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Hurricane Irma Forecast Discussion Number 33
2017-09-07 11:03:19| National Hurricane Center (Atlantic)
Issued at 500 AM AST Thu Sep 07 2017 000 WTNT41 KNHC 070903 TCDAT1 Hurricane Irma Discussion Number 33 NWS National Hurricane Center Miami FL AL112017 500 AM AST Thu Sep 07 2017 Irma has become a little less organized during the past few hours. Data from an Air Force reserve Hurricane Hunter aircraft near 0500 UTC indicated that the central pressure had risen to 921 mb and that the winds had decreased both at the 700 mb flight-level and in surface estimates from the Stepped Frequency Microwave Radiometer. Since that time, the eye has become cloud filled and the central convection has become somewhat ragged. The initial intensity is lowered to 155 kt, and this could be a little generous. The next aircraft is scheduled to reach Irma around 1200 UTC. The initial motion is 290/15. The hurricane is currently being steered by the subtropical ridge to the north, and for the next 48 h or so this motion is expected to continue with a decrease in the forward speed. After 48 h, a mid- to upper-level trough digging into the eastern United States is expected to create a break in the ridge and allow Irma to turn northward. The timing of the turn is the most important question and one still filled with uncertainty. The UKMET, UK Ensemble mean, and the NAVGEM are the models showing the latest turn, and they forecast Irma to move into the southeastern Gulf of Mexico and near the west coast of the Florida Peninsula. The ECMWF and ECMWF Ensemble mean are in the middle of the model pack and show Irma moving over the southeastern portion of the Florida Peninsula. The GFS, Canadian, and GFS Ensemble mean show the earliest turn and show Irma moving east of the coast of Florida toward the southeastern United States. The new forecast track will best follow the ECMWF, as well as the Florida State Superensemble and the HFIP Corrected Consensus, and it calls for the center to move over portions of the southeastern Florida Peninsula between 72-96 h and then across the Atlantic into southern South Carolina by 120 h. Users are again reminded not to focus on the exact track since the average NHC track errors at days 3, 4, and 5 are about 120, 175, and 225 miles, respectively. Irma should remain in warm-water, and a low shear environment for about the next three days, and thus is expected to remain a strong hurricane, most likely Category 5 or 4. Fluctuations in intensity are likely during the time due to internal eyewall replacement cycles. The large-scale models suggest that shear could increase starting at about 72 hr. However, due to the uncertainty as to whether this will happen, the intensity forecast keeps Irma at category 4 strength until landfall in Florida. The 96-120 h points now have a lower intensity due to the forecast of shear and land interaction, but Irma is still expected to be a hurricane when it reaches the southeastern United States. Since Irma is moving away from Puerto Rico and the radars there, the hourly position estimates are discontinued after this advisory. KEY MESSAGES: 1. Irma is a potentially catastrophic category 5 hurricane and will bring life-threatening wind, storm surge, and rainfall hazards to the northern coast of Hispaniola today, and the Turks and Caicos and southeastern and central Bahamas tonight and Friday. 2. Hurricane watches are in effect for the northwestern Bahamas and much of Cuba. Irma is likely to bring dangerous wind, storm surge, and rainfall to portions of these areas on Friday and Saturday. 3. The threat of direct hurricane impacts in Florida over the weekend and early next week continues to increase. Hurricane watches will likely be issued for portions of the Florida Keys and the Florida peninsula later this morning. FORECAST POSITIONS AND MAX WINDS INIT 07/0900Z 20.0N 68.3W 155 KT 180 MPH 12H 07/1800Z 20.7N 70.5W 150 KT 175 MPH 24H 08/0600Z 21.7N 73.1W 145 KT 165 MPH 36H 08/1800Z 22.3N 75.5W 140 KT 160 MPH 48H 09/0600Z 22.8N 77.4W 135 KT 155 MPH 72H 10/0600Z 24.5N 80.0W 130 KT 150 MPH 96H 11/0600Z 28.5N 80.5W 105 KT 120 MPH 120H 12/0600Z 33.0N 81.0W 75 KT 85 MPH...INLAND $$ Forecaster Beven
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