a low
confidence forecast due to uncertainty over how much land
interaction may occur as Marty approaches the coast of Mexico.

The initial motion is 015/6.  Marty is located to the west of a
mid-level ridge which extends across Central America, and to the
south of a deep-layer trough located over Mexico and the southern
United States.  The cyclone should turn northeastward at a slower
forward speed during the next 24-48 hours based on the consensus of
the track guidance.  After that time, there is some divergence in
the guidance.  The GFS moves Marty northeastward over Mexico.  On
the other hand, the ECMWF and UKMET forecast Marty to shear apart,
with the low-level center turning westward and remaining offshore.
Based on the guidance and the current trends, the new track
forecast is similar to the previous forecast in that it keeps Marty
offshore through the forecast period.  However, it is shifted
closer to the coast than the previous forecast.

A tropical storm warning may be required for portions of the coast
of Mexico later today.

FORECAST POSITIONS AND MAX WINDS

INIT  27/1500Z 14.9N 102.8W   50 KT  60 MPH
 12H  28/0000Z 15.4N 102.7W   60 KT  70 MPH
 24H  28/1200Z 16.0N 102.2W   65 KT  75 MPH
 36H  29/0000Z 16.3N 101.8W   65 KT  75 MPH
 48H  29/1200Z 16.7N 101.5W   55 KT  65 MPH
 72H  30/1200Z 17.0N 102.0W   40 KT  45 MPH
 96H  01/1200Z 17.5N 103.0W   30 KT  35 MPH
120H  02/1200Z 17.0N 104.5W   20 KT  25 MPH...POST-TROP/REMNT LOW

$$
Forecaster Beven

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