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Hurricane Irma Discussion Number  32
NWS National Hurricane Center Miami FL       AL112017
1100 PM AST Wed Sep 06 2017

Earlier radar observations from the San Juan WSR-88D showed a
concentric eyewall, and observations from an Air Force Hurricane
Hunter aircraft showed a secondary wind maximum over the northwest
quadrant.  These phenomena may be associated with some weakening,
but since the central pressure hasn't risen much the intensity is
held at 160 kt at this time.  Moreover, the concentric eyewall has
become less apparent in recent radar imagery.  Considerable
lightning activity has been noted in the eyewall of Irma, which
research has shown to sometimes be a harbinger of weakening.
Notwithstanding, 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.  The official intensity
forecast is similar to the previous one except at day 5 when
stronger shear should cause more weakening.  This forecast is
generally above the intensity model consensus.

Irma continues on a west-northwestward heading at around 14 kt.  A
ridge of high pressure over the western Atlantic should maintain
this course for the next 48 to 72 hours.  Later in the forecast
period a shortwave trough moving into the southeastern United
States is likely to turn the hurricane northwestward to
north-northwestward.  There has been a slight eastward shift in
some of the track guidance models, but since the models could shift
back to the west it is prudent to make little change to the
official forecast at this time.  In any event, users are reminded
not to focus on the exact track since the average NHC track
errors at days 4 and 5 are about 175 and 225 miles, respectively.

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 will
likely be issued for portions of the Florida Keys and the Florida
peninsula on Thursday.


FORECAST POSITIONS AND MAX WINDS

INIT  07/0300Z 19.4N  66.8W  160 KT 185 MPH
 12H  07/1200Z 20.3N  68.8W  155 KT 180 MPH
 24H  08/0000Z 21.3N  71.7W  150 KT 175 MPH
 36H  08/1200Z 22.1N  74.1W  145 KT 165 MPH
 48H  09/0000Z 22.7N  76.3W  140 KT 160 MPH
 72H  10/0000Z 24.0N  79.3W  135 KT 155 MPH
 96H  11/0000Z 27.4N  80.3W  120 KT 140 MPH
120H  12/0000Z 32.0N  81.0W   95 KT 110 MPH

$$
Forecaster Pasch

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