[Tropical] Tropical Cyclone Discussion

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WTNT44 KNHC 032057
TCDAT4

POST-TROPICAL CYCLONE HERMINE DISCUSSION NUMBER  26
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL092016
500 PM EDT SAT SEP 03 2016

Hermine has the look of an extratropical cyclone this afternoon,
with most of the deep convection situated well northeast of the
center and dry, subsiding air wrapping around the southern
semicircle.  This descending air resulted in a band of very strong
winds over portions of the Outer Banks of North Carolina today, and
those winds along with SFMR data from the aircraft support an
initial intensity of 60 kt for this advisory.  Hermine has an
expansive wind field, with 34-kt winds extending about 200 miles
from the center.  Hermine is still expected to interact with a
shortwave trough in the next couple of days over warm SSTs, which
should result in some intensification, although the global models
are a little less bullish this cycle.  The NHC forecast continues to
show Hermine at or near hurricane intensity for the next 72 hours,
with slow weakening expected thereafter.

After moving due eastward earlier today, the initial motion is now
back toward the east-northeast at around 10 kt.  Hermine should turn
northward and slow to a crawl as it merges with the upper-level
trough, and the NHC track shows only 2-3 kt of forward speed from 24
through 96 hours.  The spread in the track model guidance has
increased this cycle, with the UKMET and GFS now showing more of a
westward motion and are slower to begin moving Hermine
northeastward.  The ECMWF has trended eastward and is much faster,
taking Hermine south of Cape Cod in about 4 days, while the GFS and
UKMET are still centered offshore of New Jersey at that time. Given
the spread, and the possibility of looping motions during the
interaction with the upper trough, confidence in the details of the
track forecast remains quite low.  The new NHC track tries to
maintain some continuity with the last one, but lies to the right of
the previous one in the first 12-24 hours due to the more easterly
motion of Hermine today.  The NHC forecast is close to the
multi-model consensus through 72 hours.  After that time, the NHC
track is faster than the new consensus but slower than the previous
forecast, reflecting the increasing spread at those times and low
confidence in any particular solution.

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  03/2100Z 36.2N  73.3W   60 KT  70 MPH...POST-TROPICAL
 12H  04/0600Z 36.8N  72.0W   60 KT  70 MPH...POST-TROPICAL
 24H  04/1800Z 37.2N  71.3W   65 KT  75 MPH...POST-TROPICAL
 36H  05/0600Z 37.7N  71.6W   65 KT  75 MPH...POST-TROPICAL
 48H  05/1800Z 38.0N  72.0W   65 KT  75 MPH...POST-TROPICAL
 72H  06/1800Z 38.8N  71.9W   60 KT  70 MPH...POST-TROPICAL
 96H  07/1800Z 40.0N  71.0W   55 KT  65 MPH...POST-TROPICAL
120H  08/1800Z 41.0N  68.5W   50 KT  60 MPH...POST-TROPICAL

$$
Forecaster Brennan
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