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Hurricane Irma Discussion Number  27
NWS National Hurricane Center Miami FL       AL112017
500 PM AST Tue Sep 05 2017

Irma continues to exhibit a remarkably impressive satellite
presentation.  The intensity was increased to 160 kt on the 1800
UTC intermediate public advisory based on a couple of SFMR winds of
160 kt measured in the northeastern eyewall by the Air Force
aircraft just prior to that time.  The minimum pressure measured
by a dropsonde in the eye was 926 mb.  Irma becomes only the fifth
Atlantic basin hurricane with a peak wind speed of 160 kt or
higher.  The others are Allen (1980), the Labor Day Hurricane
of 1935, Gilbert (1988), and Wilma (2005).

The eye of Irma is within range of the Meteo France radar in the
northeastern Caribbean, and recent images show the development of
an outer eyewall, likely the beginning stages of an eyewall
replacement.  These changes in inner-core structure will likely
result in fluctuations in intensity during the next couple of days.
Otherwise, increasing upper-ocean heat content and a very favorable
upper-level pattern are expected to allow Irma to remain a category
4 or 5 hurricane during the next several days.  Once again, the NHC
forecast shows limited interaction of the hurricane with the islands
of the Greater Antilles.

Fixes from the latest satellite and radar imagery suggest that Irma
is moving a little north of due west or 280/13 kt.  A strong ridge
extending southwestward from the central Atlantic is expected to
steer Irma west-northwestward during the couple of days.  A large
mid-latitude trough over the eastern United States is forecast to
lift northeastward, allowing the ridge to build westward and keep
Irma on a westward to west-northwestward heading through Friday.
Over the weekend, a shortwave trough diving southward over the
east-central U.S. is expected to weaken the western portion of
the ridge, causing Irma to turn poleward.  The dynamical model
guidance is in good agreement through 72 hours, but there is
increasing spread thereafter.  The HWRF, UKMET, and ECMWF show a
more southerly track and a sharper turn around day 5, while the GFS
is farther north and east late in the period.  The NHC track is near
a consensus of these models and close to the HFIP corrected
consensus. Users are reminded to not focus on the exact forecast
track, especially at the longer ranges, since the average NHC track
errors are about 175 and 225 statute miles at days 4 and 5,
respectively.

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 tonight and tomorrow.
These hazards will spread into the Virgin Islands and Puerto
Rico tomorrow.  Preparations should be rushed to completion before
the arrival of tropical-storm force winds tomorrow morning in Virgin
Islands and Puerto Rico.

2. A hurricane warning is in effect for the northern coast of the
Dominican Republic, with hurricane watches for Haiti, the
southeastern Bahamas and the Turks and Caicos.  Irma is likely to
bring dangerous wind, storm surge, and rainfall to these areas from
Wednesday night through 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 beginning later this week
and this weekend from wind, storm surge, and rainfall continues to
increase in the Florida Keys and portions of the Florida Peninsula.
However, it is too soon to specify the timing and magnitude of these
impacts.

FORECAST POSITIONS AND MAX WINDS

INIT  05/2100Z 17.1N  59.8W  160 KT 185 MPH
 12H  06/0600Z 17.6N  61.8W  155 KT 180 MPH
 24H  06/1800Z 18.5N  64.6W  150 KT 175 MPH
 36H  07/0600Z 19.5N  67.3W  145 KT 165 MPH
 48H  07/1800Z 20.4N  70.1W  140 KT 160 MPH
 72H  08/1800Z 21.6N  75.3W  135 KT 155 MPH
 96H  09/1800Z 22.7N  79.3W  125 KT 145 MPH
120H  10/1800Z 24.4N  81.5W  120 KT 140 MPH

$$
Forecaster Brown

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