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Tropical Storm HELENE


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Tropical Storm Helene Discussion Number  30
NWS National Hurricane Center Miami FL       AL082018
500 PM AST Fri Sep 14 2018

The 40-45 kt of southwesterly vertical shear has really taken its
toll on Helene's cloud pattern.  A severe tilt toward the northeast
is evident in conventional and microwave imagery.  Despite the
sheared structure of the cyclone, the initial intensity is held at
60 kt due to the earlier scatterometer pass showing that value in
the northeast quadrant, and a 1603 UTC SATCON analysis that yields
an intensity estimate of 58 kt.

Little change in strength is forecast during the next 12 hours as
Helene remains over 26 to 27C sea surface temperatures.  Beyond that
brief period, the cyclone should weaken a bit as it completes
extratropical transition while passing just to the northwest of the
Azores Islands and traversing much cooler waters.  The European
models and the Canadian still show Helene being absorbed by a larger
baroclinic system in 4 days, as the cyclone quickly approaches
Ireland and the United Kingdom.  The GFS, which earlier indicated a
different solution resulting in Helene taking a more north-
northeastward to northwest track around the periphery of a
larger, complex non-tropical low, now shows Helene merging with a
front, similar to the European and Canadian scenario.

A series of recent microwave passes reveal that Helene's surface
circulation has been tracking quite a bit to the left of the
previous forecasts.  Consequently, an adjustment to the best track
was made to conform more with the microwave overpasses.  The initial
motion is estimated to be northward, or 010/16 kt.  It is also quite
possible that there maybe some binary involvement between Helene and
Tropical Storm Joyce that could be influencing a motion just to the
left of track.  In any event, a north-northeast to northeast turn is
expected over the weekend as the cyclone accelerates within the
mid-latitude westerlies.  The NHC forecast is again adjusted to the
left through 36 hours, due to the much welcomed microwave images,
and is in best agreement with TVCN multi-model consensus.

Helene is expected to be a post-tropical cyclone when it approaches
Ireland and the United Kingdom in a few days.  Interests in those
locations should consult products from their local meteorological
service for information about potential impacts from Helene.  Local
forecasts and warnings for the UK can be found on the website of the
UK Met Office at https://www.metoffice.gov.uk/.  Local forecasts
and warnings for Ireland can be found on the website of Met Eireann
at https://www.met.ie/.


FORECAST POSITIONS AND MAX WINDS

INIT  14/2100Z 34.1N  36.8W   60 KT  70 MPH
 12H  15/0600Z 36.4N  36.0W   60 KT  70 MPH
 24H  15/1800Z 38.9N  33.7W   55 KT  65 MPH
 36H  16/0600Z 41.0N  30.0W   50 KT  60 MPH
 48H  16/1800Z 43.3N  25.2W   50 KT  60 MPH...POST-TROP/EXTRATROP
 72H  17/1800Z 49.5N  12.9W   50 KT  60 MPH...POST-TROP/EXTRATROP
 96H  18/1800Z...DISSIPATED

$$
Forecaster Roberts

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