[Tropical] Tropical Cyclone Discussion

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WTNT44 KNHC 050857
TCDAT4

POST-TROPICAL CYCLONE HERMINE DISCUSSION NUMBER  32
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL092016
500 AM AST MON SEP 05 2016

Hermine remains a post-tropical cyclone with a cloud pattern more
reminiscent of an extratropical low. However, moderate convection
has been developing just north of the surface center in the northern
semicircle during the past few hours within a region of increasing
upper-level diffluence. Data from an earlier reconnaissance mission
along with recent scatterometer data suggest that the intensity
remains unchanged at 60 kt.

The initial motion is a slow northward drift 010/03 kt. Water vapor
imagery indicates that an upper-level low has developed east of the
DelMarVa peninsula and is dropping to the south of Hermine's surface
center. The latest model guidance is forecasting the surface and
mid- to upper-level lows to rotate counterclockwise around each
other for the next 24-36 hours before becoming vertically stacked by
36-48 hours. This cyclonic interaction has resulted in a significant
westward shift in the global model tracks, especially by the UKMET
and ECMWF models. As a result, the official forecast has been
shifted a little to the west of the previous advisory track, but
lies along the extreme eastern side of the guidance envelope in the
event that the models shift back to the east since such complex
interactions are difficult to forecast from cycle to cycle.

The upper-level flow across Hermine is forecast by all of the global
models to continue to become more difluent during the next 24 hours,
which should aid in the development of deep convection near the
low-level center. By 36-48 hours, the deep-layer vertical is
expected to decrease from the current 45 kt to less than 10 kt, a
pattern that is usually conducive for intensification. However,
those favorable upper-level conditions will be negated by much
cooler sea-surface temperatures of 23-25 deg C when Hermine will be
north of the Gulf Stream at that time. The global models are in very
agreement on both Hermine weakening and the 34-kt wind field
steadily shrinking by 24 hours and beyond. Further weakening is
expected through day 4, and both the ECMWF and GFS show Hermine
dissipating by day 5 over the cold waters of the North Atlantic.

The initial 50-kt wind radii have been adjusted based on data from
the reconnaissance aircraft and an earlier ASCAT overpass.

FORECAST POSITIONS AND MAX WINDS

INIT  05/0900Z 37.7N  68.3W   60 KT  70 MPH...POST-TROPICAL
 12H  05/1800Z 38.0N  69.1W   60 KT  70 MPH...POST-TROPICAL
 24H  06/0600Z 38.6N  70.2W   55 KT  65 MPH...POST-TROPICAL
 36H  06/1800Z 39.0N  70.4W   45 KT  50 MPH...POST-TROPICAL
 48H  07/0600Z 39.7N  70.1W   40 KT  45 MPH...POST-TROPICAL
 72H  08/0600Z 40.7N  67.7W   35 KT  40 MPH...POST-TROPICAL
 96H  09/0600Z 43.0N  62.7W   30 KT  35 MPH...POST-TROPICAL
120H  10/0600Z...DISSIPATED

$$
Forecaster Stewart
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