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Tropical Storm Helene Discussion Number 6
NWS National Hurricane Center Miami FL AL082018
500 PM AST Sat Sep 08 2018
During the past few hours, Helene has developed a convective band
that wraps about three-quarters of the way around the storm, and
the intensity of the inner core convection has increased. In
addition, microwave imagery shows an elliptical inner ring feature.
Satellite intensity estimates at 18Z were in the 35-50 kt range, and
given the recent increase in organization the initial intensity is
increased to 50 kt.
Helene should continue to strengthen through at least 72 h in a
light vertical shear environment over sea surface temperatures near
27C. This part of the intensity forecast has been increased over
the previous forecast and lies near the intensity consensus.
However, there remain a couple of alternate forecast scenarios. The
first is that Helene could rapidly intensify and become stronger
than currently forecast, and the rapid intensification index of the
SHIPS model has about a 35 percent chance of 25 kt of strengthening
in the next 24 h. The second is that a large plume of African dust
and associated dry air that has spread over the Cabo Verde Islands
starts entraining into the cyclone and inhibits intensification.
The former alternative is more likely than the latter at this time.
After 72 h, Helene is expected to encounter increasing southwesterly
shear associated with a mid- to upper-level trough over the central
Atlantic, which should cause the system to weaken significantly.
The initial motion is 275/11. There again is little change in the
forecast guidance or the forecast track, with Helene expected to
move westward to west-northwestward for the next 3 days or so on the
south side of the subtropical ridge, then turn northwestward in
response to the aforementioned trough.
FORECAST POSITIONS AND MAX WINDS
INIT 08/2100Z 13.6N 21.7W 50 KT 60 MPH
12H 09/0600Z 13.7N 23.5W 55 KT 65 MPH
24H 09/1800Z 14.1N 26.2W 65 KT 75 MPH
36H 10/0600Z 14.7N 29.1W 75 KT 85 MPH
48H 10/1800Z 15.5N 32.0W 85 KT 100 MPH
72H 11/1800Z 17.5N 37.0W 85 KT 100 MPH
96H 12/1800Z 19.5N 40.5W 65 KT 75 MPH
120H 13/1800Z 23.0N 42.5W 55 KT 65 MPH