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| #873935 (Received by flhurricane at: 4:58 AM 03.Sep.2016) TCDAT4 TROPICAL STORM HERMINE DISCUSSION NUMBER 24 NWS NATIONAL HURRICANE CENTER MIAMI FL AL092016 500 AM EDT SAT SEP 03 2016 Satellite, radar and surface observations continue to show that the circulation of Hermine is elongated with most of the convection well removed from the area of lowest pressure. It appears that Hermine has already begun the process of extratropical transition. Based on earlier satellite-derived winds, the initial intensity remains 50 kt. These winds are occurring over water well removed from the center in the southeast quadrant. As indicated in the previous NHC discussion, during the next day or two, Hermine is expected to undergo a complex interaction with a mid- to upper-level baroclinic trough that is developing over the eastern United States. After that time, the dynamical models forecast the upper trough to cut off directly over the surface cyclone, and the surface cyclone could regain some tropical cyclone characteristics, even though it would be under the upper-level low. By then, the strongest winds are expected to be closer to the center. Nevertheless, the dynamical guidance forecasts Hermine to strengthen during this evolution regardless of its final structure, and the NHC forecast is basically an update of the previous one. Given the uncertainty in the structure and evolution, the forecast keeps the cyclone as post-tropical after 24 hours. The initial motion is difficult to estimate given that Hermine's circulation is elongated, but cyclone appears to be moving toward the east-northeast or 060 degrees at 18 kt. The cyclone is forecast to turn more toward the northeast in about 24 hours, and then turn northward with a significant decrease in forward speed as interacts with the upper-level trough, and the steering currents weaken. After day 4, the cyclone should then move eastward with the mid-latitude flow. The track forecast is a blend between the previous NHC one and the multi-model consensus TVCN. KEY MESSAGES: 1. Hermine is expected to become a post-tropical cyclone while still producing hazardous winds and storm surge over land. NWS policy allows NHC to write advisories on and issue tropical storm watches and warnings for post-tropical cyclones, when the system continues to pose a significant threat to life and property. NHC and the NWS Eastern Region have decided that this option will be invoked for Hermine. After Hermine becomes a post-tropical cyclone, NHC will continue to issue its full suite of advisory and warning products for as long as the system remains a significant threat to land. 2. There is still considerable uncertainty as to how many of the characteristics of a tropical cyclone Hermine will have while it is off of the coast of the Mid-Atlantic and New England States. Regardless of its structure, Hermine is expected to be a vigorous storm with a large wind field that will cause wind, storm surge and surf hazards along the coast. FORECAST POSITIONS AND MAX WINDS INIT 03/0900Z 35.4N 76.5W 50 KT 60 MPH...ON THE COAST 12H 03/1800Z 36.5N 74.2W 50 KT 60 MPH...OVER WATER 24H 04/0600Z 37.3N 72.8W 50 KT 60 MPH...POST-TROPICAL 36H 04/1800Z 38.0N 72.5W 55 KT 65 MPH...POST-TROPICAL 48H 05/0600Z 38.5N 72.3W 60 KT 70 MPH...POST-TROPICAL 72H 06/0600Z 39.0N 72.0W 65 KT 75 MPH...POST-TROPICAL 96H 07/0600Z 39.5N 71.5W 55 KT 65 MPH...POST-TROPICAL 120H 08/0600Z 40.5N 70.0W 45 KT 50 MPH...POST-TROPICAL $$ Forecaster Avila |
