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Hurricane Irma Discussion Number  13
NWS National Hurricane Center Miami FL       AL112017
1100 AM AST Sat Sep 02 2017

Hurricane Irma continues to display an eye within a small central
dense overcast in the visible and infrared imagery from the GOES
and METEOSAT satellites.  Dvorak current intensity numbers from
TAFB, SAB, and the CIMSS ADT have not changed, so the intensity
remains 95 kt.

The environment in which Irma is embedded shows mixed signals for
the next few days.  While the vertical shear remains low through
Monday, the SSTs are lukewarm and mid-level humidities are dry.
(Indeed an overnight SSMIS microwave pass showed a distinct finger
of dry air wrapping around the south side of Irma not far from its
inner core.)  However, after the waters warm and the atmosphere
moistens at days 3-5, the shear is forecast to go up.  The guidance
has substantial spread between Category 2 and 4 by the end of the
forecast period.  The official intensity forecast is based upon a
blend of the LGEM/DSHP statistical techniques and the HWRF dynamical
model and is unchanged from the previous advisory.

Irma is finally moving slightly south of due west at 13 kt.  A
large, building Azores-Bermuda high should turn Irma toward the
west-southwest at a slower rate of speed during the next two days.
By days 3 to 5, Irma should be rounding the southwestern periphery
of the high and start turning back toward the west-northwest. The
track guidance is in tight agreement with this scenario and the
official forecast is a simply an update from the previous advisory.

Irma is a small hurricane, as observed by ship BATFR17 and the
overnight ASCAT scatterometer pass with tropical-storm-force winds
extending out at most about 60 nm. The official size forecast is
based upon the RVCN multi-model consensus technique.

KEY MESSAGES:

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.


FORECAST POSITIONS AND MAX WINDS

INIT  02/1500Z 18.8N  43.3W   95 KT 110 MPH
 12H  03/0000Z 18.5N  45.0W   95 KT 110 MPH
 24H  03/1200Z 18.0N  47.4W  100 KT 115 MPH
 36H  04/0000Z 17.4N  49.5W  105 KT 120 MPH
 48H  04/1200Z 17.1N  51.6W  110 KT 125 MPH
 72H  05/1200Z 17.4N  56.0W  110 KT 125 MPH
 96H  06/1200Z 19.2N  60.6W  115 KT 130 MPH
120H  07/1200Z 22.0N  66.0W  115 KT 130 MPH

$$
Forecaster Landsea

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