[Tropical] Tropical Cyclone Discussion

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WTNT44 KNHC 030855
TCDAT4

TROPICAL STORM HERMINE DISCUSSION NUMBER  24
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL092016
500 AM EDT SAT SEP 03 2016

Satellite, radar and surface observations continue to show that the
circulation of Hermine is elongated with most of the convection well
removed from the area of lowest pressure. It appears that Hermine
has already begun the process of extratropical transition. Based on
earlier satellite-derived winds, the initial intensity remains 50
kt. These winds are occurring over water well removed from the
center in the southeast quadrant.

As indicated in the previous NHC discussion, during the next day or
two, Hermine is expected to undergo a complex interaction with a
mid- to upper-level baroclinic trough that is developing over the
eastern United States. After that time, the dynamical models
forecast the upper trough to cut off directly over the surface
cyclone, and the surface cyclone could regain some tropical cyclone
characteristics, even though it would be under the upper-level low.
By then, the strongest winds are expected to be closer to the
center. Nevertheless, the dynamical guidance forecasts Hermine to
strengthen during this evolution regardless of its final structure,
and the NHC forecast is basically an update of the previous one.
Given the uncertainty in the structure and evolution, the forecast
keeps the cyclone as post-tropical after 24 hours.

The initial motion is difficult to estimate given that Hermine's
circulation is elongated, but cyclone appears to be moving toward
the east-northeast or 060 degrees at 18 kt. The cyclone is forecast
to turn more toward the northeast in about 24 hours, and then turn
northward with a significant decrease in forward speed as
interacts with the upper-level trough, and the steering currents
weaken. After day 4, the cyclone should then move eastward with the
mid-latitude flow. The track forecast is a blend between the
previous NHC one and the multi-model consensus TVCN.


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 still 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  03/0900Z 35.4N  76.5W   50 KT  60 MPH...ON THE COAST
 12H  03/1800Z 36.5N  74.2W   50 KT  60 MPH...OVER WATER
 24H  04/0600Z 37.3N  72.8W   50 KT  60 MPH...POST-TROPICAL
 36H  04/1800Z 38.0N  72.5W   55 KT  65 MPH...POST-TROPICAL
 48H  05/0600Z 38.5N  72.3W   60 KT  70 MPH...POST-TROPICAL
 72H  06/0600Z 39.0N  72.0W   65 KT  75 MPH...POST-TROPICAL
 96H  07/0600Z 39.5N  71.5W   55 KT  65 MPH...POST-TROPICAL
120H  08/0600Z 40.5N  70.0W   45 KT  50 MPH...POST-TROPICAL

$$
Forecaster Avila

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