000
WTPZ42 KNHC 281446
TCDEP2

TROPICAL STORM MARTY DISCUSSION NUMBER   8
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP172015
1000 AM CDT MON SEP 28 2015

Marty continues to produce a large area of deep convection,
although the overall convective appearance is a little stretched.
Microwave data show that, while the center is embedded within the
convection, the southwesterly portion of the circulation is
partially exposed due to the southwesterly shear.  The initial wind
speed remains 60 kt, a blend of the TAFB and SAB Dvorak estimates.
An Air Force Reserve Hurricane Hunter aircraft should be in the
area for the afternoon advisory to get a better estimate of the
winds.

The storm is drifting northward, caught in an area of weak steering
currents while 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 northeastward during the next
day or so, but the model guidance remains in disagreement on how
close Marty will get to the coast of Mexico during the next 36-48
hours.  The GFS and HWRF take Marty inland, while the GFS and ECMWF
ensemble means keep the tropical cyclone farther offshore. Usually
with such different guidance from reasonable models it makes good
sense to stay close to the model consensus, which results in little
net change to the NHC forecast during the first 36 to 48 hours.
After that time, the mid- to 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, and the official forecast is a little farther
westward at those times.

Little change in strength is expected with Marty over the next day
or so while the storm remains in an environment of moderate
west-southwesterly shear.  Increasing vertical shear and potentially
cooler SSTs due to upwelling are expected to cause a gradual
weakening to begin on Tuesday.  This trend should then continue
throughout the remainder of the forecast period while Marty remains
in an area of moderate-to-strong southwesterly shear.  Most of the
guidance, except the ECMWF, shows the cyclone dissipating by day
5 due to the persistent shear.  The latest NHC forecast is a blend
of the previous one and the intensity consensus, although I
consider it more uncertain than normal at long range due to the
track uncertainities.


FORECAST POSITIONS AND MAX WINDS

INIT  28/1500Z 16.1N 103.0W   60 KT  70 MPH
 12H  29/0000Z 16.3N 102.9W   60 KT  70 MPH
 24H  29/1200Z 16.6N 102.5W   55 KT  65 MPH
 36H  30/0000Z 16.8N 102.4W   50 KT  60 MPH
 48H  30/1200Z 17.0N 102.8W   45 KT  50 MPH
 72H  01/1200Z 17.3N 105.0W   35 KT  40 MPH
 96H  02/1200Z 17.5N 106.5W   30 KT  35 MPH
120H  03/1200Z 18.0N 108.0W   25 KT  30 MPH...POST-TROP/REMNT LOW

$$
Forecaster Blake