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Hurricane HELENE


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Hurricane Helene Discussion Number  13
NWS National Hurricane Center Miami FL       AL082018
1100 AM AST Mon Sep 10 2018

Helene's cloud pattern has become much better organized during the
past few hours, with a clear 20 n mi wide eye seen in Meteosat
infrared imagery, and a clear eye noted in a 1135 UTC AMSU overpass.
A Dvorak classifications from TAFB and SAB were both T5.0/90 knots
at 1200 UTC, and that is the initial intensity for this advisory.
Helene will remain in an environment that should support some
additional strengthening for the next 12 hours or so, with very low
shear and SSTs of 27C or above. After that time, the SSTs cool
quickly and the shear increases dramatically by 36 hours, which
should induce steady weakening. Late in the period, the shear
continues, and while SSTs increase the atmosphere dries out, and the
intensity is held steady at day 5. The new NHC intensity forecast is
above most of the guidance through 24 hours to account for the
recent intensification of the storm, but then is close to or
slightly above HCCA and IVCN.

The initial motion estimate is 285/14. The subtropical ridge north
of Helene will gradually weaken as an upper-level trough digs
southward around 40W. This pattern will cause Helene to slow down
and gradually recurve during the forecast period, with a faster
northward motion expected at days 4 and 5 as Helene interacts with
the aforementioned upper-level trough. While there is significant
variability between the various global models in the exact
evolution of the upper-level pattern, the track model guidance is
in generally good agreement on this evolution, although the UKMET,
its ensemble mean and the GEFS mean are notable outliers to the
right. The new NHC track was not changed much from the previous one
and lies near the middle of the guidance envelope and is close to
HCCA.

FORECAST POSITIONS AND MAX WINDS

INIT  10/1500Z 14.6N  30.0W   90 KT 105 MPH
 12H  11/0000Z 15.2N  32.0W  100 KT 115 MPH
 24H  11/1200Z 15.9N  34.3W  100 KT 115 MPH
 36H  12/0000Z 16.8N  36.1W   90 KT 105 MPH
 48H  12/1200Z 18.0N  37.7W   75 KT  85 MPH
 72H  13/1200Z 21.5N  39.5W   60 KT  70 MPH
 96H  14/1200Z 27.0N  40.5W   50 KT  60 MPH
120H  15/1200Z 32.5N  40.5W   50 KT  60 MPH

$$
Forecaster Brennan

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