Tropical Storm Helene Discussion Number   7
NWS National Hurricane Center Miami FL       AL082018
1100 PM AST Sat Sep 08 2018

Helene continues this evening with a large area of convective
bands that wrap about three-quarters around the center.  A blend of
the subjective Dvorak classifications from TAFB and SAB with
the CIMSS' SATCON gives an intensity of 50 kt, unchanged from the
previous advisory. The tropical storm is situated just south of the
upper-level subtropical ridge, which is inducing weak southeasterly
vertical shear.  That along with warm 28C waters and a moist,
unstable atmosphere should support at least steady intensification
for the next two days.  By day three, however, the shear will
increase out of the southwest as Helene moves close to an
upper-level low. Simultaneously, the SSTs will drop to near 26C and
the mid-troposphere should dry.  The new intensity forecast shows a
peak of 90 kt at 48 h, with gradual to steady weakening thereafter.
This is based upon the IVCN multi-model consensus (minus the
much weaker HMON solution) with a higher peak intensity than the
previous advisory but a slightly faster decay.

Helene's initial position was aided by the availability of a 2222Z
ASCAT pass, which showed it to be somewhat farther south than
earlier estimated.  The initial motion is analyzed to be 260/11,
as it is steered by a deep-layer ridge to its north.  The
tropical cyclone should gradually turn toward the west-northwest by
Monday, then toward the north-northwest by Wednesday.  The new
track forecast - based on the TVCN multimodel consensus - is
slightly south of the previous advisory throughout the forecast
period, mainly because of the southward initial position.

The aforementioned scatterometer pass allowed for a more accurate
assessment of the initial size, which is somewhat larger than
analyzed earlier.  The new wind radii prediction indicates a larger
Helene than the previous advisory, and is based upon the RVCN
multi-model consensus.

FORECAST POSITIONS AND MAX WINDS

INIT  09/0300Z 13.2N  22.8W   50 KT  60 MPH
12H  09/1200Z 13.3N  24.7W   60 KT  70 MPH
24H  10/0000Z 13.8N  27.4W   70 KT  80 MPH
36H  10/1200Z 14.5N  30.2W   80 KT  90 MPH
48H  11/0000Z 15.3N  33.0W   90 KT 105 MPH
72H  12/0000Z 17.1N  37.4W   85 KT 100 MPH
96H  13/0000Z 19.5N  40.5W   65 KT  75 MPH
120H  14/0000Z 22.5N  42.5W   50 KT  60 MPH

$$
Forecaster Landsea