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Hurricane Sergio Discussion Number  11
NWS National Hurricane Center Miami FL       EP212018
900 PM MDT Mon Oct 01 2018

A just-received GMI overpass indicates that Sergio has become better
organized with formation of a well-defined eye and eyewall under a
convective overcast with cloud tops to -85C.  Satellite intensity
estimates are in the 55-65 kt range, and based on the improved
structure, the intensity will be set at the upper end of that range.
Thus, Sergio is upgraded to a hurricane.

The intensity forecast is low confidence.  The better structure of
Sergio is now conducive for rapid intensification.  However, the
large-scale models forecast northeasterly vertical shear to increase
to 20-25 kt by 24 h, which should be strong enough to stop rapid
development, although the guidance suggests at least slow
strengthening during this time.  The intensity forecast follows the
guidance trend in calling for gradual strengthening, but there could
be a 12-h or so burst of rapid intensification before the shear gets
too strong.  The shear should subside after about 36 h while Sergio
is still over warm water, and the forecast shows the system becoming
a major hurricane near the 72 h point in response to the more
favorable conditions.  After 72 h, a combination of decreasing
sea-surface temperatures along the forecast track and entrainment of
drier air should lead to gradual weakening.

The initial motion is 265/12.  As stated in the previous discussion,
a westward motion at about the same forward speed is expected during
the next day or so while the storm remains on the south side of a
mid-level ridge. A turn to the west-northwest is forecast by late
Tuesday, followed by a northwestward motion on Wednesday as the
western portion of the mid-level ridge erodes due to a large scale
mid- to upper-level trough near the southwestern United States.  By
the end of the forecast period, a ridge is expected to build to the
north of Sergio, which should cause a turn back toward the west. The
track guidance has changed little since the last advisory, and the
new NHC track is close to both the previous forecast track and the
consensus models.

FORECAST POSITIONS AND MAX WINDS

INIT  02/0300Z 10.9N 112.0W   65 KT  75 MPH
 12H  02/1200Z 11.2N 113.7W   75 KT  85 MPH
 24H  03/0000Z 11.7N 115.6W   80 KT  90 MPH
 36H  03/1200Z 12.6N 117.2W   85 KT 100 MPH
 48H  04/0000Z 13.7N 118.5W   95 KT 110 MPH
 72H  05/0000Z 16.0N 121.0W  100 KT 115 MPH
 96H  06/0000Z 17.0N 124.0W   95 KT 110 MPH
120H  07/0000Z 17.5N 127.0W   85 KT 100 MPH

$$
Forecaster Beven

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