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TROPICAL STORM IAN DISCUSSION NUMBER   3
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL102016
1100 PM AST MON SEP 12 2016

Ian remains a sheared tropical cyclone, with the strongest
convection displaced to the northeast and east of the center.
However, moderate convection has recently developed over and just
north of the center due to the vertical wind shear's backing
from a southwesterly to southerly direction. Data from a recent
partial ASCAT-B overpass indicate that the 34-kt wind field has
expanded to at least 200 nmi in the northeastern quadrant, and that
peak winds have also increased to more than 35 kt. Dvorak satellite
intensity estimates are T3.0/45 kt from TAFB and T2.5/35 kt from
SAB, and an average of these values yields an advisory intensity of
40 kt.

Ian has maintained a steady motion of 330/11 kt. The latest NHC
model guidance is in excellent agreement on the storm continuing to
move north-northwestward through a break in the Bermuda-Azores High
for the next 24 hours, followed by a turn to the north at 36-48
hours. After that, Ian is expected to gradually accelerate as the
cyclone gets captured by a strong mid-latitude shortwave trough. The
new official forecast track is essentially just an update of the
previous advisory track and lies close to the GFS-ECMWF model
consensus, and the multi-model consensus TVCN.

The vertical shear is forecast to decrease to 15-20 kt in 36 to 48
hours when an upper-level low, currently located about 400 nmi
northwest of the cyclone, is forecast by the GFS, UKMET, and ECMWF
models to move over and become colocated with Ian's low-level
center. The combination of decreasing vertical wind shear and some
infusion of baroclinic energy associated with this complex
interaction is expected to produce at least some slight
strengthening by days 2-3. Around 120 hours, Ian is forecast to
interact with a frontal zone over the north Atlantic and be
transformed into an extratropical cyclone.

FORECAST POSITIONS AND MAX WINDS

INIT  13/0300Z 23.5N  51.5W   40 KT  45 MPH
 12H  13/1200Z 24.9N  52.4W   40 KT  45 MPH
 24H  14/0000Z 26.9N  53.4W   40 KT  45 MPH
 36H  14/1200Z 29.4N  54.1W   45 KT  50 MPH
 48H  15/0000Z 31.8N  54.0W   50 KT  60 MPH
 72H  16/0000Z 38.0N  49.7W   50 KT  60 MPH
 96H  17/0000Z 48.0N  38.0W   50 KT  60 MPH
120H  18/0000Z 56.5N  25.2W   50 KT  60 MPH...POST-TROP/EXTRATROP

$$
Forecaster Stewart

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