ZCZC MIATCDAT5 ALL
TTAA00 KNHC DDHHMM

TROPICAL STORM IAN DISCUSSION NUMBER   4
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL102016
500 AM AST TUE SEP 13 2016

Ian is a strongly sheared tropical storm.  The low-level center of
the cyclone is completely exposed with the main area of deep
convection located about 120 n mi northeast of the center.  This
asymmetric cloud pattern is the result of about 30 kt of
southwesterly vertical wind shear.  The initial intensity is held
at 40 kt for this advisory, in agreement with the earlier ASCAT
data.  This estimate is a little higher than the latest Dvorak
classifications.

Satellite fixes indicate that Ian has slowed down during the last
several hours.  The initial motion estimate, using a 12-h average,
is 335/8 kt.  The tropical cyclone is situated between a mid- to
upper-level low to its west and a mid-level high to its east.  This
pattern is expected to persist for the next couple of days, and
should cause Ian to move north-northwestward to northward with a
slight increase in forward speed.  After that time, a shortwave
trough is expected to approach the tropical storm, and it will
likely cause Ian to turn northeastward and accelerate as it becomes
embedded in the mid-latitude flow.  The models are in fair agreement
in this overall scenario, and only small changes were made to the
previous NHC track forecast.  This prediction is in best agreement
with the various consensus aids.

The strong shear currently affecting Ian is expected to persist for
another 24 hours, so little change in strength is predicted during
that time.  The shear is forecast to lessen some in a couple of days
while Ian is still over warm water.  These environmental conditions,
combined with some baroclinic forcing, could allow for slight
strengthening in the 36- to 72-h time period.  After that time, Ian
is expected to move back into a strong shear environment and over
progressively colder water, which should lead to extratropical
transition in about 4 days.  The NHC intensity forecast is largely
the same as the previous one, and is in best agreement with the
SHIPS guidance during its tropical phase and the global models
during the extratropical portion.

FORECAST POSITIONS AND MAX WINDS

INIT  13/0900Z 24.0N  51.7W   40 KT  45 MPH
 12H  13/1800Z 25.4N  52.5W   40 KT  45 MPH
 24H  14/0600Z 27.8N  53.4W   40 KT  45 MPH
 36H  14/1800Z 30.4N  53.8W   45 KT  50 MPH
 48H  15/0600Z 33.2N  53.1W   50 KT  60 MPH
 72H  16/0600Z 40.7N  46.1W   50 KT  60 MPH
 96H  17/0600Z 50.7N  32.0W   50 KT  60 MPH...POST-TROP/EXTRATROP
120H  18/0600Z 58.5N  18.5W   45 KT  50 MPH...POST-TROP/EXTRATROP

$$
Forecaster Cangialosi

NNNN