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Hurricane Irma Discussion Number  18
NWS National Hurricane Center Miami FL       AL112017
500 PM AST Sun Sep 03 2017

The satellite presentation of Irma has degraded slightly since
this morning with the eye becoming less defined.  There is evidence
of some northerly flow beneath the cirrus outflow, which may be
disrupting the inner core and preventing Irma from strengthening.
The latest consensus of the Dvorak satellite estimates suggest that
the 100 kt initial intensity could be a little generous, but with a
NOAA aircraft headed into the hurricane it is best to wait until
data from that mission is received before making any adjustment to
the initial wind speed.

Irma has been moving more westward since the previous advisory, but
the longer-term motion estimate is still south of due west or
260/12 kt.  A strong high pressure ridge over the central Atlantic
should steer Irma westward to west-southwestward during the next
couple of days.  After that time, a turn toward the west-northwest
should occur as Irma approaches the western portion of the Atlantic
ridge.  The cross-track spread of the guidance is still relatively
small through day 5, but the ECMWF, UKMET, and HWRF are on the
southern side of the guidance envelope, with the GFS near the
middle of the envelope.  The latest NHC track is once again near
the consensus of these typically reliable models, which is between
the southern edge of the guidance and the TCVN multi-model
consensus.  The updated track is not very different from the
previous advisory, except at day 5 where it is slightly west of
the previous forecast.

Users are reminded to not focus on the exact forecast track since
strong winds and heavy rainfall extend well away from the center.
In fact, ASCAT data that arrived after the issuance of the previous
advisory indicated that the size of the tropical-storm-force wind
field has expanded, especially over the northern semicircle.  As a
result, the initial and forecast wind radii have been adjusted
accordingly.


KEY MESSAGES:
1. Irma is expected to impact the northeastern Leeward Islands by
the middle of this week as a major hurricane, accompanied by
dangerous wind, storm surge, and rainfall impacts, along with rough
surf and rip currents.  Hurricane watches have been issued for
portions of the Leeward Islands and additional hurricane or tropical
storm watches may be required on Monday.  Residents in these areas
should monitor the progress of Irma and listen to advice given by
officials.

2. Irma is expected to remain a dangerous major hurricane through
the upcoming week and could directly affect the British and U.S.
Virgin Islands, Puerto Rico, Hispaniola, the Turks and Caicos, and
the Bahamas. Residents in all of these areas should monitor the
progress of Irma and listen to advice given by officials.  Tropical
storm or hurricane watches could be issued for the British and U.S.
Virgin Islands and Puerto Rico on Monday.

3. It is too early to determine what direct impacts Irma might 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  03/2100Z 17.6N  49.8W  100 KT 115 MPH
 12H  04/0600Z 17.2N  51.4W  105 KT 120 MPH
 24H  04/1800Z 16.8N  53.5W  110 KT 125 MPH
 36H  05/0600Z 16.8N  55.8W  115 KT 130 MPH
 48H  05/1800Z 17.3N  58.2W  120 KT 140 MPH
 72H  06/1800Z 19.1N  63.5W  120 KT 140 MPH
 96H  07/1800Z 21.2N  68.8W  115 KT 130 MPH
120H  08/1800Z 23.0N  73.5W  115 KT 130 MPH

$$
Forecaster Brown

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