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Hurricane Sergio Discussion Number  20
NWS National Hurricane Center Miami FL       EP212018
200 AM PDT Thu Oct 04 2018

Sergio's intensity is estimated to have increased just a little
more this morning, and it remains a powerful category 4 hurricane.
Subjective Dvorak estimates are T6.5/127 kt from TAFB and T6.0/115
kt from SAB, and the initial intensity is raised to 120 kt, which
is a blend of these numbers and very close to the latest objective
ADT estimate.  Sergio's maximum winds are expected to change little
over the next 12-24 hours, and the hurricane's intensity could very
well be modulated by difficult-to-forecast internal processes such
as eyewall replacement cycles.  After 48 hours, an increase in
shear and gradually cooling sea surface temperatures should result
in some weakening, especially when Sergio moves more slowly at the
end of the forecast period and possibly upwells some cooler water
from below.  The updated NHC intensity forecast is nudged downward
during the first 48 hours to follow the latest consensus aids, but
no changes were made to the previous forecast beyond 48 hours.

The hurricane's initial motion is northwestward, or 320/7 kt, with
Sergio located along the southwestern periphery of a mid-level
ridge extending westward from Mexico.  This ridge is expected to
build westward and connect to another mid-level ridge located near
the central Pacific Ocean, forcing Sergio to turn westward, or even
south of due west, between 36-72 hours.  By days 4 and 5, a large
trough near the west coast of the United States should have greater
influence, causing Sergio to slow down and turn northward by the
end of the forecast period.  The track guidance is tightly
clustered for much of the forecast period, although a notable
southward shift among the guidance, especially from the ECMWF,
required a bit of a southward shift in the official forecast as
well.


FORECAST POSITIONS AND MAX WINDS

INIT  04/0900Z 14.4N 118.8W  120 KT 140 MPH
 12H  04/1800Z 15.1N 119.7W  120 KT 140 MPH
 24H  05/0600Z 15.8N 121.0W  115 KT 130 MPH
 36H  05/1800Z 16.0N 122.2W  105 KT 120 MPH
 48H  06/0600Z 15.8N 123.5W   95 KT 110 MPH
 72H  07/0600Z 15.5N 126.2W   90 KT 105 MPH
 96H  08/0600Z 16.0N 128.5W   80 KT  90 MPH
120H  09/0600Z 17.5N 128.5W   75 KT  85 MPH

$$
Forecaster Berg

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