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Hurricane IRMA (Text)


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Hurricane Irma Discussion Number  14
NWS National Hurricane Center Miami FL       AL112017
500 PM AST Sat Sep 02 2017

Hurricane Irma continues to display an eye within a small central
dense overcast, although the eye has been going through periods
where it becomes less well defined.  The satellite intensity
estimates from TAFB and SAB remain 90 kt, while the latest CIMMS
ADT and satellite consensus technique estimates ARE 95-105 KT.
Since there has been little overall change in organization since
the last advisory, the initial intensity remains 95 kt.

The environment in which Irma is embedded continues to show mixed
signals during the forecast period, and the intensity guidance
responds to this by ranging between little and slow intensification.
The hurricane is currently suffering some impact of sea surface
temperatures of about 27C and mid-level dry air entrainment.  Later
in the period, Irma should encounter warmer water and increasing
moisture at a time when the vertical wind shear may be increasing.
Given the uncertainty on when all of the ingredients may come
together, the new intensity forecast is the same as the previous
forecast and calls for gradual intensification through the next
5 days.  An alternative forecast scenario is that Irma gets
significantly stronger than forecast near the end of the forecast
period if the shear is less than currently expected.

The initial motion remains 265/13.  A large and building subtropical
ridge should steer Irma generally west-southwestward during the next
two days or so.  Between 72-120 h, Irma should be rounding the
southwestern periphery of the ridge and start turning back toward
the west-northwest.  While the track guidance remains in good
agreement with this scenario, from 72-120 h there has been a
westward shift of the guidance that results in the new forecast
track coming 30-60 n mi closer to the Leeward and Virgin Islands
than in the previous advisory.  This latter portion of the track
lies near the center of the guidance envelope,  but with the ECMWF
and corrected consensus models to the south and the GFS to the
north.

While Irma is currently a small hurricane, the size guidance
suggests it should grow in size during the next 72 h.  This will
affect how soon watches may be issued for portions of the Leeward
and Virgin Islands.


KEY MESSAGES:

1. Irma is expected to be a major hurricane when it moves closer to
the Lesser Antilles early next week, producing rough surf and rip
currents.  Irma could also cause dangerous wind, storm surge, and
rainfall impacts on some islands, although it is too soon to specify
where and when those hazards could occur.  Residents in the Lesser
Antilles should monitor the progress of Irma through the weekend and
listen to any advice given by local officials.

2. It is much too early to determine what direct impacts Irma will
have on the Bahamas and 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  02/2100Z 18.5N  44.6W   95 KT 110 MPH
 12H  03/0600Z 18.0N  46.5W   95 KT 110 MPH
 24H  03/1800Z 17.4N  48.8W  100 KT 115 MPH
 36H  04/0600Z 17.0N  50.9W  105 KT 120 MPH
 48H  04/1800Z 16.8N  53.0W  110 KT 125 MPH
 72H  05/1800Z 17.5N  57.5W  110 KT 125 MPH
 96H  06/1800Z 19.5N  62.5W  115 KT 130 MPH
120H  07/1800Z 22.0N  68.0W  115 KT 130 MPH

$$
Forecaster Beven

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Page last modified: Saturday, 16-Dec-2017 12:09:22 UTC