734 
 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