Record Setting Category Five Hurricane Irma May Effect The Region Early Next Week

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Hurricane Irma Discussion Number  31
NWS National Hurricane Center Miami FL       AL112017
500 PM AST Wed Sep 06 2017

The eye of Irma passed over the northernmost Virgin Islands earlier
this afternoon and it is now located just to their northwest.  The
satellite and radar presentation of the hurricane remains extremely
impressive.  An Air Force Reserve reconnaissance aircraft reported
peak SFMR winds of 156 kt and flight-level winds of 164 kt during
its mission this afternoon.  Although there have been no SFMR or
flight-level winds as high as what was observed yesterday, the
initial intensity remains 160 kt, due to the potential of
undersampling.  Dropsonde observations in the eye indicated that the
pressure rose a few millibars this morning, but the most recent
aircraft report shows that the pressure has fallen to 914 mb.

The hurricane remains on a west-northwestward motion at about 14
kt.  A high pressure ridge over the western Atlantic is forecast to
keep Irma on a west-northwestward course over the next 48 to 72
hours.  The track guidance is still in good agreement during that
period, and little change to the NHC forecast was required.  By the
weekend, a shortwave trough diving southward over the east-central
United States is expected to cause Irma to turn northwestward and
northward.  The 12Z guidance has generally shifted slightly
westward, closer to the previous NHC forecast.  As a result, little
overall change was made to the 4-5 day track forecast.  The NHC
track is once again close to the HFIP corrected consensus model.
This is also near the 12Z GEFS ensemble mean, but a little east of
the latest ECMWF ensemble mean.  Users are reminded that the average
NHC track errors at days 4 and 5 are about 175 and 225 miles,
respectively.

Low vertical wind shear and warm waters along the forecast track of
Irma should allow it to remain a very powerful hurricane during the
next several days, and the intensity forecast is again near the
upper-end of the guidance and is the same as the previous advisory
through 96 hours.  Increasingly southwesterly shear and potential
land interaction late in the period is expected to cause some
decrease in Irma's strength by day 5.

Efforts to provide the forecast models with as much data as possible
continue, with 6-hourly NWS balloon launches across much of the
continental United States, and the NOAA G-IV aircraft currently
sampling the environment around the storm.

KEY MESSAGES:

1. Irma is a potentially catastrophic category 5 hurricane and will
bring life-threatening wind, storm surge, and rainfall hazards to
Puerto Rico tonight, the northern coast of Hispaniola Thursday, and
the Turks and Caicos and southeastern and central Bahamas Thursday
and Friday.

2. Hurricane watches are in effect for the northwestern Bahamas and
much of Cuba.  Irma is likely to bring dangerous wind, storm surge,
and rainfall to portions of these areas on Friday and Saturday.

3. The threat of direct hurricane impacts in Florida over the
weekend and early next week has increased.  Hurricane watches could
be issued for portions of the Florida Keys and the Florida peninsula
on Thursday.

FORECAST POSITIONS AND MAX WINDS

INIT  06/2100Z 18.8N  65.4W  160 KT 185 MPH
 12H  07/0600Z 19.6N  67.6W  155 KT 180 MPH
 24H  07/1800Z 20.7N  70.4W  150 KT 175 MPH
 36H  08/0600Z 21.5N  73.1W  145 KT 165 MPH
 48H  08/1800Z 22.2N  75.6W  140 KT 160 MPH
 72H  09/1800Z 23.2N  79.0W  135 KT 155 MPH
 96H  10/1800Z 26.0N  80.2W  125 KT 145 MPH
120H  11/1800Z 31.0N  81.0W  105 KT 120 MPH

$$
Forecaster Brown

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