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Hurricane IRMA


Hurricane Irma Discussion Number  15
NWS National Hurricane Center Miami FL       AL112017
1100 PM AST Sat Sep 02 2017

The cloud pattern of Irma has not changed significantly in structure
today.  The eye continues to become apparent and then hide under
the convective canopy, and this has been the observed pattern for
the past 24 hours or so. Dvorak estimates go up and down with the
presence of the eye, but an average of these numbers supports an
initial intensity of 95 kt.  A NOAA Hurricane Hunter plane will help
with the intensity estimate on Sunday.

I hesitate to speculate too much about the environment that Irma is
embedded within.  All of the standard ingredients necessary for
strengthening are forecast to be at least marginally favorable, but
none are expected to be hostile for intensification.  The NHC
forecast, which in fact is similar to the previous one, continues to
be a blend of the statistical models and the explosive strengthening
shown by the regional hurricane and global models.

The subtropical ridge building to the north of Irma has been
steering the hurricane toward the west or 260 degrees at 12
kt.  The ridge is forecast to amplify even more, and this flow
pattern will force the hurricane to dive west-southwestward for a
couple of days.  Irma should then begin to gain latitude once it
reaches the southwestern edge of the ridge in about 3 days.  The
confidence in the track forecast is high for the next 72 hours since
all of the reliable guidance is basically on top of each other.
After 3 days, when the hurricane is forecast to be approaching the
northern Leeward Islands, the guidance envelope spreads out and
becomes bounded by the southernmost tracks of the HWRF, HCCA and the
ECMWF models, and by the northernmost GFS and UK models.  The
confidence beyond 3 days is then much lower.  Tonight's NHC forecast
was adjusted a just little to the south of the previous one due to
another small shift of the guidance envelope.  The forecast is
basically on top of the multi-model consensus TVCX.

While Irma is currently a small hurricane, the size guidance
suggests it should grow in size during the next 72 h.  This will
affect how soon watches may be issued for portions of the Leeward
and Virgin Islands.


1. Irma is expected to be a major hurricane when it moves closer to
the Lesser Antilles early next week, producing rough surf and rip
currents.  Irma could also cause dangerous wind, storm surge, and
rainfall impacts on some islands, although it is too soon to specify
where and when those hazards could occur.  Residents in the Lesser
Antilles should monitor the progress of Irma through the weekend and
listen to any advice given by local officials.

2. It is much too early to determine what direct impacts Irma will
have on the Bahamas and the continental United States.  Regardless,
everyone in hurricane-prone areas should ensure that they have their
hurricane plan in place, as we are now near the peak of the season.


INIT  03/0300Z 18.3N  46.2W   95 KT 110 MPH
 12H  03/1200Z 17.8N  48.0W   95 KT 110 MPH
 24H  04/0000Z 17.2N  50.1W  100 KT 115 MPH
 36H  04/1200Z 16.7N  52.3W  105 KT 120 MPH
 48H  05/0000Z 16.5N  54.5W  110 KT 125 MPH
 72H  06/0000Z 17.7N  59.3W  110 KT 125 MPH
 96H  07/0000Z 19.9N  64.5W  115 KT 130 MPH
120H  08/0000Z 22.5N  69.5W  115 KT 130 MPH

Forecaster Avila