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Hurricane Irma Discussion Number  11
NWS National Hurricane Center Miami FL       AL112017
1100 PM AST Fri Sep 01 2017

After developing a nearly clear eye during the afternoon hours,
Irma appears to have once again temporarily peaked.  A WindSat pass
around 2100 UTC hinted at the beginning of another eyewall
replacement cycle, which would be consistent with the observed
cooling of the eye.  However, the small size of the hurricane's
inner core relative to the resolution of the microwave instrument
makes it impossible for me to say for sure.  The initial intensity
has been lowered slightly to 100 kt, but it should be stressed that
this is probably just another fluctuation, in what will likely
be a long string of small changes in intensity over the next
several days.  Since we do not have the ability to predict
such changes, the NHC forecast shows very gradual intensification
throughout the forecast period, given the warm SSTs and increasing
moisture content along the forecast track.  The NHC forecast is near
the intensity consensus, but slightly favors the higher dynamical
models.

The initial motion remains 275/12 kt.  The hurricane has moved a
little farther north than previously expected, and the track has
been adjusted in that direction.  Overall, the reasoning behind the
track forecast has not changed, and Irma is still expected to turn
west-southwestward on Saturday due to a building ridge over the
central Atlantic.  After about 72 h, there remains a large
north-south spread in the guidance, with the GFS continuing to show
a weaker ridge (and a northern track), while the ECMWF shows a
stronger ridge/southern track.  The ECMWF has performed better for
Irma thus far, so my forecast continues to favor that solution.

KEY MESSAGES:

1. Irma is expected to be a major hurricane when it approaches 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 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.

FORECAST POSITIONS AND MAX WINDS

INIT  02/0300Z 19.1N  40.5W  100 KT 115 MPH
 12H  02/1200Z 19.0N  42.3W  100 KT 115 MPH
 24H  03/0000Z 18.5N  44.6W  105 KT 120 MPH
 36H  03/1200Z 17.9N  46.9W  110 KT 125 MPH
 48H  04/0000Z 17.3N  49.0W  110 KT 125 MPH
 72H  05/0000Z 16.8N  53.3W  110 KT 125 MPH
 96H  06/0000Z 18.0N  57.5W  115 KT 130 MPH
120H  07/0000Z 20.5N  62.5W  115 KT 130 MPH

$$
Forecaster Zelinsky

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