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Hurricane IRMA (Text)


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Hurricane Irma Discussion Number  33...Corrected
NWS National Hurricane Center Miami FL       AL112017
500 AM AST Thu Sep 07 2017

Corrected to modify Key Messages to reflect the issuance of a
hurricane warning for the northwestern Bahamas.

Irma has become a little less organized during the past few hours.
Data from an Air Force reserve Hurricane Hunter aircraft near 0500
UTC indicated that the central pressure had risen to 921 mb and
that the winds had decreased both at the 700 mb flight-level and in
surface estimates from the Stepped Frequency Microwave Radiometer.
Since that time, the eye has become cloud filled and the central
convection has become somewhat ragged.  The initial intensity is
lowered to 155 kt, and this could be a little generous.  The next
aircraft is scheduled to reach Irma around 1200 UTC.

The initial motion is 290/15.  The hurricane is currently being
steered by the subtropical ridge to the north, and for the next
48 h or so this motion is expected to continue with a decrease in
the forward speed.  After 48 h, a mid- to upper-level trough
digging into the eastern United States is expected to create a
break in the ridge and allow Irma to turn northward.  The timing of
the turn is the most important question and one still filled with
uncertainty.  The UKMET, UK Ensemble mean, and the NAVGEM are the
models showing the latest turn, and they forecast Irma to move into
the southeastern Gulf of Mexico and near the west coast of the
Florida Peninsula.  The ECMWF and ECMWF Ensemble mean are in the
middle of the model pack and show Irma moving over the southeastern
portion of the Florida Peninsula.  The GFS, Canadian, and GFS
Ensemble mean show the earliest turn and show Irma moving east of
the coast of Florida toward the southeastern United States.  The
new forecast track will best follow the ECMWF, as well as the
Florida State Superensemble and the HFIP Corrected Consensus, and
it calls for the center to move over portions of the southeastern
Florida Peninsula between 72-96 h and then across the Atlantic into
southern South Carolina by 120 h.   Users are again reminded not to
focus on the exact track since the average NHC track errors at days
3, 4, and 5 are about 120, 175, and 225 miles, respectively.

Irma should remain in warm-water, and a low shear environment for
about the next three days, and thus is expected to remain a strong
hurricane, most likely Category 5 or 4.  Fluctuations in intensity
are likely during the time due to internal eyewall replacement
cycles.  The large-scale models suggest that shear could increase
starting at about 72 hr.  However, due to the uncertainty as to
whether this will happen, the intensity forecast keeps Irma at
category 4 strength until landfall in Florida.  The 96-120 h points
now have a lower intensity due to the forecast of shear and land
interaction, but Irma is still expected to be a hurricane when it
reaches the southeastern United States.

Since Irma is moving away from Puerto Rico and the radars there, the
hourly position estimates are discontinued after this advisory.


KEY MESSAGES:

1. Irma is a potentially catastrophic category 5 hurricane and will
bring life-threatening wind, storm surge, and rainfall hazards to
the northern coast of Hispaniola today, the Turks and Caicos
tonight, and the Bahamas tonight through Saturday.

2. A hurricane watch is in effect 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 continues to increase.  Hurricane
watches will likely be issued for portions of the Florida Keys and
the Florida peninsula later this morning.


FORECAST POSITIONS AND MAX WINDS

INIT  07/0900Z 20.0N  68.3W  155 KT 180 MPH
 12H  07/1800Z 20.7N  70.5W  150 KT 175 MPH
 24H  08/0600Z 21.7N  73.1W  145 KT 165 MPH
 36H  08/1800Z 22.3N  75.5W  140 KT 160 MPH
 48H  09/0600Z 22.8N  77.4W  135 KT 155 MPH
 72H  10/0600Z 24.5N  80.0W  130 KT 150 MPH
 96H  11/0600Z 28.5N  80.5W  105 KT 120 MPH
120H  12/0600Z 33.0N  81.0W   75 KT  85 MPH...INLAND

$$
Forecaster Beven/Brennan

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