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Hurricane Irma Discussion Number  26
NWS National Hurricane Center Miami FL       AL112017
1100 AM AST Tue Sep 05 2017

Irma is an extremely impressive hurricane in both infrared and
visible satellite images.  Experimental GOES-16 one-minute visible
satellite pictures show a distinct 25-30 n mi wide eye with several
mesovortices rotating within with eye.  The aircraft have not
sampled the northeastern eyewall where the strongest winds were
measured shortly before 1200 UTC this morning, but the Air Force
plane will be entering the eye in that quadrant momentarily.  A peak
SFMR wind of 154 kt was reported, with a few others of 149-150 kt.
Based on these data the initial intensity is set at 155 kt for this
advisory. This makes Irma the strongest hurricane in the Atlantic
basin outside of the Caribbean Sea and Gulf of Mexico in the NHC
records.

Irma is expected to remain within low vertical wind shear, a moist
mid-level atmosphere, and high upper-ocean heat content as it moves
west-northwestward during the next several days. These conditions
should allow the hurricane to remain very intense throughout much
of the forecast period, however, fluctuations in intensity are
likely to occur as eyewall replacement cycles take place.  The NHC
intensity forecast is near the upper-end of the guidance and assumes
little overall interaction of Irma with the islands of the Greater
Antilles.

Irma continues to move westward at about 12 kt, and a strong
subtropical ridge centered over the central Atlantic should steer
Irma generally westward today. The ridge is expected to remain in
place over the western Atlantic during the next several days and
Irma is forecast to move west-northwestward throughout the most of
remainder of the forecast period.  Around day 5, a shortwave trough
dropping southward over the central United States is expected to
begin eroding the western portion of the ridge, allowing a Irma to
gain some latitude.  The new NHC track forecast is close to the
HFIP corrected consensus model and is very similar to the previous
forecast.

Since Irma is a large hurricane, users are reminded to not focus on
the exact forecast track since tropical-storm and hurricane-force
winds and life-threatening storm surge extend far from the center.
Residents in the Leeward Islands should complete their preparations
very soon as the weather will begin to deteriorate over the
easternmost Leeward Islands later this afternoon.


KEY MESSAGES:

1. Irma is a potentially catastrophic category 5 hurricane and will
bring life-threatening wind, storm surge, and rainfall hazards to
portions of the northeastern Leeward Islands beginning later today
and the Virgin Islands and Puerto Rico beginning tomorrow.
Preparations should be rushed to completion before the arrival of
tropical-storm force winds later today in the Leeward Islands and
tomorrow morning in Virgin Islands and Puerto Rico.

2. Hurricane watches have been issued for portions of the Dominican
Republic and Haiti, the southeastern Bahamas and Turks and Caicos,
and Irma could bring dangerous wind, storm surge, and rainfall to
those areas on Thursday and Friday.

3. Irma could directly affect the remainder of the Bahamas and Cuba
as an extremely dangerous major hurricane later this week. Residents
in these areas should monitor the progress of Irma and listen to
advice given by officials.

4. The chance of direct impacts from Irma later this week and this
weekend is increasing in the Florida Keys and portions of the
Florida Peninsula. However, it is too soon to specify the timing
and magnitude of the impacts. Elsewhere, it is too early to
determine what direct impacts Irma might have on the continental
United States. Everyone in hurricane-prone areas should ensure that
they have their hurricane plan in place.

FORECAST POSITIONS AND MAX WINDS

INIT  05/1500Z 16.8N  58.4W  155 KT 180 MPH
 12H  06/0000Z 17.2N  60.3W  155 KT 180 MPH
 24H  06/1200Z 18.1N  63.0W  150 KT 175 MPH
 36H  07/0000Z 19.1N  65.9W  145 KT 165 MPH
 48H  07/1200Z 20.1N  68.7W  140 KT 160 MPH
 72H  08/1200Z 21.4N  74.0W  135 KT 155 MPH
 96H  09/1200Z 22.7N  78.3W  130 KT 150 MPH
120H  10/1200Z 24.4N  81.2W  125 KT 145 MPH

$$
Forecaster Brown

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