[Tropical] Tropical Cyclone Discussion

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WTNT44 KNHC 021455
TCDAT4

TROPICAL STORM HERMINE DISCUSSION NUMBER  21
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL092016
1100 AM EDT FRI SEP 02 2016

Hermine is moving over southeastern Georgia and has weakened
steadily since landfall.  Convective tops have warmed, but several
land stations have shown sustained winds near tropical storm force
near the center with some gusts close to 50 kt.  Stronger winds are
also found over the Atlantic coastal waters, with buoy 41008
reporting winds around 40 kt in the last hour.  Based on this, the
initial intensity is set to 45 kt for this advisory.  The central
pressure, based on surface data, is about 989 mb.  While the center
of Hermine will remain over land for the next 24 hours or so, I
don't expect the winds over water to decrease much, so the NHC
forecast shows no change during that time.  Strengthening through a
combination of diabatic and baroclinic processes is shown after the
system moves offshore.  The GFS, ECMWF, and UKMET all show Hermine
interacting with a potent upper-level shortwave trough in 2-3 days,
and the system could re-strengthen to near hurricane force at that
time.  Slow weakening is expected later in the period, but Hermine
is expected to remain a dangerous cyclone through 5 days.  The NHC
intensity forecast is based largely on global model guidance, and is
a bit higher than the previous one.

The initial motion estimate is 040/16.  Hermine should continue
moving northeastward for the next 36 to 48 hours ahead of an
approaching mid-latitude trough.  After that time, the cyclone
should turn northward and nearly stall out as it interacts with the
upper-level shortwave off the mid-Atlantic coast.  A slow
northeastward motion is shown at days 4 and 5, but there is a fair
bit of model spread late in the period and confidence in the details
of the track forecast at those times remains low.

Note that the NHC forecast shows Hermine as a post-tropical cyclone
in about 48 hours, but model fields do not show it undergoing a
full extratropical transition, as it instead appears to acquire a
warm seclusion structure.  There is a possibility that the system
could regain some tropical characteristics in 4-5 days, but this
remains uncertain.

Based on the latest forecast, tropical storm watches and warnings
have been expanded northward along the U.S. mid-Atlantic coast.

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  02/1500Z 31.9N  82.0W   45 KT  50 MPH...INLAND
 12H  03/0000Z 33.5N  79.7W   45 KT  50 MPH...INLAND
 24H  03/1200Z 35.4N  76.3W   45 KT  50 MPH...INLAND
 36H  04/0000Z 36.7N  73.9W   55 KT  65 MPH...OVER WATER
 48H  04/1200Z 37.5N  72.6W   55 KT  65 MPH...POST-TROPICAL
 72H  05/1200Z 38.1N  72.6W   65 KT  75 MPH...POST-TROPICAL
 96H  06/1200Z 38.8N  71.9W   60 KT  70 MPH...POST-TROPICAL
120H  07/1200Z 39.3N  71.5W   50 KT  60 MPH...POST-TROPICAL

$$
Forecaster Brennan
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