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TROPICAL STORM IAN DISCUSSION NUMBER   7
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL102016
1100 PM AST TUE SEP 13 2016

Ian's cloud pattern has changed little in organization since this
afternoon.  The low-level center remains exposed to the south of a
fan-shaped mass of deep convection.  This structure is consistent
with a deep-layer southwesterly vertical shear of nearly 40 kt
associated with a mid- to upper-tropospheric cyclone to the
west of the cyclone. A 0018Z ASCAT pass showed reliable surface
winds near 45 kt well north of the center, which agrees with a TAFB
satellite classifciation of T3.0.  On this basis, the initial
intensity is increased to 45 kt.

The initial motion estimate has been nudged a little more to the
right than earlier today and is now 345/11 kt.  Ian should continue
to move generally northward into a weakness in the subtropical ridge
over the central Atlantic during the next 24 hours or so.  A
significant acceleration toward the northeast should occur in about
36 hours when a shortwave trough moving through Atlantic Canada
approaches Ian.  The NHC track forecast is adjusted to the right of
the previous one, especially after 24 hours, and is in agreement
with the the latest track consensus aids.

The SHIPS output indicates that the shear should decrease,
beginning in about 12 hours, roughly around the time that Ian
becomes co-located with the upper low to its west.  The shear is
forecast to reach a relative minimum 24 hours later while the flow
aloft gradually becomes more diffluent.  This baroclinic forcing is
expected to result in some intensification while Ian remains over
warm waters, though the cyclone could possess attributes of both a
tropical and an extratropical cyclone. Extratropical transition is
indicated in 2 to 3 days, with absorption by a larger extratropical
storm system over the North Atlantic likely around 72 hours.  The
NHC intensity forecast is about the same as the previous one and
near the multi-model consensus.

FORECAST POSITIONS AND MAX WINDS

INIT  14/0300Z 27.6N  52.8W   45 KT  50 MPH
 12H  14/1200Z 29.8N  53.2W   45 KT  50 MPH
 24H  15/0000Z 32.3N  53.1W   50 KT  60 MPH
 36H  15/1200Z 35.2N  51.3W   55 KT  65 MPH
 48H  16/0000Z 39.4N  47.0W   55 KT  65 MPH
 72H  17/0000Z 49.7N  33.7W   55 KT  65 MPH...POST-TROP/EXTRATROP
 96H  18/0000Z...ABSORBED

$$
Forecaster Kimberlain

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