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Post-Tropical Cyclone HERMINE Forecast Discussion Number 27
2016-09-04 04:54:14| National Hurricane Center (Atlantic)
Issued at 1100 PM EDT SAT SEP 03 2016 000 WTNT44 KNHC 040254 TCDAT4 POST-TROPICAL CYCLONE HERMINE DISCUSSION NUMBER 27 NWS NATIONAL HURRICANE CENTER MIAMI FL AL092016 1100 PM EDT SAT SEP 03 2016 Hermine has changed little in structure since the last advisory, with most of the deep convection situated well northeast of the center and dry, subsiding air wrapping around the southern semicircle. Earlier data from an Air Force Reserve Hurricane Hunter aircraft showed that the central pressure had risen to 998 mb, and that 55-60 kt surface winds were present about 70 n mi northwest of the center. Based on this, the initial intensity remains 60 kt. Hermine continues to move east-northeastward with the initial motion of 075/11. During the next 72 hours, a mid- to upper-level trough currently moving through the northeastern United States will move close to, and then over the top of, the post-tropical cyclone. During this evolution, Hermine is expected to move slowly northward and then northwestward as it makes at least a partial cyclonic loop. While the track guidance is in fair agreement for the first 48-72 hours, there remains significant spread later in the period on how fast the cyclone will move out to sea after it merges with the trough. The UKMET is very slow to move the system out, while the ECMWF is much faster. The GFS is between these extremes, and the current run shows a little faster motion than the previous run. The new forecast track is similar to the previous forecast through 72 hours, and then is somewhat faster in moving the system to the east-northeast. Overall, there remains low confidence in any particular model solution, and it should be noted that the GFS and UKMET both bring the center closer to land than the official forecast. Little change in strength is likely tonight. On Sunday and Monday, the interaction with the upper-level trough is expected to provide more favorable conditions for strengthening while the system is over sea surface temperatures of 28C or warmer, and the dynamical models all show some deepening during this period. Based on this, the intensity forecast calls for Hermine to become a hurricane-force low. The guidance is in good agreement that the system should slowly weaken after 48 hours, and this is indicated in the forecast as well. 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 04/0300Z 36.5N 72.1W 60 KT 70 MPH...POST-TROPICAL 12H 04/1200Z 37.1N 71.2W 60 KT 70 MPH...POST-TROPICAL 24H 05/0000Z 37.6N 71.2W 65 KT 75 MPH...POST-TROPICAL 36H 05/1200Z 37.9N 71.5W 65 KT 75 MPH...POST-TROPICAL 48H 06/0000Z 38.3N 71.8W 65 KT 75 MPH...POST-TROPICAL 72H 07/0000Z 39.0N 71.5W 60 KT 70 MPH...POST-TROPICAL 96H 08/0000Z 40.0N 70.0W 55 KT 65 MPH...POST-TROPICAL 120H 09/0000Z 41.0N 66.5W 50 KT 60 MPH...POST-TROPICAL $$ Forecaster Beven
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