[Tropical] Tropical Cyclone Discussion

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WTNT44 KNHC 030300
TCDAT4

TROPICAL STORM HERMINE DISCUSSION NUMBER  23
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL092016
1100 PM EDT FRI SEP 02 2016

Radar imagery and surface observations indicate that the
circulation of Hermine has become elongated from east-northeast to
west-southwest over the past few hours.  This is likely in response
to the tropical cyclone encroaching on a developing frontal
boundary that extends from eastern North Carolina eastward into the
Atlantic.  However, buoy reports from the Atlantic southeast of the
center remain in the 40-45 kt range, so the initial intensity
remains 45 kt.

During the forecast period, Hermine is expected to undergo a complex
interaction with a mid- to upper-level baroclinic trough that is
developing over the eastern United States.  During the first 36-48
hours, the cyclone is likely to start extratropical transition as it
tries to merge with the frontal boundary.  From 48-96 hours, the
dynamical models forecast the upper trough to cut off directly over
the surface cyclone, and as this happens they forecast the surface
cyclone to acquire a structure that resembles a tropical cyclone
with the strongest winds close to the center.  This suggests the
possibility that Hermine could regain some tropical cyclone
characteristics even though it would be under the upper-level low.
For all of this complexity, the dynamical guidance forecast Hermine
to strengthen during this evolution regardless of its final
structure, so the new intensity forecast is an update of the
previous forecast.  Given the uncertainty in the structure and
evolution, the forecast keeps the cyclone as post-tropical after 24
hours.

The initial motion is 055/19 as Hermine is now embedded in
deep-layer southwesterly flow ahead of the above mentioned
baroclinic trough.  During the next 24-36 hours, the cyclone should
decelerate and gradually turn more toward the north.  The dynamical
models agree that the surface center should make at least a partial
cyclonic loop from 48-96 hours as it moves under the upper-level
low.  After 96 hours, there is spread in the guidance, as the GFS
shows a very slow motion while the ECMWF/CMC/UKMET move the system
somewhat faster toward the east-northeast.  The new forecast track
shows a little more bend back toward the west than the previous
track, then it is a little slower to move the system to the
east-northeast later in the period.  It should be noted that the
GFS and ECMWF both bring the center of Hermine closer to the coast
than the current forecast, and if this trend continues it may
require some adjustment to the track in later advisories.

KEY MESSAGES:

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.

2. There is considerable uncertainty as to how many of the
characteristics of a tropical cyclone Hermine will have while it is
off of the coast of the Mid-Atlantic and New England States.
Regardless of its structure, Hermine is expected to be a vigorous
storm with a large wind field that will cause wind, storm surge and
surf hazards along the coast.


FORECAST POSITIONS AND MAX WINDS

INIT  03/0300Z 34.1N  78.4W   45 KT  50 MPH...INLAND
 12H  03/1200Z 35.5N  76.0W   45 KT  50 MPH...INLAND
 24H  04/0000Z 36.8N  73.7W   50 KT  60 MPH...OVER WATER
 36H  04/1200Z 37.8N  72.6W   55 KT  65 MPH...POST-TROPICAL
 48H  05/0000Z 38.4N  72.8W   60 KT  70 MPH...POST-TROPICAL
 72H  06/0000Z 38.5N  73.0W   65 KT  75 MPH...POST-TROPICAL
 96H  07/0000Z 39.5N  71.5W   60 KT  70 MPH...POST-TROPICAL
120H  08/0000Z 40.5N  70.0W   50 KT  60 MPH...POST-TROPICAL

$$
Forecaster Beven
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