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Tropical Storm Irma Discussion Number   4
NWS National Hurricane Center Miami FL       AL112017
500 AM AST Thu Aug 31 2017

Irma continues to become better organized with a developing CDO and
a few banding features.  The overall cloud pattern is fairly
symmetric, with fair upper-level outflow over all but the
southeastern portion of the circulation.  The current intensity
is set at 60 kt, which is the mean of Dvorak estimates from TAFB
and SAB.  The environment ahead of Irma appears to be mostly
conducive for intensification, with increasingly warm SSTs.
Vertical shear is expected to remain low as an upper-level trough
to the west-northwest of the tropical cyclone lifts out to the
north and west.  The only noticeable inhibiting factor is slightly
drier mid-level air in 2-5 days, but this will probably not be much
of a deterrent for strengthening.  The official intensity forecast
follows the latest consensus of the models, and is similar to the
previous NHC prediction.

Based on geostationary satellite fixes, the initial motion is just
slightly north of west or 280/10 kt.  Most of the track guidance
shows a west-northwestward motion for the next day or so.  Irma
will remain situated to the south of a well-defined
mid-tropospheric ridge through the forecast period.  Much of the
guidance indicates that Irma will turn toward a slightly south of
westward heading in a couple of days, presumably in response to some
building of high pressure to the north and northwest.  The official
track forecast also shows this, and remains on the southern side of
the guidance suite.  This is mainly a blend of the GFS and ECMWF
solutions.

FORECAST POSITIONS AND MAX WINDS

INIT  31/0900Z 16.5N  32.9W   60 KT  70 MPH
 12H  31/1800Z 17.0N  34.2W   70 KT  80 MPH
 24H  01/0600Z 17.6N  36.0W   80 KT  90 MPH
 36H  01/1800Z 18.0N  37.9W   85 KT 100 MPH
 48H  02/0600Z 18.2N  40.1W   90 KT 105 MPH
 72H  03/0600Z 17.5N  45.0W  100 KT 115 MPH
 96H  04/0600Z 16.7N  49.5W  105 KT 120 MPH
120H  05/0600Z 16.0N  53.5W  105 KT 120 MPH

$$
Forecaster Pasch

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