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TROPICAL STORM MARTY DISCUSSION NUMBER  11
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP172015
400 AM CDT TUE SEP 29 2015

Conventional satellite imagery shows a rapid decay of the deep
convection overnight, and that convection is now separated to the
east of the partially exposed surface circulation center. The 20-30
kt of westerly shear indicated on the UW-CIMSS analysis is
drastically affecting the vertical structure of the cyclone as well,
as seen in microwave imagery. As a result, the initial intensity is
decreased to 55 kt based on a compromise of the subjective and
objective satellite intensity estimates.  Strong, persistent,
vertical shear and possibly upwelling of cooler waters should cause
the cyclone to quickly weaken to a depression in 36 hours, and
degenerate into a remnant low in 2 days. With the exception of the
ECMWF and UKMET models, which show the cyclone lingering as a
remnant low beyond day 4, all of the other large- scale models, and
the statistical-dynamical guidance, show dissipation in 4 days or
less. The NHC forecast follows the latter scenario, and also shows
the cyclone weakening to a remnant low by 48 hours.

Marty has been moving a bit erratically during the past 6 hours, and
the best motion estimate is a temporary drift toward the east.
Deterministic and ensemble guidance indicate that this motion,
within the weak steering flow produced by a mid-level trough to the
northwest of the cyclone, should continue through today.  Afterward,
Marty is expected to turn back toward the northwest and west-
northwest as a shallow, weaker cyclone, during the next 36 hours as
a low to mid-level high pressure ridge builds over southern Mexico.
The official forecast is weighted heavily on the TVCX multi-model
consensus and is a little bit faster and to the south of the
previous advisory.

FORECAST POSITIONS AND MAX WINDS

INIT  29/0900Z 16.4N 101.7W   55 KT  65 MPH
 12H  29/1800Z 16.5N 101.4W   45 KT  50 MPH
 24H  30/0600Z 16.7N 101.7W   35 KT  40 MPH
 36H  30/1800Z 16.9N 102.4W   30 KT  35 MPH
 48H  01/0600Z 17.2N 103.4W   25 KT  30 MPH...POST-TROP/REMNT LOW
 72H  02/0600Z 17.2N 105.0W   25 KT  30 MPH...POST-TROP/REMNT LOW
 96H  03/0600Z...DISSIPATED

$$
Forecaster Roberts

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