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Tropical Storm SERGIO


Tropical Storm Sergio Discussion Number   6
NWS National Hurricane Center Miami FL       EP212018
300 PM MDT Sun Sep 30 2018

Satellite imagery over the past several hours as well as microwave
and scatterometer data indicate that the center of Sergio is
farther south than previously analyzed. The deep convection near
the center has become ragged in appearance and elongated from
east-west.  However, microwave data continues to show a well-defined
mid-level circulation, and satellite intensity estimates are
unanimously supporting 55 kt, which will be the initial intensity
for this advisory.

The initial motion estimate is now 260/11 kt, partially due to
adjustment of the center farther south. Otherwise, there are no
changes in the forecast track philosophy from the previous advisory.
For the next couple of days, Sergio should move generally westward
or just south of due west around the southern periphery of a large
deep-layer subtropical ridge extending from Mexico westward
across the eastern Pacific. By 48 hours, a mid- to upper-level
trough moving southeastward along and offshore of the western U.S.
will erode the western part of the ridge. This will allow the
cyclone to turn west-northwestward to northwestward from 48- to
96-hours with a gradual decrease in forward speed. Very near the end
of the forecast period, the trough should shift eastward and low-
to mid-level ridging should build to the northwest of the cyclone.
This pattern change should induce a more westward turn by 120 hours.

Despite the current ragged appearance of the cyclone, the
environment of low-shear, high-moisture, and warm waters are
expected to support a period of rapid intensification sometime
within the next 36 hours or so. By 36 hours, northeasterly shear is
expected to develop over the storm and increase to 25 kt by 48
hours. This should at least limit the rate of intensification if not
weaken the cyclone slightly from 48- to 72-hours. After that time,
global models indicate that the shear will lessen to under 10 kt for
the remainder of the forecast period. There may be a window of time
beginning after 72 hours where some additional strengthening could
occur. By 96 hours, the system will then be moving over waters of
26-27 C very near a gradient of cooler SSTs just to the north. The
current intensity forecast calls for a steadying of intensity after
the shear abates, then a slow weakening trend commences by the end
of the period due to cooler SSTs. Other than a slight delay in
Sergio becoming a hurricane, the intensity forecast is essentially
the same as the previous forecast and remains close to the
dynamical model consensus. Based on this intensity forecast, Sergio
is still forecast to become a major hurricane on Tuesday.


INIT  30/2100Z 11.5N 106.3W   55 KT  65 MPH
 12H  01/0600Z 11.4N 108.1W   65 KT  75 MPH
 24H  01/1800Z 11.2N 110.4W   80 KT  90 MPH
 36H  02/0600Z 11.4N 112.6W   95 KT 110 MPH
 48H  02/1800Z 11.9N 114.5W  105 KT 120 MPH
 72H  03/1800Z 13.8N 117.5W  100 KT 115 MPH
 96H  04/1800Z 16.0N 120.0W  100 KT 115 MPH
120H  05/1800Z 17.3N 122.0W   90 KT 105 MPH

Forecaster Latto/Cangialosi