ZCZC MIATCDAT5 ALL
TTAA00 KNHC DDHHMM

TROPICAL STORM IAN DISCUSSION NUMBER  12
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL102016
500 AM AST THU SEP 15 2016

Ian continues to have a structure somewhat resembling a subtropical
cyclone, with the low-level center situated beneath an upper-level
low and a cloud pattern featuring a broken band of convection.
However, there is still some moderate to deep convection not too far
from the center, and the most recent AMSU pass still showed a deep
warm core structure.  The initial intensity remains 45 kt in
agreement with the latest satellite classification of ST3.5 from
TAFB.  Ian is running out of time to strengthen via diabatic
processes, and most of the strengthening shown here is expected to
be due to baroclinic effects, especially given the current
convective structure.  By 24 hours, strong shear and cool SSTs
should result in Ian being post-tropical, and the cyclone should be
fully extratropical by 36 hours.  The intensity forecast is based
mainly on the global models and guidance from the Ocean Prediction
Center, and shows the post-tropical cyclone intensifying to near
hurricane strength before it's absorbed by a larger extratropical
low in 48 hours.

The initial motion estimate is 025/17.  Ian should accelerate
northeastward ahead of a large upper-level trough moving eastward
across the northwestern Atlantic during the next 36 hours before
being absorbed.  The new NHC track forecast is similar to the
previous one and near the middle of the guidance envelope.

The initial 34-kt wind radii were adjusted based on a partial ASCAT
pass from around 0100 UTC.

FORECAST POSITIONS AND MAX WINDS

INIT  15/0900Z 35.5N  52.4W   45 KT  50 MPH
 12H  15/1800Z 38.3N  49.8W   50 KT  60 MPH
 24H  16/0600Z 43.0N  43.7W   55 KT  65 MPH...POST-TROPICAL
 36H  16/1800Z 49.1N  35.5W   60 KT  70 MPH...POST-TROP/EXTRATROP
 48H  17/0600Z...ABSORBED BY ANOTHER EXTRATROPICAL LOW

$$
Forecaster Brennan

NNNN