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Hurricane Irma Forecast Discussion Number 13
2017-09-02 16:56:18| National Hurricane Center (Atlantic)
Issued at 1100 AM AST Sat Sep 02 2017 000 WTNT41 KNHC 021456 TCDAT1 Hurricane Irma Discussion Number 13 NWS National Hurricane Center Miami FL AL112017 1100 AM AST Sat Sep 02 2017 Hurricane Irma continues to display an eye within a small central dense overcast in the visible and infrared imagery from the GOES and METEOSAT satellites. Dvorak current intensity numbers from TAFB, SAB, and the CIMSS ADT have not changed, so the intensity remains 95 kt. The environment in which Irma is embedded shows mixed signals for the next few days. While the vertical shear remains low through Monday, the SSTs are lukewarm and mid-level humidities are dry. (Indeed an overnight SSMIS microwave pass showed a distinct finger of dry air wrapping around the south side of Irma not far from its inner core.) However, after the waters warm and the atmosphere moistens at days 3-5, the shear is forecast to go up. The guidance has substantial spread between Category 2 and 4 by the end of the forecast period. The official intensity forecast is based upon a blend of the LGEM/DSHP statistical techniques and the HWRF dynamical model and is unchanged from the previous advisory. Irma is finally moving slightly south of due west at 13 kt. A large, building Azores-Bermuda high should turn Irma toward the west-southwest at a slower rate of speed during the next two days. By days 3 to 5, Irma should be rounding the southwestern periphery of the high and start turning back toward the west-northwest. The track guidance is in tight agreement with this scenario and the official forecast is a simply an update from the previous advisory. Irma is a small hurricane, as observed by ship BATFR17 and the overnight ASCAT scatterometer pass with tropical-storm-force winds extending out at most about 60 nm. The official size forecast is based upon the RVCN multi-model consensus technique. 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 02/1500Z 18.8N 43.3W 95 KT 110 MPH 12H 03/0000Z 18.5N 45.0W 95 KT 110 MPH 24H 03/1200Z 18.0N 47.4W 100 KT 115 MPH 36H 04/0000Z 17.4N 49.5W 105 KT 120 MPH 48H 04/1200Z 17.1N 51.6W 110 KT 125 MPH 72H 05/1200Z 17.4N 56.0W 110 KT 125 MPH 96H 06/1200Z 19.2N 60.6W 115 KT 130 MPH 120H 07/1200Z 22.0N 66.0W 115 KT 130 MPH $$ Forecaster Landsea
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