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Tropical Storm HELENE


ZCZC MIATCDAT3 ALL
TTAA00 KNHC DDHHMM

Tropical Storm Helene Discussion Number   4
NWS National Hurricane Center Miami FL       AL082018
500 AM AST Sat Sep 08 2018

Helene's structure appears to be quickly improving this morning.
The initial intensity has been increased to 40 kt based on a blend
of the latest subjective Dvorak intensity estimates and the
UW-CIMSS SATCON.  Several microwave passes overnight indicate that
the cyclone has become more vertically aligned, likely due to a
reformation of the center beneath the strongest convection.  In
fact, a 0733 UTC SSMIS image shows the early development of a low-
to mid-level eye, which is often a precursor to intensification.

Nearly all of the intensity guidance seems to have picked up on the
improved structure of Helene, and the models show far more
intensification than before, especially through 48 h.  Consequently,
a significant change has been made to the intensity forecast, which
now calls for Helene to become a hurricane on Sunday while it is
near the Cabo Verde islands.  Once the cyclone passes the islands,
the environment is expected to remain favorable for intensification
for at least 72 h.  By the end of the forecast period, all of the
guidance shows some weakening due primarily to increased shear
associated with an approaching mid- to upper-level trough over the
central Atlantic.  Given the recent microwave imagery signature, low
shear, and warm SSTs, its possible that the current forecast is
still too conservative, especially for the first 36 h of the
forecast, and further adjustments could be required if it becomes
clear that Helene is intensifying at a faster rate than expected.

Smoothing through the possible reformation of the center, Helene
appears to be moving west with an initial motion around 275/10 kt.
Little change was required to the track forecast, which has been
nudged only slightly south for the first 48 h of the forecast, in
line with the latest track model consensus.  A westward motion
should continue for the next couple of days, as Helene is steered
by a mid-level ridge to the north.  By day 5, the aforementioned
trough over the central Atlantic should force Helene to turn toward
the northwest.

FORECAST POSITIONS AND MAX WINDS

INIT  08/0900Z 13.7N  19.6W   40 KT  45 MPH
 12H  08/1800Z 13.9N  21.2W   50 KT  60 MPH
 24H  09/0600Z 14.1N  23.5W   60 KT  70 MPH
 36H  09/1800Z 14.7N  26.3W   70 KT  80 MPH
 48H  10/0600Z 15.4N  29.3W   75 KT  85 MPH
 72H  11/0600Z 17.2N  35.1W   80 KT  90 MPH
 96H  12/0600Z 19.0N  39.5W   70 KT  80 MPH
120H  13/0600Z 22.0N  42.5W   65 KT  75 MPH

$$
Forecaster Zelinsky

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