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ZCZC MIATCDAT1 ALL TTAA00 KNHC DDHHMM Hurricane Irma Discussion Number 36 NWS National Hurricane Center Miami FL AL112017 1100 PM EDT Thu Sep 07 2017 Microwave images and data from an Air Force Reserve Hurricane Hunter aircraft indicate that Irma is developing a concentric eyewall structure, and the plane reported an elliptical eye. The highest flight-level and SFMR winds measured by the aircraft were 146 kt and 139 kt, respectively, so the initial intensity is lowered slightly to 145 kt. Irma's central pressure has fallen a bit despite the slight reduction in winds, counterbalanced by an expansion of the hurricane-force wind field as observed by the plane. If Irma has developed concentric eyewalls, then some additional gradual weakening or fluctuations in intensity are possible over the next day or two, despite a seemingly favorable atmospheric and oceanic environment. The intensity models depict a very gradual decrease in Irma's maximum winds up until a possible landfall in Florida, but the hurricane is nonetheless expected to still be at or near category 4 strength at that time. After landfall, a fairly quick decay in maximum winds is expected due to land interaction and increased shear, although Irma's large wind field is likely to still produce hurricane-force winds over a large area. Irma continues to move west-northwestward at 290/14 kt, steered by the western extent of the subtropical ridge. Irma should maintain this trajectory but slow down during the next 36 hours, and then turn north-northwestward toward a break in the ridge by 72 hours. Although there was a westward shift in the 18Z GFS, this model's new track puts it very close to the previous NHC forecast. As such, no major changes to the NHC track were required on this forecast through 72 hours, and the projected path lies between the GFS and ECMWF, closest to the TVCN consensus and HCCA. After 72 hours, the models are showing a more pronounced turn toward the northwest when Irma interacts with a shortwave diving southeastward from the Central Plains, and the NHC forecast has been adjusted accordingly at the end of the forecast period. KEY MESSAGES: 1. Irma is a potentially catastrophic category 5 hurricane and will continue to bring life-threatening wind, storm surge, and rainfall hazards to the Turks and Caicos Islands and the Bahamas through Saturday. Heavy rainfall is still possible across portions of Hispaniola through Friday. Hurricane conditions will also spread over portions of the north coast of Cuba, especially over the adjacent Cuban Keys through Saturday. 2. Severe hurricane conditions are expected over portions of the Florida peninsula and the Florida Keys beginning late Saturday. Irma could make landfall in southern Florida as a dangerous major hurricane, and bring life-threatening storm surge and wind impacts to much of the state. A Hurricane Warning is in effect for southern Florida, the Florida Keys, Lake Okeechobee, and Florida Bay, while Hurricane Watches have been issued northward into central Florida. 3. A Storm Surge Warning is in effect for southern Florida and the Florida Keys. A Storm Surge Warning means there is a danger of life-threatening inundation, from rising water moving inland from the coastline, during the next 36 hours in the indicated locations. For a depiction of areas at risk, please see the National Weather Service Storm Surge Watch/Warning Graphic, available at hurricanes.gov. This is a life-threatening situation. Persons located within these areas should take all necessary actions to protect life and property from rising water and the potential for other dangerous conditions. Promptly follow evacuation and other instructions from local officials. A Storm Surge Watch has been issued north of the Storm Surge Warning for portions of the central Florida coast. 4. There is a chance of direct impacts in portions of Georgia, South Carolina, and North Carolina, but it is too early to specify the magnitude and location of these impacts. FORECAST POSITIONS AND MAX WINDS INIT 08/0300Z 21.3N 72.4W 145 KT 165 MPH 12H 08/1200Z 21.8N 74.4W 145 KT 165 MPH 24H 09/0000Z 22.3N 76.7W 140 KT 160 MPH 36H 09/1200Z 22.9N 78.6W 135 KT 155 MPH 48H 10/0000Z 23.7N 79.9W 135 KT 155 MPH 72H 11/0000Z 27.2N 81.1W 100 KT 115 MPH...INLAND 96H 12/0000Z 32.0N 83.5W 50 KT 60 MPH...INLAND 120H 13/0000Z 35.5N 86.5W 30 KT 35 MPH...INLAND $$ Forecaster Berg NNNN