[Tropical] Tropical Cyclone Discussion

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WTNT44 KNHC 040859
TCDAT4

POST-TROPICAL CYCLONE HERMINE DISCUSSION NUMBER  28
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL092016
500 AM EDT SUN SEP 04 2016

The overall structure of Hermine has changed little during the past
6 hours. A ring of shallow convection has developed within 60-100
n mi northwest through northeast of the center, possibly due to that
portion of the circulation passing over the Gulf Stream, where water
temperatures are 29-30C. An earlier ASCAT-B pass indicated that
winds had decreased to 50-55 kt, and recent data from an Air Force
Reserve reconnaissance aircraft confirms that Hermine has peak
winds of 55 kt.

The initial motion estimate is 065/10 kt. Hermine is expected to
move slowly northward and perhaps erratically during the next 36
hours as the post-tropical cyclone begins to interact with an
approaching strong mid-/upper-level shortwave trough that is
gradually becoming more negatively tilted based on water vapor
imagery. By 36-48 hours, Hermine is expected to become vertically
stacked beneath a cut-off low, which could result in the cyclone
briefly stalling south of southern New England before lifting out to
the northeast by 72 hours. The models are in fairly good agreement
on this developing track scenario through about 36 hours, but then
diverge significantly after that with the UKMET model turning
Hermine farther west closer to the New Jersey coast while the ECMWF
lifts out Hermine more quickly. The GFS solution lies between these
two extremes, and the new forecast track closely follows that model.
However, there remains low confidence in any particular model.

Little change in strength is likely today. However, by late tonight
and on Monday, the models indicate that at least 6 deg C of cooling
in the mid-/upper-levels will occur when Hermine's low-level
circulation moves underneath the cut-off low. The combination of the
much cooler air aloft over SSTs of at least 27-28C should generate
strong instability and some inner-core convection, possibly
resulting in a Hermine making the transition to a subtropical
cyclone and strengthening back to hurricane force. The guidance is
in good agreement that the system should slowly weaken after 48
hours when the system will be moving over much cooler water north
of the Gulf Stream. The intensity forecast closely follows a blend
of the GFS-ECMWF model solutions.

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  04/0900Z 36.8N  70.8W   55 KT  65 MPH
 12H  04/1800Z 37.2N  70.3W   55 KT  65 MPH...POST-TROPICAL
 24H  05/0600Z 37.7N  70.6W   60 KT  70 MPH...POST-TROPICAL
 36H  05/1800Z 38.0N  71.0W   65 KT  75 MPH...POST-TROPICAL
 48H  06/0600Z 38.5N  70.8W   65 KT  75 MPH...POST-TROPICAL
 72H  07/0600Z 39.2N  70.0W   60 KT  70 MPH...POST-TROPICAL
 96H  08/0600Z 40.3N  68.0W   55 KT  65 MPH...POST-TROPICAL
120H  09/0600Z 41.5N  63.5W   50 KT  60 MPH...POST-TROPICAL

$$
Forecaster Stewart

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