[Tropical] Tropical Cyclone Discussion

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WTNT42 KNHC 130851
TCDAT2

Hurricane Ophelia Discussion Number  17
NWS National Hurricane Center Miami FL       AL172017
500 AM AST Fri Oct 13 2017

Ophelia's eye has become cloud filled during the past several hours,
and an 0330 UTC GCOM microwave pass indicated that the hurricane
only had about half an eyewall, with nearly all of the deep
convection located over the eastern part of the circulation.
However, the initial intensity remains 90 kt based on Dvorak CI
numbers of 5.0 from TAFB and SAB.

The initial motion estimate is a little faster toward the
east-northeast, or 070/7 kt, with Ophelia embedded in the flow ahead
of a large mid-latitude trough pushing east of Atlantic Canada.  As
this trough swings eastward over the north Atlantic, Ophelia is
expected to continue accelerating and turn northeastward by day 3
and north-northeastward by day 4.  There is high confidence in
the track forecast for the first 48 hours while Ophelia remains a
tropical cyclone.  Although there is some model divergence after 48
hours once Ophelia becomes extratropical, the global models remain
relatively tightly clustered through day 4, and they all agree that
Ophelia will track very close to the western shores of the British
Isles on days 3 and 4.  As such, very little change was made to the
NHC official forecast compared with the previous advisory.  There
is significantly more spread in Ophelia's track by day 5, with the
GFS moving Ophelia over the North Sea while the ECMWF and UKMET
turn the cyclone eastward over Norway.  However, NHC's graphical
product suite cannot handle forecast points east of the Prime
Meridian, so the official forecast now cuts off after day 4.

Vertical shear over Ophelia may decrease a bit over the next 24
hours, and the hurricane will remain over marginally warm waters
for the next day or two.  Hence, only slight weakening is
anticipated through 48 hours, and Ophelia is forecast to remain a
tropical cyclone during that period.  Ophelia is then expected to
merge with an approaching cold front and develop into a warm
seclusion by day 3 to the southwest of Ireland, with baroclinic
forcing likely helping to maintain the cyclone's intensity for
about a day.  Some weakening is anticipated after day 3 while
Ophelia moves near the British Isles, but strong winds are becoming
increasingly likely over portions of Ireland and United Kingdom
regardless of the cyclone's exact intensity.

While the NHC track continues to keep the center of Ophelia south
and east of the Azores, tropical-storm-force winds are possible
throughout the Azores by Saturday night due to an approaching
front.  In addition, the wind field of Ophelia will likely expand as
the cyclone begins extratropical transition, and any deviation to
the left of the forecast track could bring stronger winds 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 transition to a hurricane-force post-
tropical cyclone by Monday before it moves near Ireland and the
United Kingdom.  While post-tropical Ophelia will likely bring some
direct impacts from wind and heavy rain to portions of these areas,
as well as dangerous marine conditions, given the forecast
uncertainty at these time ranges it is too soon to determine the
exact magnitude, timing and location of the impacts.  Residents in
Ireland and the United Kingdom should monitor the progress of
Ophelia for the next several days.  For more information on local
impacts, residents of 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  13/0900Z 31.1N  33.9W   90 KT 105 MPH
 12H  13/1800Z 31.8N  32.5W   85 KT 100 MPH
 24H  14/0600Z 33.1N  29.7W   85 KT 100 MPH
 36H  14/1800Z 35.0N  25.5W   80 KT  90 MPH
 48H  15/0600Z 37.9N  20.8W   80 KT  90 MPH
 72H  16/0600Z 47.9N  13.7W   75 KT  85 MPH...POST-TROP/EXTRATROP
 96H  17/0600Z 57.5N   6.0W   50 KT  60 MPH...POST-TROP/EXTRATROP
120H  18/0600Z...EAST OF THE PRIME MERIDIAN

$$
Forecaster Berg
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