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Hurricane Irma Forecast Discussion Number 6
2017-08-31 22:32:35| National Hurricane Center (Atlantic)
Issued at 500 PM AST Thu Aug 31 2017 000 WTNT41 KNHC 312032 TCDAT1 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. FORECAST POSITIONS AND MAX WINDS 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
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