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Hurricane Ophelia Discussion Number  20
NWS National Hurricane Center Miami FL       AL172017
1100 PM AST Fri Oct 13 2017

The eye of Hurricane Ophelia continues to be remarkably distinct on
satellite and remains surrounded by a ring of deep convection. An
average of objective and subjective Dvorak numbers still support an
initial intensity of 85 kt. The hurricane has managed to move
through an area of relatively low shear and maintain its intensity
so far. This is about to change in a day or so when a cold front
reaches the circulation of the hurricane and the shear increases
substantially. By then, any increase in intensity should be
triggered by baroclinic forces as the cyclone becomes extratropical.
The NHC forecast calls for Ophelia to become extratropical in about
48 hours and dissipate or absorbed by another large low beyond 4
days.

Given that the cyclone 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.

Now that the cyclone is well embedded in the mid-latitude
westerlies, it is moving faster toward the east-northeast or 060
degrees at 17 kt.  An additional increase in forward speed is
anticipated as a large high-latitude trough accelerates the westerly
flow. Track models are in very good agreement and this increases the
confidence in the NHC forecast which is in the middle of the tight
guidance envelope.

Although the center of Ophelia is not expected to reach Ireland or
the UK for another 2-3 days, 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 Saturday night primarily due to an approaching cold front.
However, any track deviation to the left could bring stronger winds
associated with Ophelia's circulation to 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 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.


FORECAST POSITIONS AND MAX WINDS

INIT  14/0300Z 33.0N  30.6W   85 KT 100 MPH
 12H  14/1200Z 34.3N  27.9W   80 KT  90 MPH
 24H  15/0000Z 36.5N  23.2W   80 KT  90 MPH
 36H  15/1200Z 40.3N  17.7W   75 KT  85 MPH
 48H  16/0000Z 46.0N  14.0W   75 KT  85 MPH...POST-TROP/EXTRATROP
 72H  17/0000Z 56.0N   7.0W   50 KT  60 MPH...POST-TROP/EXTRATROP
 96H  18/0000Z 62.0N   1.0W   40 KT  45 MPH...POST-TROP/EXTRATROP
120H  19/0000Z...DISSIPATED

$$
Forecaster Avila

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