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Tropical Storm HELENE


ZCZC MIATCDAT3 ALL
TTAA00 KNHC DDHHMM

Tropical Storm Helene Discussion Number   5
NWS National Hurricane Center Miami FL       AL082018
1100 AM AST Sat Sep 08 2018

The structure of Helene continues to improve, with convective
banding wrapping around almost completely around the center of the
cyclone in visible satellite imagery.  Infrared imagery suggests
this convection is not very strong at the moment, though, and this
may be why recent scatterometer data indicates that the storm has
not strengthened since the last advisory.  The initial intensity
remains 40 kt based on the scatterometer winds.

All indications are that Helene should strengthen through at least
72 h in a light vertical shear environment over sea surface
temperatures near 27C.  This part of the intensity forecast is
similar to the previous forecast and lies near the intensity
consensus.  However, there are a couple of alternate forecast
scenarios.  The first is that Helene could rapidly intensify and
become stronger than currently forecast.  The second is that a
large plume of African dust and associated dry air that has spread
over the northern Cabo Verde Islands starts entraining into the
cyclone and inhibits intensification.  The former alternative seems
more likely than the latter at this time, and if RI begins later
intensity forecasts will need to be increased.  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.

Helene's center appears to have again re-formed, this time a little
to the south.  The somewhat uncertain initial motion is 265/11.
Other than a slight nudge to the south due to the initial position,
there is little change in the forecast track for Helene.  The
cyclone should moved westward to west-northwestward for the next
3-4 days on the south side of the subtropical ridge, then turn
northwestward in response to the aforementioned trough.

FORECAST POSITIONS AND MAX WINDS

INIT  08/1500Z 13.4N  20.6W   40 KT  45 MPH
 12H  09/0000Z 13.6N  22.4W   50 KT  60 MPH
 24H  09/1200Z 13.9N  24.9W   60 KT  70 MPH
 36H  10/0000Z 14.6N  27.8W   70 KT  80 MPH
 48H  10/1200Z 15.3N  30.8W   80 KT  90 MPH
 72H  11/1200Z 17.0N  36.0W   80 KT  90 MPH
 96H  12/1200Z 19.0N  40.0W   70 KT  80 MPH
120H  13/1200Z 22.5N  42.5W   60 KT  70 MPH

$$
Forecaster Beven

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