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Tropical Storm Irma Discussion Number   1
NWS National Hurricane Center Miami FL       AL112017
1100 AM AST Wed Aug 30 2017

Satellite images indicate that the low pressure area in the far
eastern Atlantic has become much better organized since yesterday,
with many curved bands around the center.  ASCAT data showed peak
winds of about 42 kt, and after considering the small size of the
circulation and some undersampling due to the resolution of the
instrument, the initial wind speed is set to 45 kt.  Global models
indicate that the upper-level winds are likely to be favorable for
strengthening of Irma during the next several days.  However, Irma
will be moving over more marginal water temperatures and into drier
mid-level conditions, which should temper the intensification rate.
The NHC solution is a blend of the intensity consensus and the
statistical-dynamical hurricane models SHIPS and LGEM.  At the end
of the period, the forecast could turn out to be conservative if the
very conducive environment shown in most of the global models
emerges.

The initial motion estimate is 280/11.  A ridge over the eastern
Atlantic is forecast to steer Irma westward over the next few days.
Thereafter, the ridge builds southwestward, which will likely cause
the storm to move, somewhat unusually, toward the west-southwest.
The official forecast puts more weight on the global models than the
regional hurricane models, which appear to have a northward bias on
this cycle.  Thus, the NHC track prediction is on the southwestern
side of the guidance envelope, although not as far in that direction
as the ECMWF or its ensemble mean.

FORECAST POSITIONS AND MAX WINDS

INIT  30/1500Z 16.4N  30.3W   45 KT  50 MPH
 12H  31/0000Z 16.7N  31.9W   50 KT  60 MPH
 24H  31/1200Z 17.3N  33.8W   55 KT  65 MPH
 36H  01/0000Z 17.9N  35.7W   60 KT  70 MPH
 48H  01/1200Z 18.2N  37.6W   65 KT  75 MPH
 72H  02/1200Z 18.7N  41.9W   70 KT  80 MPH
 96H  03/1200Z 17.7N  46.5W   75 KT  85 MPH
120H  04/1200Z 17.0N  51.0W   80 KT  90 MPH

$$
Forecaster Blake

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