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Chill falls over the Atlantic as the official season heads towards close
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| #874053 (Received by flhurricane at: 5:01 AM 04.Sep.2016) TCDAT4 POST-TROPICAL CYCLONE HERMINE DISCUSSION NUMBER 28 NWS NATIONAL HURRICANE CENTER MIAMI FL AL092016 500 AM EDT SUN SEP 04 2016 The overall structure of Hermine has changed little during the past 6 hours. A ring of shallow convection has developed within 60-100 n mi northwest through northeast of the center, possibly due to that portion of the circulation passing over the Gulf Stream, where water temperatures are 29-30C. An earlier ASCAT-B pass indicated that winds had decreased to 50-55 kt, and recent data from an Air Force Reserve reconnaissance aircraft confirms that Hermine has peak winds of 55 kt. The initial motion estimate is 065/10 kt. Hermine is expected to move slowly northward and perhaps erratically during the next 36 hours as the post-tropical cyclone begins to interact with an approaching strong mid-/upper-level shortwave trough that is gradually becoming more negatively tilted based on water vapor imagery. By 36-48 hours, Hermine is expected to become vertically stacked beneath a cut-off low, which could result in the cyclone briefly stalling south of southern New England before lifting out to the northeast by 72 hours. The models are in fairly good agreement on this developing track scenario through about 36 hours, but then diverge significantly after that with the UKMET model turning Hermine farther west closer to the New Jersey coast while the ECMWF lifts out Hermine more quickly. The GFS solution lies between these two extremes, and the new forecast track closely follows that model. However, there remains low confidence in any particular model. Little change in strength is likely today. However, by late tonight and on Monday, the models indicate that at least 6 deg C of cooling in the mid-/upper-levels will occur when Hermine's low-level circulation moves underneath the cut-off low. The combination of the much cooler air aloft over SSTs of at least 27-28C should generate strong instability and some inner-core convection, possibly resulting in a Hermine making the transition to a subtropical cyclone and strengthening back to hurricane force. The guidance is in good agreement that the system should slowly weaken after 48 hours when the system will be moving over much cooler water north of the Gulf Stream. The intensity forecast closely follows a blend of the GFS-ECMWF model solutions. 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 and into midweek. 2. Small changes in the meandering track of Hermine could result in large differences in the impacts along the mid-Atlantic coast during the next several days. In addition, there could be multiple occurrences of tropical storm conditions in some locations within the warning area during this time. 3. 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. 4. 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, Flooding Graphics prior to today's 11 am EDT advisory understated the inundation risk from the Carolinas northward. The NWS is using the GFS ensemble system instead of P-surge for ongoing issuances of the Potential Storm Surge Flooding Graphic to provide a more realistic depiction of the threat. 5. The Prototype Storm Surge Watch/Warning Graphic accounts for the current wind structure of Hermine, and therefore accurately identifies those areas at risk for life-threatening storm surge. This graphic will also continue to be produced for Hermine. FORECAST POSITIONS AND MAX WINDS INIT 04/0900Z 36.8N 70.8W 55 KT 65 MPH 12H 04/1800Z 37.2N 70.3W 55 KT 65 MPH...POST-TROPICAL 24H 05/0600Z 37.7N 70.6W 60 KT 70 MPH...POST-TROPICAL 36H 05/1800Z 38.0N 71.0W 65 KT 75 MPH...POST-TROPICAL 48H 06/0600Z 38.5N 70.8W 65 KT 75 MPH...POST-TROPICAL 72H 07/0600Z 39.2N 70.0W 60 KT 70 MPH...POST-TROPICAL 96H 08/0600Z 40.3N 68.0W 55 KT 65 MPH...POST-TROPICAL 120H 09/0600Z 41.5N 63.5W 50 KT 60 MPH...POST-TROPICAL $$ Forecaster Stewart |
