[Tropical] Tropical Cyclone Discussion

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WTNT45 KNHC 140837
TCDAT5

TROPICAL STORM IAN DISCUSSION NUMBER   8
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL102016
500 AM AST WED SEP 14 2016

The cloud pattern of Ian has changed little during the past several
hours.  The low-level center of the tropical storm is completely
exposed and located more than 100 n mi south of the main area of
deep convection.  This poor structure is the result of about 30 kt
of south-southwesterly vertical wind shear.  The initial intensity
is held at 45 kt, in agreement with the earlier scatterometer data
and a Dvorak CI-number from TAFB.

Ian is moving northward at about 13 kt, and is being steered
by the southerly flow between a mid- to upper-level low to its west
and a ridge to its east.  A northward to north-northeastward motion
at about the same forward speed is forecast during the next 24 hours
as the steering pattern more or less persists.  After that time, a
shortwave trough is expected to move east of Atlantic Canada, and
that should cause Ian to turn northeastward and sharply accelerate
until the cyclone becomes absorbed by a large extratropical low in 3
to 4 days. The models are in good agreement, and the official
forecast lies closest to a consensus of the GFS and ECMWF models.

The strong shear that has been affecting Ian since genesis is
expected to let up some during the next 24 to 36 hours.  The
predicted lower shear, fairly warm water, and baroclinic forcing
should allow Ian to strengthen a little during the next day or two.
Beyond a couple of days, Ian is forecast to move over sharply
cooler sea surface temperatures and into an environment of strong
shear.  These conditions should cause the cyclone to lose
its tropical characteristics in 2 to 3 days.  The NHC intensity
forecast is an update of the previous one, and lies near the middle
of the guidance envelope.

FORECAST POSITIONS AND MAX WINDS

INIT  14/0900Z 29.3N  53.0W   45 KT  50 MPH
 12H  14/1800Z 31.3N  53.2W   50 KT  60 MPH
 24H  15/0600Z 34.0N  52.3W   50 KT  60 MPH
 36H  15/1800Z 37.2N  49.2W   55 KT  65 MPH
 48H  16/0600Z 41.6N  43.6W   55 KT  65 MPH
 72H  17/0600Z 53.4N  28.0W   55 KT  65 MPH...POST-TROP/EXTRATROP
 96H  18/0600Z...DISSIPATED

$$
Forecaster Cangialosi
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