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Hurricane Irma Forecast Discussion Number 12
2017-09-02 10:50:43| National Hurricane Center (Atlantic)
Issued at 500 AM AST Sat Sep 02 2017 000 WTNT41 KNHC 020850 TCDAT1 Hurricane Irma Discussion Number 12 NWS National Hurricane Center Miami FL AL112017 500 AM AST Sat Sep 02 2017 Irma appears to have weakened a little during the last several hours. The eye has become cloud filled once again, and the convective pattern is not as impressive as it was yesterday. A blend of the latest Dvorak classifications from TAFB/SAB and ADT values from CIMSS at the University of Wisconsin support lowering the initial wind speed a little to 95 kt. It is interesting to note that a ship (BATFR17) passed within 50 n mi to the west of the center of Irma and has only reported winds of about 40 kt, indicating that the core of Irma is compact. The observed fluctuations in strength during the past day or so are likely to continue for about another day while Irma remains over marginally warm waters and in fairly close proximity to dry air. Eyewall replacement cycles, like the one observed yesterday, could occur, but forecasting the timing and duration of these are not possible. After 24 hours, Irma is expected to move over progressively warmer waters and into a more moist environment. These more favorable conditions combined with low to moderate wind shear should allow the hurricane to strengthen. The NHC intensity forecast follows the consensus aids HCCA and IVCN, and it is fairly similar to the previous advisory. Irma is now moving due west at 12 kt. A subtropical high pressure system to the north of the hurricane is expected to strengthen and build westward during the next couple of days. This pattern should cause Irma to move west-southwestward during that time. Thereafter, a turn back to the west and then west-northwest is predicted in the 3-5 day time period when Irma moves on the south and southwest sides of the high. Although the models agree on the overall scenario, there remains about 200 n mi north-south spread among the best-performing models on day 5. The NHC track forecast has been adjusted to the south at the longer-range points, and it is about halfway between the latest runs of the GFS and ECMWF models. 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 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 02/0900Z 19.0N 41.8W 95 KT 110 MPH 12H 02/1800Z 18.8N 43.7W 100 KT 115 MPH 24H 03/0600Z 18.3N 46.1W 100 KT 115 MPH 36H 03/1800Z 17.7N 48.3W 105 KT 120 MPH 48H 04/0600Z 17.1N 50.4W 110 KT 125 MPH 72H 05/0600Z 17.0N 54.7W 110 KT 125 MPH 96H 06/0600Z 18.4N 59.3W 115 KT 130 MPH 120H 07/0600Z 21.0N 64.5W 115 KT 130 MPH $$ Forecaster Cangialosi
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