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TROPICAL STORM IAN DISCUSSION NUMBER   6
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL102016
500 PM AST TUE SEP 13 2016

Ian continues to be sheared, and the low-level center is estimated
to be near the southern edge of a ragged-looking area of deep
convection.  Since there has been no appreciable increase in the
organization of the tropical cyclone, the advisory intensity is held
at 40 kt, which is the mean of Dvorak intensity estimates from TAFB
and SAB.  Given that the shear is forecast to remain strong enough
to inhibit strengthening for the next 24 hours or so, with some
relaxation thereafter, the official intensity forecast shows some
slow strengthening beginning tomorrow night.  The official forecast
is in good agreement with the latest LGEM guidance.  In 72 hours,
the global models predict the cyclone to have a frontal structure
over the north Atlantic, so the official forecast shows Ian as
extratropical by that time.

The storm has sped up a bit, and the initial motion is now near
340/14 kt.  Over the next day or so, Ian will continue to move
through a weakness in the mid-level subtropical ridge.  Later, when
the cyclone moves into the mid-latitude westerlies, a shortwave
trough approaching from the west should cause Ian to accelerate
north-northeastward to northeastward.  The official track forecast
is faster than the previous one, but not quite as fast as the
latest ECMWF and GFS solutions.  This is close to the multi-model
consensus and the FSU Superensemble.

FORECAST POSITIONS AND MAX WINDS

INIT  13/2100Z 27.0N  52.7W   40 KT  45 MPH
 12H  14/0600Z 29.1N  53.3W   40 KT  45 MPH
 24H  14/1800Z 31.8N  53.6W   40 KT  45 MPH
 36H  15/0600Z 34.5N  53.0W   45 KT  50 MPH
 48H  15/1800Z 37.5N  50.5W   50 KT  60 MPH
 72H  16/1800Z 48.5N  37.0W   50 KT  60 MPH...POST-TROP/EXTRATROP
 96H  17/1800Z 58.0N  22.0W   50 KT  60 MPH...POST-TROP/EXTRATROP
120H  18/1800Z 64.5N   8.0W   45 KT  50 MPH...POST-TROP/EXTRATROP

$$
Forecaster Pasch

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