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TROPICAL STORM MARTY DISCUSSION NUMBER   6
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP172015
1000 PM CDT SUN SEP 27 2015

Recent infrared and microwave satellite imagery indicate that a
strong burst of deep convection has developed in the northwestern
quadrant of the circulation, with very cold cloud tops having spread
southwestward over the previously exposed low-level circulation
center.  This increase in Marty's cloud pattern suggests that the
cyclone has strengthened despite moderate to strong westerly
vertical wind shear. The initial intensity has been increased to 55
kt based on a Dvorak satellite intensity estimate of T3.5/55 kt from
both TAFB and SAB.

The initial motion is an uncertain 360/02 kt. Overall, Marty doesn't
appear to have moved much, if at all, since the previous advisory.
Steering currents are weak since the cyclone is embedded in the base
of a broad deep-layer trough that extends from the Texas coast
southward across central Mexico and into the eastern North Pacific.
Only a slow north or northeastward drift if expected for the next
two days. The high terrain of southern Mexico is likely helping to
induce a narrow ridge just offshore the Pacific coast, which should
prevent the cyclone from gaining much latitude. The global and
regional models, excluding the GFS model, maintain some degree of
low- to mid-level ridging throughout the forecast period, which
gradually forces Marty on a westward track over water after 48
hours. In contrast, the GFS model marches the cyclone inland over
Mexico in about 48 hours, but this is considered to be an outlier
scenario since the GFS-Ensemble Mean keeps Marty well offshore
similar to the ECMWF and the other model solutions. The official
forecast track is essentially just an update of the previous track
through 72 hours, and then is a little slower than the previous
advisory after that as per the latest TVCE consensus model solution.

The overall atmospheric and oceanic environments surrounding Marty
are expected to essentially remain unchanged for the next 24 hours
or so. As a result, little change in intensity is expected during
that time. In the 36-48 hour time period, however, the global models
are forecasting the vertical wind shear to back around to the
southwest and increase to around 25 kt, which should induce steady
weakening through at least 72 hours. By 96 hours, the shear is
forecast to sharply decrease to around 10 kt or less, possibly
giving Marty a chance to re-strengthen on days 4 and 5 if a decent
low-level circulation still remains at that time. The new NHC
intensity forecast follows the trend of the previous advisory in
calling for Marty to become a depression by around 96 hours and a
remnant low by 120 hours. However, the remnant low forecast is still
somewhat uncertain.

Given that Marty has slowed down considerably and is expected to
move somewhat little slower than indicated in the previous advisory,
a tropical storm warning is not necessary at this time. However, a
tropical storm warning could be required for portions of the coast
of Mexico by Monday morning.

FORECAST POSITIONS AND MAX WINDS

INIT  28/0300Z 15.3N 103.0W   55 KT  65 MPH
 12H  28/1200Z 15.7N 102.6W   55 KT  65 MPH
 24H  29/0000Z 16.2N 102.2W   55 KT  65 MPH
 36H  29/1200Z 16.6N 102.0W   55 KT  65 MPH
 48H  30/0000Z 16.8N 102.3W   45 KT  50 MPH
 72H  01/0000Z 16.9N 103.5W   35 KT  40 MPH
 96H  02/0000Z 17.0N 105.0W   30 KT  35 MPH
120H  03/0000Z 17.2N 106.0W   25 KT  30 MPH...POST-TROP/REMNT LOW

$$
Forecaster Stewart

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