[Tropical] Tropical Cyclone Discussion

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WTNT44 KNHC 020300
TCDAT4

HURRICANE HERMINE DISCUSSION NUMBER  19
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL092016
1100 PM EDT THU SEP 01 2016

Reports from an Air Force Reserve Hurricane Hunter aircraft
indicate that Hermine has gradually intensified since the last
forecast advisory.  The plane measured 86-kt flight-level winds
southeast of the center at 850 mb with SFMR and dropsonde data
suggesting winds of at least 65 kt.  Based on these data and a
earlier 983 mb central pressure, the initial intensity is set at 70
kt.

The initial motion remains 030/12.  The flow on the eastern side of
a mid-level trough over the southeastern United States should cause
Hermine to move north-northeastward to northeastward with an
increase in forward speed during the next 36 hours.  The track
guidance is tightly clustered during this period, and this part of
the forecast track is an update of the previous track.  At 48 hours
and beyond, Hermine is expected to interact with a baroclinic trough
over the northeastern United States.  The track guidance become
rather divergent during this period, with the GFS and ECMWF models
showing a looping track close to the coast, while the UKMET and
Canadian models show a slow motion somewhat farther offshore.  The
new forecast track compromises between these solutions and shows a
slow northeastward motion during this time.  Regardless of the exact
track, Hermine should linger for several days near the northeastern
U. S. coast as a vigorous low pressure system.

A little more strengthening is possible during the last few hours
before landfall.  After landfall, Hermine is expected to weaken as
it crosses the southeastern United States, eventually emerging from
the North Carolina coast as a tropical storm.  The cyclone is
expected to re-intensify as an extratropical low during its
interaction with the baroclinic trough.  The forecast intensities
have been raised for this part of the forecast based on global model
forecasts, and it is possible that they are a little conservative.
Later in the forecast period, vertical shear decreases, and the
cyclone could be situated over marginally warm waters.  Therefore
there is the possibility of the system regaining some tropical
characteristics in 4-5 days, although this remains speculative.

KEY MESSAGES:

1. As landfall occurs, it is important to not focus on the exact
forecast track.  Hermine is an extremely asymmetric storm, with a
large extent of dangerous winds, life-threatening storm surge, and
flooding rains well to the east and southeast of the path of the
center.

2. 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  02/0300Z 29.7N  84.3W   70 KT  80 MPH
 12H  02/1200Z 31.5N  82.8W   60 KT  70 MPH
 24H  03/0000Z 33.6N  79.7W   45 KT  50 MPH...INLAND
 36H  03/1200Z 35.6N  75.7W   45 KT  50 MPH...INLAND
 48H  04/0000Z 37.0N  73.2W   50 KT  60 MPH...POST-TROP/EXTRATROP
 72H  05/0000Z 38.0N  72.5W   55 KT  65 MPH...POST-TROP/EXTRATROP
 96H  06/0000Z 39.0N  72.0W   55 KT  65 MPH...POST-TROP/EXTRATROP
120H  07/0000Z 39.5N  70.5W   50 KT  60 MPH...POST-TROP/EXTRATROP

$$
Forecaster Beven
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