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Tropical Storm Irma Forecast Discussion Number 4
2017-08-31 10:44:57| National Hurricane Center (Atlantic)
Issued at 500 AM AST Thu Aug 31 2017 000 WTNT41 KNHC 310844 TCDAT1 Tropical Storm Irma Discussion Number 4 NWS National Hurricane Center Miami FL AL112017 500 AM AST Thu Aug 31 2017 Irma continues to become better organized with a developing CDO and a few banding features. The overall cloud pattern is fairly symmetric, with fair upper-level outflow over all but the southeastern portion of the circulation. The current intensity is set at 60 kt, which is the mean of Dvorak estimates from TAFB and SAB. The environment ahead of Irma appears to be mostly conducive for intensification, with increasingly warm SSTs. Vertical shear is expected to remain low as an upper-level trough to the west-northwest of the tropical cyclone lifts out to the north and west. The only noticeable inhibiting factor is slightly drier mid-level air in 2-5 days, but this will probably not be much of a deterrent for strengthening. The official intensity forecast follows the latest consensus of the models, and is similar to the previous NHC prediction. Based on geostationary satellite fixes, the initial motion is just slightly north of west or 280/10 kt. Most of the track guidance shows a west-northwestward motion for the next day or so. Irma will remain situated to the south of a well-defined mid-tropospheric ridge through the forecast period. Much of the guidance indicates that Irma will turn toward a slightly south of westward heading in a couple of days, presumably in response to some building of high pressure to the north and northwest. The official track forecast also shows this, and remains on the southern side of the guidance suite. This is mainly a blend of the GFS and ECMWF solutions. FORECAST POSITIONS AND MAX WINDS INIT 31/0900Z 16.5N 32.9W 60 KT 70 MPH 12H 31/1800Z 17.0N 34.2W 70 KT 80 MPH 24H 01/0600Z 17.6N 36.0W 80 KT 90 MPH 36H 01/1800Z 18.0N 37.9W 85 KT 100 MPH 48H 02/0600Z 18.2N 40.1W 90 KT 105 MPH 72H 03/0600Z 17.5N 45.0W 100 KT 115 MPH 96H 04/0600Z 16.7N 49.5W 105 KT 120 MPH 120H 05/0600Z 16.0N 53.5W 105 KT 120 MPH $$ Forecaster Pasch
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