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Hurricane Irma Forecast Discussion Number 21
2017-09-04 16:49:30| National Hurricane Center (Atlantic)
Issued at 1100 AM AST Mon Sep 04 2017 000 WTNT41 KNHC 041449 TCDAT1 Hurricane Irma Discussion Number 21 NWS National Hurricane Center Miami FL AL112017 1100 AM AST Mon Sep 04 2017 The satellite presentation of Irma has improved markedly over the past 24 hours, with the eye becoming larger and much more distinct. A NOAA Hurricane Hunter aircraft has reported peak 700-mb flight level winds of 117 kt, SFMR winds of 107 kt, and dropsonde data that support a minimum pressure of around 944 mb. These data support an initial intensity of 105 kt. The aircraft also reported concentric eyewalls and a double wind maximum during the last couple of passes through the center, indicating that an eyewall replacement cycle has likely begun. Irma is expected to remain within a very favorable environment for strengthening during the next several days and additional intensification appears likely. However, eyewall replacement cycles are likely to cause some fluctuations in intensity during that time. The NHC forecast brings the hurricane to category 4 strength within the next 24 hours, and then maintains Irma as a powerful hurricane during the next 5 days, assuming that the core of the cyclone does not move over any of the Greater Antilles. Irma has been moving west-southwestward or 255/11 kt to the south of a strong mid-level ridge over the central Atlantic. The hurricane should turn westward later today or tonight, then west-northwestward Tuesday as it reaches the southwestern portion of the ridge. As mentioned in the previous NHC discussion, a large mid-latitude trough is expected to amplify over the eastern United States during the next few days. The global models are unanimous in lifting the trough out to the northeast by late in the week, allowing the Atlantic ridge to build westward on days 3 through 5. The track guidance has again shifted westward and southwestward at days 4 and 5, but the models remain in very good agreement through the forecast period. The updated NHC track forecast has been adjusted southwestward late in the period, and lies very near the consensus of the ECMWF, GFS, and HWRF models. Six hourly upper-air soundings will begin at 1800 UTC today over the central United States to better sample the upstream mid-latitude trough. In addition, the NOAA G-IV aircraft will begin sampling the environment around Irma this afternoon and evening, and these data will be included in tonight's 0000 UTC model runs. Users are reminded to not focus on the exact forecast track since strong winds and heavy rainfall extend well away from the center. In addition, average NHC track errors are about 175 and 225 statute miles at days 4 and 5, respectively. KEY MESSAGES: 1. Irma is expected to affect the northeastern Leeward Islands as a dangerous major hurricane, accompanied by life-threatening wind, storm surge, and rainfall impacts, along with rough surf and rip currents. Hurricane warnings have been issued for portions of the Leeward Islands. Preparations should be rushed to completion, as tropical-storm force winds are expected to first arrive in the hurricane warning area by late Tuesday. 2. Irma could directly affect the British and U.S. Virgin Islands and Puerto Rico as a dangerous major hurricane later this week. Hurricane watches have been issued for these areas, and tropical- storm-force winds could arrive in these areas by early Wednesday. 3. Irma could directly affect Hispaniola, the Turks and Caicos, the Bahamas, and Cuba as a dangerous major hurricane later this week. Residents in these areas should monitor the progress of Irma and listen to advice given by officials. 4. There is an increasing chance of seeing some impacts from Irma in the Florida Peninsula and the Florida Keys later this week and this weekend. In addition, rough surf and dangerous marine conditions will begin to affect the southeastern U.S. coast by later this week. Otherwise, it is still too early to determine what direct impacts Irma might have on the continental United States. However, 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 04/1500Z 16.8N 53.3W 105 KT 120 MPH 12H 05/0000Z 16.5N 54.8W 115 KT 130 MPH 24H 05/1200Z 16.7N 57.1W 120 KT 140 MPH 36H 06/0000Z 17.3N 59.6W 120 KT 140 MPH 48H 06/1200Z 18.2N 62.4W 125 KT 145 MPH 72H 07/1200Z 19.9N 68.0W 120 KT 140 MPH 96H 08/1200Z 21.2N 73.3W 115 KT 130 MPH 120H 09/1200Z 22.0N 77.8W 115 KT 130 MPH...NEAR THE COAST OF CUBA $$ Forecaster Brown
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