[Tropical] Tropical Cyclone Discussion

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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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