Hurricane OPHELIA (Text)


Hurricane Ophelia Discussion Number  21
NWS National Hurricane Center Miami FL       AL172017
500 AM AST Sat Oct 14 2017

Ophelia's 20-nmi-diameter eye has continued to become more distinct
and cloud-free, with the eye temperature now reaching 15 deg C.
Satellite intensity estimates range from T4.5/77 kt from TAFB to
T5.0/90 kt from SAB and T5.5/102 kt from UW-CIMSS ADT. For now, the
initial intensity will remain at 85 kt, which is an average of the
available intensity estimates.

The initial motion estimate is 060/21 kt. Ophelia is embedded within
deep-layer southwesterly flow on the east side of a broad
mid-latitude trough. The global and regional models remain in
excellent agreement on the trough amplifying over the next 3-4 days,
which will cause the hurricane to accelerate toward the northeast at
forward speeds near 30 kt by 48 hours. The tight clustering of the
NHC model guidance, which shows very little cross-track or
along-track spread, increases the confidence in the official track.
As a result, no significant changes were made to the previous
advisory, and the new forecast track remains near the middle of the
guidance envelope, close to the HCCA and TVCX consensus models.

Ophelia is expected to remain in relatively low vertical wind shear
environment for the next 12 hours or so, which should help the
hurricane retain much of its current intensity during that time,
even though SSTs are only going to be 24-25C. However, upper-level
temperatures that are still about 2 deg C cooler than normal, which
will help to create sufficient instability to continue to drive the
development of inner-core convection. By 36 hours or so, the shear
is forecast to increase to 30-40 kt and the troposphere is expected
to become stable as sea-surface temperatures decrease to less than
20 deg C. However, even those SST values are about 2 deg C warmer
than normal for this time of the year. Those above-average ocean
temperatures are forecast to combine with strong baroclinic energy
associated with a potent, negatively tilted, upper-level trough,
causing Ophelia to transition into a powerful extratropical low
pressure system. By 48 hours, the post-tropical cyclone is forecast
to maintain sustained hurricane-force winds as it approaches
Ireland, with stronger winds expected over higher terrain.

Given that Ophelia is forecast to become extratropical, the wind
field should expand, resulting in impacts over portions of the
British Isles regardless of its exact location or strength.  By 96
hours, Ophelia should have weakened due to the interaction with
land, causing the surface circulation to become ill-defined, and
dissipation is expected shortly thereafter.

Although the center of Ophelia is not forecast to reach Ireland or
the UK for another 48-60 hours, wind and rain effects 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
beginning tonight, primarily due to an approaching cold front.
However, any track deviation to the west could bring stronger winds
associated with Ophelia's circulation to those islands. Interests
in the Azores should refer to products issued by the Azores Weather
Forecast and Watch Center.


1. Ophelia is expected to become a hurricane-force post-tropical
cyclone by Monday before 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.


INIT  14/0900Z 33.9N  28.6W   85 KT 100 MPH
 12H  14/1800Z 35.3N  25.4W   80 KT  90 MPH
 24H  15/0600Z 38.3N  20.4W   80 KT  90 MPH
 36H  15/1800Z 43.0N  16.0W   75 KT  85 MPH...POST-TROP/EXTRATROP
 48H  16/0600Z 48.4N  12.5W   75 KT  85 MPH...POST-TROP/EXTRATROP
 72H  17/0600Z 57.4N   6.3W   50 KT  60 MPH...POST-TROP/EXTRATROP
 96H  18/0600Z 63.1N    .5W   40 KT  45 MPH...POST-TROP/EXTRATROP
120H  19/0600Z...DISSIPATED

Forecaster Stewart


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Page last modified: Sunday, 31-Dec-2017 12:09:53 UTC