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| #873895 (Received by flhurricane at: 11:02 PM 02.Sep.2016) TCDAT4 TROPICAL STORM HERMINE DISCUSSION NUMBER 23 NWS NATIONAL HURRICANE CENTER MIAMI FL AL092016 1100 PM EDT FRI SEP 02 2016 Radar imagery and surface observations indicate that the circulation of Hermine has become elongated from east-northeast to west-southwest over the past few hours. This is likely in response to the tropical cyclone encroaching on a developing frontal boundary that extends from eastern North Carolina eastward into the Atlantic. However, buoy reports from the Atlantic southeast of the center remain in the 40-45 kt range, so the initial intensity remains 45 kt. During the forecast period, Hermine is expected to undergo a complex interaction with a mid- to upper-level baroclinic trough that is developing over the eastern United States. During the first 36-48 hours, the cyclone is likely to start extratropical transition as it tries to merge with the frontal boundary. From 48-96 hours, the dynamical models forecast the upper trough to cut off directly over the surface cyclone, and as this happens they forecast the surface cyclone to acquire a structure that resembles a tropical cyclone with the strongest winds close to the center. This suggests the possibility that Hermine could regain some tropical cyclone characteristics even though it would be under the upper-level low. For all of this complexity, the dynamical guidance forecast Hermine to strengthen during this evolution regardless of its final structure, so the new intensity forecast is an update of the previous forecast. Given the uncertainty in the structure and evolution, the forecast keeps the cyclone as post-tropical after 24 hours. The initial motion is 055/19 as Hermine is now embedded in deep-layer southwesterly flow ahead of the above mentioned baroclinic trough. During the next 24-36 hours, the cyclone should decelerate and gradually turn more toward the north. The dynamical models agree that the surface center should make at least a partial cyclonic loop from 48-96 hours as it moves under the upper-level low. After 96 hours, there is spread in the guidance, as the GFS shows a very slow motion while the ECMWF/CMC/UKMET move the system somewhat faster toward the east-northeast. The new forecast track shows a little more bend back toward the west than the previous track, then it is a little slower to move the system to the east-northeast later in the period. It should be noted that the GFS and ECMWF both bring the center of Hermine closer to the coast than the current forecast, and if this trend continues it may require some adjustment to the track in later advisories. 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 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 03/0300Z 34.1N 78.4W 45 KT 50 MPH...INLAND 12H 03/1200Z 35.5N 76.0W 45 KT 50 MPH...INLAND 24H 04/0000Z 36.8N 73.7W 50 KT 60 MPH...OVER WATER 36H 04/1200Z 37.8N 72.6W 55 KT 65 MPH...POST-TROPICAL 48H 05/0000Z 38.4N 72.8W 60 KT 70 MPH...POST-TROPICAL 72H 06/0000Z 38.5N 73.0W 65 KT 75 MPH...POST-TROPICAL 96H 07/0000Z 39.5N 71.5W 60 KT 70 MPH...POST-TROPICAL 120H 08/0000Z 40.5N 70.0W 50 KT 60 MPH...POST-TROPICAL $$ Forecaster Beven |
