[Tropical] Tropical Cyclone Discussion

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WTNT42 KNHC 142037
TCDAT2

Hurricane Ophelia Discussion Number  23
NWS National Hurricane Center Miami FL       AL172017
500 PM AST Sat Oct 14 2017

Ophelia continues to have an impressive appearance in satellite
imagery.  The eye has remained clear all afternoon, surrounded by a
very symmetric ring of cold cloud tops.  Since the cloud pattern has
not changed significantly over the past few hours, the intensity has
been held at 100 kt. Microwave imagery and scatterometer retrievals
indicate that beneath the cold canopy, Ophelia's structure is
beginning to deteriorate, at least slightly. A GMI overpass around
1700 UTC indicated that the vortex is beginning to tilt toward the
east with height, probably a result of increasing southwesterly
shear associated with a large upper-level trough to the west.
Furthermore, a pair of earlier ASCAT passes suggested that an
approaching cold front is already infringing on the NW quadrant of
the circulation, within about 80 n mi of Ophelia's eye.  Given the
close proximity of the cold front and upper-level trough,
extratropical transition will likely begin within about 12 hours.  A
favorable baroclinic environment may allow the cyclone to deepen as
the wind field increases in size, so only a slight decrease of the
maximum wind is expected through 36 hours.  After that time, the
post-tropical cyclone will likely occlude and interact with land,
causing a faster rate of weakening and resulting in the eventual
dissipation of the surface circulation in about 96 hours.

Ophelia continues to pick up forward speed and the initial motion
estimate is now 055/24 kt.  The hurricane remains embedded within
the southwesterly flow ahead of the aforementioned mid-latitude
trough, and this will keep Ophelia on a northeast or north-northeast
heading through the next 72 hours.  The track guidance remains in
fairly good agreement, though it has shifted southward a little for
the first 24 hours.  The new NHC track forecast has been adjusted in
that direction, and is close to the GFS/ECMWF consensus throughout
the forecast period.

Although the center of Ophelia is not forecast to reach Ireland or
the UK until Monday, wind and rains will arrive well in advance of
the cyclone center. Individuals in those locations should consult
products from their local meteorological service for more
information on local impacts.

Tropical-storm-force winds are possible throughout the Azores
after Ophelia passes to the south and east tonight as a cold front
moves through the islands.  Interests in the Azores should refer to
products issued by the Azores Weather Forecast and Watch Center.

KEY MESSAGES:

1. Ophelia is expected to be a powerful extratropical cyclone with
hurricane force winds Monday while it moves near Ireland and the
United Kingdom.  Direct impacts from wind and heavy rain in portions
of these areas are likely, along with dangerous marine conditions.
For more details on the magnitude, timing, and location of impacts
from post-tropical Ophelia, residents in Ireland should refer to
products issued by Met Eireann, and residents in the United Kingdom
should refer to products issued by the Met Office.


FORECAST POSITIONS AND MAX WINDS

INIT  14/2100Z 35.9N  23.7W  100 KT 115 MPH
 12H  15/0600Z 38.0N  20.0W   95 KT 110 MPH
 24H  15/1800Z 42.8N  15.3W   85 KT 100 MPH
 36H  16/0600Z 48.8N  12.3W   75 KT  85 MPH...POST-TROP/EXTRATROP
 48H  16/1800Z 54.0N   9.3W   60 KT  70 MPH...POST-TROP/EXTRATROP
 72H  17/1800Z 59.5N   3.5W   40 KT  45 MPH...POST-TROP/EXTRATROP
 96H  18/1800Z...DISSIPATED

$$
Forecaster Zelinsky
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