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Hurricane Irma Forecast Discussion Number 18
2017-09-03 22:52:50| National Hurricane Center (Atlantic)
Issued at 500 PM AST Sun Sep 03 2017 000 WTNT41 KNHC 032052 TCDAT1 Hurricane Irma Discussion Number 18 NWS National Hurricane Center Miami FL AL112017 500 PM AST Sun Sep 03 2017 The satellite presentation of Irma has degraded slightly since this morning with the eye becoming less defined. There is evidence of some northerly flow beneath the cirrus outflow, which may be disrupting the inner core and preventing Irma from strengthening. The latest consensus of the Dvorak satellite estimates suggest that the 100 kt initial intensity could be a little generous, but with a NOAA aircraft headed into the hurricane it is best to wait until data from that mission is received before making any adjustment to the initial wind speed. Irma has been moving more westward since the previous advisory, but the longer-term motion estimate is still south of due west or 260/12 kt. A strong high pressure ridge over the central Atlantic should steer Irma westward to west-southwestward during the next couple of days. After that time, a turn toward the west-northwest should occur as Irma approaches the western portion of the Atlantic ridge. The cross-track spread of the guidance is still relatively small through day 5, but the ECMWF, UKMET, and HWRF are on the southern side of the guidance envelope, with the GFS near the middle of the envelope. The latest NHC track is once again near the consensus of these typically reliable models, which is between the southern edge of the guidance and the TCVN multi-model consensus. The updated track is not very different from the previous advisory, except at day 5 where it is slightly west of the previous forecast. Users are reminded to not focus on the exact forecast track since strong winds and heavy rainfall extend well away from the center. In fact, ASCAT data that arrived after the issuance of the previous advisory indicated that the size of the tropical-storm-force wind field has expanded, especially over the northern semicircle. As a result, the initial and forecast wind radii have been adjusted accordingly. KEY MESSAGES: 1. Irma is expected to impact the northeastern Leeward Islands by the middle of this week as a major hurricane, accompanied by dangerous wind, storm surge, and rainfall impacts, along with rough surf and rip currents. Hurricane watches have been issued for portions of the Leeward Islands and additional hurricane or tropical storm watches may be required on Monday. Residents in these areas should monitor the progress of Irma and listen to advice given by officials. 2. Irma is expected to remain a dangerous major hurricane through the upcoming week and could directly affect the British and U.S. Virgin Islands, Puerto Rico, Hispaniola, the Turks and Caicos, and the Bahamas. Residents in all of these areas should monitor the progress of Irma and listen to advice given by officials. Tropical storm or hurricane watches could be issued for the British and U.S. Virgin Islands and Puerto Rico on Monday. 3. It is too early to determine what direct impacts Irma might 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 03/2100Z 17.6N 49.8W 100 KT 115 MPH 12H 04/0600Z 17.2N 51.4W 105 KT 120 MPH 24H 04/1800Z 16.8N 53.5W 110 KT 125 MPH 36H 05/0600Z 16.8N 55.8W 115 KT 130 MPH 48H 05/1800Z 17.3N 58.2W 120 KT 140 MPH 72H 06/1800Z 19.1N 63.5W 120 KT 140 MPH 96H 07/1800Z 21.2N 68.8W 115 KT 130 MPH 120H 08/1800Z 23.0N 73.5W 115 KT 130 MPH $$ Forecaster Brown
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