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TROPICAL STORM MARTY DISCUSSION NUMBER  12
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP172015
1000 AM CDT TUE SEP 29 2015

Satellite imagery and data from the Mexican radar at Acapulco
indicate that Marty continues to lose organization due to the
effects of 30 kt of westerly shear.  The convection is occurring in
episodic bursts, and the low-level center is located near the
southwestern edge of the bursts.  The initial intensity is decreased
to 50 kt based on 1200 UTC satellite intensity estimates from TAFB
and SAB and the subsequent decay in the cloud pattern.

Marty continues to drift erratically with the initial motion now
120/3.  The mid- to upper-level trough north of the system is
starting to weaken and split, with an upper-level low developing
northwest of Marty and a mid-level ridge developing north of the
storm.  This evolution should result in Marty beginning a west-
northwestward to westward motion during the next 24-48 hours, and
all the guidance that does not quickly dissipate the cyclone
supports this scenario.  The new forecast track is a little south of
the previous track based on the initial position, and it is also
faster in moving Marty westward.  However, it is slower than the
model consensus.  On the new forecast track, it is less likely that
the center of Marty will make landfall on the coast of Mexico.

The dynamical models forecast the current shear to continue for the
next 36-48 hours, which should cause Marty to steadily, if not
rapidly, weaken.  The new intensity forecast, which is in best
agreement with the LGEM model, is an update of the previous forecast
in calling for Marty to weaken to a tropical depression in 24-36
hours and then degenerate into a remnant low by 48 hours.  It
should be noted that this forecast lies on the upper edge of the
intensity guidance envelope, so Marty could weaken faster than
currently forecast.  One extra day has been added to the remnant low
phase based on the forecasts from the ECMWF, UKMET, and Canadian
models.


FORECAST POSITIONS AND MAX WINDS

INIT  29/1500Z 16.2N 101.6W   50 KT  60 MPH
 12H  30/0000Z 16.4N 101.6W   45 KT  50 MPH
 24H  30/1200Z 16.6N 102.2W   35 KT  40 MPH
 36H  01/0000Z 16.8N 103.3W   30 KT  35 MPH
 48H  01/1200Z 16.9N 104.1W   25 KT  30 MPH...POST-TROP/REMNT LOW
 72H  02/1200Z 17.0N 106.0W   25 KT  30 MPH...POST-TROP/REMNT LOW
 96H  03/1200Z 17.0N 107.5W   20 KT  25 MPH...POST-TROP/REMNT LOW
120H  04/1200Z...DISSIPATED

$$
Forecaster Beven

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