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Hurricane Irma Forecast Discussion Number 44
2017-09-10 04:58:08| National Hurricane Center (Atlantic)
Issued at 1100 PM EDT Sat Sep 09 2017 000 WTNT41 KNHC 100258 TCDAT1 Hurricane Irma Discussion Number 44 NWS National Hurricane Center Miami FL AL112017 1100 PM EDT Sat Sep 09 2017 Irma's nearly 24-hour traversal of the north coast of Cuba appears to have affected the hurricane's structure and intensity. An earlier NOAA Hurricane Hunter aircraft and a more recent Air Force reconnaissance flight both measured maximum flight-level winds near 105 kt and surface winds near 95 kt. The planes have reported a double eyewall structure, which has also been observed in WSR-88D Doppler radar data from Miami and Key West. Irma's intensity has been conservatively lowered to 105 kt, and I'd rather wait to lower the winds further until we've seen the full data set from the Air Force mission. Irma has stuttered near the north coast of Cuba for the past few hours, which may be a harbinger of the north-northwestward turn that we've been waiting for. In any event, Irma appears to be moving very slowly toward the northwest, or 305/5 kt, very gradually shifting away from the north coast of Cuba. With the hurricane located near a break in the subtropical ridge, it should turn north-northwestward soon and accelerate near or along the west coast of Florida during the next 36-48 hours. Because of Irma's hesitation to move northwestward, the new track guidance has shifted ever so slightly westward, and the new NHC track is just a little left of the previous one. Although it is likely that the eye will move near or over the Lower Keys Sunday morning, the hurricane's angle of approach to the west coast of Florida makes it very difficult to pinpoint exactly where Irma will cross the Florida Gulf coast. If an eyewall replacement occurs within the next 6-12 hours, Irma has an opportunity to restrengthen a bit while it moves across the Straits of Florida. After that time, however, southwesterly shear is expected to increase to 25-30 kt in about 24 hours and then continue increasing through 48 hours. This shear, in addition to Irma's core interacting with Florida, is likely to cause a weakening trend after 24 hours. The new intensity guidance calls for an adjustment from the previous forecast, although it conservatively lies near the upper bound of the reliable models. Irma is still expected to approach the Florida Keys and Florida west coast as a major hurricane, and combined with its large size, will produce significant storm surge flooding in those areas. KEY MESSAGES: 1. Irma is expected bring life-threatening wind and storm surge to the Florida Keys and southwestern Florida as an extremely dangerous major hurricane tonight through Sunday. Preparations in the Florida Keys and southwest Florida should be complete since tropical-storm- force winds are already affecting portions of the coast. 2. There is an imminent danger of life-threatening storm surge flooding in portions of central and southern Florida, including the Florida Keys, where a Storm Surge Warning is in effect. The threat of catastrophic storm surge flooding is highest along the southwest coast of Florida, where 10 to 15 feet of inundation above ground level is expected. This is a life-threatening situation, and everyone in these areas should immediately follow any evacuation instructions from local officials. 3. Irma will bring life-threatening wind impacts to much of Florida regardless of the exact track of the center. Wind hazards from Irma are also expected to spread northward through much of Georgia and portions of South Carolina and Alabama. 4. Irma is expected to produce very heavy rain and inland flooding. Total rain accumulations of 10 to 20 inches, with isolated amounts of between 20 and 25 inches, are expected over the Florida Keys, the Florida peninsula, and southeast Georgia from Saturday through Monday. Significant river flooding is possible in these areas. Early next week Irma will also bring periods of heavy rain to much of the southeast United States where an average of 2 to 6 inches is forecast, with isolated higher amounts, from North and South Carolina to Tennessee and eastern Alabama. This includes some mountainous areas which are more prone to flash flooding. Residents throughout the southeast states should remain aware of the flood threat and stay tuned to forecasts and warnings. FORECAST POSITIONS AND MAX WINDS INIT 10/0300Z 23.5N 81.0W 105 KT 120 MPH 12H 10/1200Z 24.7N 81.7W 110 KT 125 MPH 24H 11/0000Z 26.8N 82.4W 105 KT 120 MPH 36H 11/1200Z 29.5N 83.4W 80 KT 90 MPH...ON THE COAST 48H 12/0000Z 32.2N 85.0W 55 KT 65 MPH...INLAND 72H 13/0000Z 35.3N 89.0W 25 KT 30 MPH...POST-TROP/INLAND 96H 14/0000Z 37.0N 88.5W 20 KT 25 MPH...POST-TROP/INLAND 120H 15/0000Z...DISSIPATED $$ Forecaster Berg
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