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TROPICAL STORM MARTY DISCUSSION NUMBER   3
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP172015
400 AM CDT SUN SEP 27 2015

ASCAT data from a few hours ago indicate that Marty's maximum winds
have increased to 45 kt.  The center is embedded beneath a
persistent burst of deep convection, and strong convective bands
trail to the south and southwest of the central dense overcast.
The structure suggests there could be a little bit of westerly
shear, which UW-CIMSS and the SHIPS diagnostics are showing to be
about 10-15 kt.

Marty is over sea surface temperatures of at least 30 degrees
Celsius, and the vertical shear is expected to be steady for the
next 24 hours or so.  These conditions should allow the cyclone to
continue strengthening, and in fact, rapid intensification is a
possibility.  The SHIPS RI guidance is showing a 64 percent chance
of a 25-kt increase in winds during the next 24 hours, and the
SHIPS and LGEM models bring Marty at or just below hurricane
intensity at that time.  The dynamical models are a little bit
less aggressive with the future intensity, but given that Marty is
already a little stronger from the get-go, the more aggressive
statistical guidance seems to be the more likely scenario.
Vertical shear is expected to increase by 36 hours, which is
expected to lead to a quick weakening trend during the latter part
of the forecast period.  The updated NHC intensity forecast is
adjusted upward a bit through 48 hours to account for the higher
initial intensity and the latest guidance, and is actually very
similar to the Florida State Superensemble.

Marty is located to the west of a mid-level ridge which extends
across Central America, and to the south of a deep-layer trough
located over Mexico and the southern United States.  This pattern
is steering the cyclone northward, or 360/5 kt.  Marty should turn
northeastward but slow down during the next 48 hours as the
steering currents weaken.  After the vertical shear increases,
possibly displacing the deep convection away from the center, the
circulation is expected to stall and then turn westward between
days 3 through 5, keeping the center just offshore of the coast of
Mexico.  The GFS and the NAVGEM are the only models that bring
Marty inland over Mexico, but these solutions do not appear likely
at this time, especially since even the GFS ensemble mean stays
offshore.  Despite these model differences, no significant changes
were required to the NHC track forecast from the previous advisory.

FORECAST POSITIONS AND MAX WINDS

INIT  27/0900Z 14.3N 103.1W   45 KT  50 MPH
 12H  27/1800Z 15.0N 102.9W   55 KT  65 MPH
 24H  28/0600Z 15.6N 102.5W   65 KT  75 MPH
 36H  28/1800Z 16.0N 102.1W   65 KT  75 MPH
 48H  29/0600Z 16.2N 101.8W   60 KT  70 MPH
 72H  30/0600Z 16.5N 101.6W   40 KT  45 MPH
 96H  01/0600Z 17.0N 103.0W   30 KT  35 MPH
120H  02/0600Z 16.5N 105.0W   20 KT  25 MPH...POST-TROP/REMNT LOW

$$
Forecaster Berg

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