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Hurricane HELENE


Hurricane Helene Discussion Number  16
NWS National Hurricane Center Miami FL       AL082018
500 AM AST Tue Sep 11 2018

An Advanced Microwave Technology Sounder (ATMS) 165GHZ overpass on
Helene indicated a well-developed closed eyewall and a deep
convective outer band with associated cold cloud tops of -76C
wrapping around the south through east quadrants of the cyclone.
The initial intensity is held at 95 kt for this advisory and is
based on a blend of the subjective and objective T-numbers.

There still appears to be a small window of opportunity for Helene
to strengthen, during the next 12 hours or so, before the ocean and
the upper wind environment become less conducive.  Afterward, the
SSTs decrease significantly and the vertical shear increases, which
should induce gradual weakening.  By day 3, the sea surface
temperatures increase, however, the southwesterly shear persists and
the mid-level atmospheric moisture decreases.  Interestingly
enough, the global models show Helene either maintaining tropical
storm strength or even intensifying by the end of the period,
possibly due to some mid-latitude dynamic forcing influences.  It's
also worth noting that the Florida State Cyclone Phase Evolution
analysis and forecast product shows the system retaining a
relatively symmetric warm core through the entire forecast.

The initial motion is estimated to be west-northwestward, or 300/12
kt.  A turn more toward the northwest is forecast early on
Wednesday.  Beyond mid-period, the large-scale models agree that a
mid- to upper tropospheric trough over the northeast Atlantic will
dig southward inducing a weakness in the eastern portion of the
subtropical ridge.  In response to this change in the synoptic
steering pattern, Helene should slowly turn northward and then
north-northeastward during the next 72 to 96 hours, followed by a
turn toward the northeast on day 5.  The official forecast is close
to the previous one, and lies near the TVCN consensus model and the
GFS ensemble mean.


INIT  11/0900Z 16.0N  33.6W   95 KT 110 MPH
 12H  11/1800Z 16.8N  35.0W  100 KT 115 MPH
 24H  12/0600Z 18.0N  36.5W   85 KT 100 MPH
 36H  12/1800Z 19.5N  37.6W   75 KT  85 MPH
 48H  13/0600Z 21.4N  38.3W   65 KT  75 MPH
 72H  14/0600Z 25.7N  38.8W   55 KT  65 MPH
 96H  15/0600Z 31.4N  36.6W   50 KT  60 MPH
120H  16/0600Z 37.3N  31.2W   50 KT  60 MPH

Forecaster Roberts