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HURRICANE MARTY DISCUSSION NUMBER  10
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP172015
1000 PM CDT MON SEP 28 2015

Marty's convective cloud pattern has been steadily shrinking over
the past few hours, accompanied by a sharp decrease in cloud top
temperatures. In addition, a 29/0002 UTC AMSU overpass indicates
that the low-level and mid-/upper-level circulations are starting to
decouple, with the low-level center lagging back to the west as a
result of strong westerly vertical wind shear impinging on the small
hurricane. The initial intensity has been lowered to 65 kt based on
a blend of satellite intensity estimates of T4.0/65 from TAFB and
T3.5/55 kt from SAB, and a UW-CIMSS ADT value of T4.0.

Microwave data over the past few hours indicate that Marty has
slowed and turned toward the east-northeast, and is now moving
070/03 kt. There remains a distinct bifurcation in the model
guidance with the GFS, HRWF, UKMET, NAVGEM, moving a stronger Marty
steadily east-northeastward and inland over Mexico in 24-36 hours,
whereas the ECMWF, GFS-Ensemble Mean, GFDL, and GFDN models keep a
weaker Marty offshore before turning it westward at around 48 hours.
Given the recent sharp downward trend in the convective structure of
Marty, along with an expected increase in the deep-layer vertical
wind shear to 25-30 kt by 18-24 hours, the official forecast leans
toward the weaker and more westward model solutions, which is
consistent with the previous advisory and the consensus model TVCE.

With vertical shear values of 30 kt or greater expected in the
12-36 hour period, steady weakening appears likely during that
time period. After that time, additional weakening is likely due to
the expected shallow vortex having a difficult time regenerating and
maintaining deep convection within a progressively drier mid-level
environment. Proximity to the mountainous landmass of south-central
Mexico could also provide a potential weakening factor. The new NHC
intensity forecast follows the weakening trend of the previous
advisory, and lies close to the IVCN intensity consensus model.

FORECAST POSITIONS AND MAX WINDS

INIT  29/0300Z 16.7N 102.0W   65 KT  75 MPH
 12H  29/1200Z 16.8N 101.6W   65 KT  75 MPH
 24H  30/0000Z 16.9N 101.5W   55 KT  65 MPH
 36H  30/1200Z 17.2N 101.5W   50 KT  60 MPH
 48H  01/0000Z 17.4N 102.1W   40 KT  45 MPH
 72H  02/0000Z 17.6N 103.0W   30 KT  35 MPH
 96H  03/0000Z 18.0N 106.8W   20 KT  25 MPH...POST-TROP/REMNT LOW
120H  04/0000Z...DISSIPATED

$$
Forecaster Stewart

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