[Tropical] Tropical Cyclone Discussion

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WTNT44 KNHC 040254
TCDAT4

POST-TROPICAL CYCLONE HERMINE DISCUSSION NUMBER  27
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL092016
1100 PM EDT SAT SEP 03 2016

Hermine has changed little in structure since the last advisory,
with most of the deep convection situated well northeast of the
center and dry, subsiding air wrapping around the southern
semicircle.  Earlier data from an Air Force Reserve Hurricane
Hunter aircraft showed that the central pressure had risen to 998
mb, and that 55-60 kt surface winds were present about 70 n mi
northwest of the center.  Based on this, the initial intensity
remains 60 kt.

Hermine continues to move east-northeastward with the initial
motion of 075/11.  During the next 72 hours, a mid- to upper-level
trough currently moving through the northeastern United States will
move close to, and then over the top of, the post-tropical cyclone.
During this evolution, Hermine is expected to move slowly northward
and then northwestward as it makes at least a partial cyclonic loop.
While the track guidance is in fair agreement for the first 48-72
hours, there remains significant spread later in the period on how
fast the cyclone will move out to sea after it merges with the
trough.  The UKMET is very slow to move the system out, while the
ECMWF is much faster.  The GFS is between these extremes, and the
current run shows a little faster motion than the previous run.
The new forecast track is similar to the previous forecast through
72 hours, and then is somewhat faster in moving the system to the
east-northeast.  Overall, there remains low confidence in any
particular model solution, and it should be noted that the GFS and
UKMET both bring the center closer to land than the official
forecast.

Little change in strength is likely tonight.  On Sunday and Monday,
the interaction with the upper-level trough is expected to provide
more favorable conditions for strengthening while the system is
over sea surface temperatures of 28C or warmer, and the dynamical
models all show some deepening during this period. Based on this,
the intensity forecast calls for Hermine to become a hurricane-force
low.  The guidance is in good agreement that the system should
slowly weaken after 48 hours, and this is indicated in the forecast
as well.

KEY MESSAGES:

1. The slow motion and large wind field associated with Hermine will
result in a long duration of hazardous conditions along much of the
mid-Atlantic coast extending into southern New England through the
holiday weekend and into midweek.

2. Small changes in the meandering track of Hermine could result in
large differences in the impacts along the mid-Atlantic coast during
the next several days.  In addition, there could be multiple
occurrences of tropical storm conditions in some locations within
the warning area during this time.

3. Although Hermine has become a post-tropical cyclone, NHC will
continue to issue its full suite of advisory and warning products as
long as the system remains a significant threat to land areas.

4. P-surge, the model that drives the Potential Storm Surge Flooding
Graphic, is designed for a wind field typical of a tropical cyclone.
The wind field of Hermine is very poorly represented by the P-surge
model and as a result, Flooding Graphics prior to today's 11 am
EDT advisory understated the inundation risk from the Carolinas
northward.  The NWS is using the GFS ensemble system instead of
P-surge for ongoing issuances of the Potential Storm Surge Flooding
Graphic to provide a more realistic depiction of the threat.

5. The Prototype Storm Surge Watch/Warning Graphic accounts for
the current wind structure of Hermine, and therefore accurately
identifies those areas at risk for life-threatening storm surge.
This graphic will also continue to be produced for Hermine.


FORECAST POSITIONS AND MAX WINDS

INIT  04/0300Z 36.5N  72.1W   60 KT  70 MPH...POST-TROPICAL
 12H  04/1200Z 37.1N  71.2W   60 KT  70 MPH...POST-TROPICAL
 24H  05/0000Z 37.6N  71.2W   65 KT  75 MPH...POST-TROPICAL
 36H  05/1200Z 37.9N  71.5W   65 KT  75 MPH...POST-TROPICAL
 48H  06/0000Z 38.3N  71.8W   65 KT  75 MPH...POST-TROPICAL
 72H  07/0000Z 39.0N  71.5W   60 KT  70 MPH...POST-TROPICAL
 96H  08/0000Z 40.0N  70.0W   55 KT  65 MPH...POST-TROPICAL
120H  09/0000Z 41.0N  66.5W   50 KT  60 MPH...POST-TROPICAL

$$
Forecaster Beven
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