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| #873965 (Received by flhurricane at: 11:01 AM 03.Sep.2016) TCDAT4 POST-TROPICAL CYCLONE HERMINE DISCUSSION NUMBER 25 NWS NATIONAL HURRICANE CENTER MIAMI FL AL092016 1100 AM EDT SAT SEP 03 2016 Satellite imagery indicates that Hermine has become a post-tropical cyclone, with the coldest convective tops now located more than 200 n mi northeast of the exposed center. Despite this change in structure, surface data from the Outer Banks indicate that some strong winds persist near the center, and the initial intensity is set to 55 kt for this advisory. During the next 48 to 72 hours, Hermine will interact with a strong mid-latitude shortwave trough and all of the global models show the system re-intensifying during that time and a redevelopment of a stronger inner core, albeit one situated underneath an upper-level low. Regardless of its final structure, Hermine is expected to remain a dangerous cyclone through the 5 day period. The initial motion estimate is a somewhat uncertain 060/14. Hermine should continue moving northeastward in deep-layer southwesterly flow through 24 hours and then meander generally northward from 36 to 72 hours while the cyclone deepens beneath the upper-level low. Late in the period, the guidance is in generally good agreement showing a steadier motion toward the northeast, although there is significant spread. The new NHC forecast is generally close to the previous one and is near a consensus of the GFS and ECMWF through 3 days, and then favors the guidance that is a bit faster and farther north at days 4 and 5. KEY MESSAGES: 1. The slow motion and large wind field associated with Hermine will result in a long duration of hazardous conditions along much of the mid-Atlantic coast extending into southern New England through the holiday weekend. 2. Although Hermine has become a post-tropical cyclone, NHC will continue to issue its full suite of advisory and warning products as long as the system remains a significant threat to land areas. 3. P-surge, the model that drives the Potential Storm Surge Flooding Graphic, is designed for a wind field typical of a tropical cyclone. The wind field of Hermine is very poorly represented by the P-surge model and as a result, recent Flooding Graphics have understated the inundation risk from the Carolinas northward. NHC will be discontinuing runs of the P-surge model for Hermine with this advisory. The NWS is attempting to substitute the GFS ensemble system for P-surge for the next issuance of the Potential Storm Surge Flooding Graphic, to provide a more realistic depiction of the threat. If this effort is unsuccessful, issuance of the Potential Storm Surge Flooding Graphic for Hermine will also be discontinued. 4. The Prototype Storm Surge Watch/Warning Graphic does account for the current wind structure of Hermine, and therefore accurately identifies those areas at risk for life-threatening storm surge. This graphic will continue to be produced for Hermine. FORECAST POSITIONS AND MAX WINDS INIT 03/1500Z 36.1N 75.2W 55 KT 65 MPH...POST-TROPICAL 12H 04/0000Z 37.1N 73.0W 60 KT 70 MPH...POST-TROPICAL 24H 04/1200Z 37.9N 71.6W 60 KT 70 MPH...POST-TROPICAL 36H 05/0000Z 38.4N 71.6W 65 KT 75 MPH...POST-TROPICAL 48H 05/1200Z 38.7N 71.9W 65 KT 75 MPH...POST-TROPICAL 72H 06/1200Z 39.5N 71.5W 65 KT 75 MPH...POST-TROPICAL 96H 07/1200Z 40.4N 70.2W 60 KT 70 MPH...POST-TROPICAL 120H 08/1200Z 41.0N 67.5W 50 KT 60 MPH...POST-TROPICAL $$ Forecaster Brennan |
