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Tropical Storm HERMINE Forecast Discussion Number 22
2016-09-02 22:46:06| National Hurricane Center (Atlantic)
Issued at 500 PM EDT FRI SEP 02 2016 000 WTNT44 KNHC 022045 TCDAT4 TROPICAL STORM HERMINE DISCUSSION NUMBER 22 NWS NATIONAL HURRICANE CENTER MIAMI FL AL092016 500 PM EDT FRI SEP 02 2016 While the cloud pattern of Hermine has become elongated north and east of the center, and the central convective tops have warmed, numerous surface observations of 40-kt winds along the South Carolina coast support maintaining an initial intensity of 45 kt for this advisory. While the center of Hermine remains over land for the next 12 to 18 hours, little change in intensity is expected, as the strongest winds should remain over the coastal waters of the Carolinas. After the center moves offshore, intensification is expected through a combination of diabatic and baroclinic processes. The global models continue to show Hermine interacting with a potent upper-level shortwave trough in 2-3 days, and the system is expected to restrengthen to near hurricane force in 48 to 72 hours. As the upper-level forcing moves away, slow weakening is expected later in the period. The NHC intensity forecast is above the tropical cyclone guidance suite and is based largely on global models. The initial motion estimate is 055/17, as Hermine is currently embedded in deep-layer southwesterly flow ahead of a positively- tilted mid-latitude trough. During the next 36 to 48 hours the cyclone should gradually slow down and then turn more poleward as the shortwave trough amplifies over the central Appalachians and mid-Atlantic states. This trough interacts with and superimposes itself on top of Hermine between 48 and 72 hours. Not surprisingly, there are differences in the details of how the model guidance handles the motion of Hermine during this time, with the ECMWF and ECMWF ensemble mean along the western side of the guidance by 72 hours, and the GFS, HWRF, and UKMET farther to the east. Note that the interaction between Hermine and the upper trough could result in some looping motions that are not captured by the 12 and 24 hour spacing of the official NHC forecast points. Late in the forecast period, a northeastward motion is expected as the upper trough slides eastward. Spread continues in the guidance at those times as well, with the HWRF joining the ECMWF on the left side of the guidance envelope at those times. Overall, the new NHC forecast has been adjusted a bit to the left and is close to the middle of the guidance envelope. This track is near the GEFS ensemble mean and a blend of the GFS and ECMWF. Needless to say, small changes in the eventual track of Hermine could result in large changes in impacts along the mid-Atlantic coast. The NHC forecast shows Hermine as a post-tropical cyclone in 36 hours, but model fields do not show it undergoing a full extratropical transition, as it instead appears to acquire a warm seclusion structure with a strong low-level vortex underneath an upper-level low. There is a possibility that the system could regain some tropical characteristics in 3-5 days, but this remains uncertain. It is important to remind users that Hermine is expected to remain a dangerous cyclone through the forecast period regardless of the details of its structure. Based on the latest forecast, tropical storm warnings have been expanded northward along the U.S. mid-Atlantic coast to Sandy Hook, New Jersey. KEY MESSAGE: 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. FORECAST POSITIONS AND MAX WINDS INIT 02/2100Z 33.0N 80.5W 45 KT 50 MPH...INLAND 12H 03/0600Z 34.5N 77.9W 45 KT 50 MPH...INLAND 24H 03/1800Z 36.1N 75.0W 50 KT 60 MPH...OVER WATER 36H 04/0600Z 37.1N 73.1W 55 KT 65 MPH...POST-TROPICAL 48H 04/1800Z 37.8N 72.5W 60 KT 70 MPH...POST-TROPICAL 72H 05/1800Z 38.4N 72.8W 65 KT 75 MPH...POST-TROPICAL 96H 06/1800Z 39.7N 71.0W 60 KT 70 MPH...POST-TROPICAL 120H 07/1800Z 40.5N 69.5W 50 KT 60 MPH...POST-TROPICAL $$ Forecaster Brennan
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