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Hurricane IRMA


Hurricane Irma Discussion Number   6
NWS National Hurricane Center Miami FL       AL112017
500 PM AST Thu Aug 31 2017

Irma has become an impressive hurricane with intense eyewall
convection surrounding a small eye.  Satellite estimates continue to
rapidly rise, and the Dvorak classifications from both TAFB & SAB
support an initial wind speed of 100 kt.  This is a remarkable 50-kt
increase from yesterday at this time.

Microwave and satellite data suggest that an eyewall replacement
cycle could be starting.  This isn't surprising given how small the
eye is, and will probably be the first of many eyewall cycles for
this hurricane.  Overall, Irma should be in a low-shear environment
for several days, with the intensity controlled by eyewall cycles
and the moderately warm SSTs along the path.  Thus the forecast
intensity is leveled off for the next 2 days.  After the weekend,
Irma should be moving over much warmer water, with SSTs forecast to
be 29C at the end of the period.  All indications are for Irma to be
strengthening by the end of the forecast period, with the NHC
prediction adjusted slightly upward from the previous one, in line
with the extremely low pressures forecast by the global and regional
hurricane models at that time.

Irma continues moving west-northwestward, now at about 10 kt.
There has been no change to the forecast philosophy, with the
hurricane likely to turn westward and west-southwestward over the
next few days due to a building ridge over the central Atlantic.  At
long range, however, model guidance is not in good agreement on the
strength of the ridge, resulting in some significant north-south
differences in the global models.  I am inclined to stay on the
southwestern side of the model guidance, given the rather consistent
forecasts of the ECMWF and its ensemble.  In addition, the strongest
members of the recent ensembles are on the southern side on the
consensus, giving some confidence in that approach.


INIT  31/2100Z 17.3N  34.8W  100 KT 115 MPH
 12H  01/0600Z 17.8N  36.2W  105 KT 120 MPH
 24H  01/1800Z 18.2N  38.3W  105 KT 120 MPH
 36H  02/0600Z 18.3N  40.7W  105 KT 120 MPH
 48H  02/1800Z 17.9N  42.9W  105 KT 120 MPH
 72H  03/1800Z 16.8N  47.5W  110 KT 125 MPH
 96H  04/1800Z 16.0N  52.0W  115 KT 130 MPH
120H  05/1800Z 16.5N  56.5W  120 KT 140 MPH

Forecaster Blake