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Tropical Storm SERGIO


Tropical Storm Sergio Discussion Number   8
NWS National Hurricane Center Miami FL       EP212018
300 AM MDT Mon Oct 01 2018

Sergio's convective pattern in infrared satellite imagery has waxed
and waned since the previous advisory. A previously large, solid
curved band has become broken during the past couple of hours, but
the comma head or CDO of the cloud band has increased in size while
cloud tops have cooled to near -80C. Passive microwave imagery
indicates that within the comma head feature an 18-20-nmi-wide
closed eye has developed in the low-levels, while the mid- and
upper-level eye remains open and ragged. Subjective intensity
estimates remain T3.5/55 kt from TAFB and SAB. However, based on the
well-defined, low-level eye, the intensity has been nudged upward to
60 kt for this advisory.

The initial motion is now 270/12 kt, based primarily on microwave
satellite fix positions. Sergio is forecast to move generally
westward for the next 36 hours or so, under the influence of a
strong, deep-layer ridge located to the north of the cyclone. By 48
hours, a mid- to upper-level trough moving southeastward along and
offshore of the western U.S. is expected to gradually erode the
western part of the ridge, allowing Sergio turn west-northwestward
on day 2 and move northwestward on days 3-5.  The new NHC track
forecast is similar to the previous advisory track, and lies near a
blend of the consensus models TCVA, TVCE, and HCCA.

Both the statistical and dynamical intensity models have been too
bullish on Sergio strengthening during the past 36 hours. Although
the environment is quite moist with near 80 percent mid-level
humidity values, a pronounced dry intrusion eroding the inner-core
convection remains. By the time the dry air mixes out, modest
northerly vertical wind shear is expected to hinder development in
the 24-48 hour period. By 72 hours and beyond, the shear is
forecast to decrease below 10 kt, but Sergio will be moving over
cooler waters at that time. Given the mixed environmental signals,
the official intensity is on the conservative side and remains well
below the stronger HCCA and FSSE models, and closer to the simple
consensus model IVCN.


INIT  01/0900Z 11.5N 108.6W   60 KT  70 MPH
 12H  01/1800Z 11.4N 110.3W   70 KT  80 MPH
 24H  02/0600Z 11.5N 112.5W   85 KT 100 MPH
 36H  02/1800Z 12.0N 114.5W   95 KT 110 MPH
 48H  03/0600Z 12.8N 116.1W  100 KT 115 MPH
 72H  04/0600Z 14.9N 118.8W  100 KT 115 MPH
 96H  05/0600Z 16.8N 121.2W   95 KT 110 MPH
120H  06/0600Z 17.9N 123.2W   90 KT 105 MPH

Forecaster Stewart