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Tropical Storm Sergio Discussion Number   4
NWS National Hurricane Center Miami FL       EP212018
400 AM CDT Sun Sep 30 2018

Microwave and infrared satellite images, along with scatterometer
wind data, indicate that Sergio has continued to become better
organized, with strong convection now more concentrated near the
center. However, scatterometer data showed that the inner-core wind
field is still rather loose and not particularly well-defined quite
yet. The initial intensity has been increased to 45 kt based on a
Dvorak satellite intensity estimate of T3.0/45 kt from TAFB and
SAB, which is supported by a 0536 UTC ASCAT pass that contained a
few 45-kt vectors east of the center.

The initial motion estimate is 270/10 kt. There is no significant
change to the previous forecast track or reasoning. For the next 48
hours or so, Sergio should move generally westward along the
southern periphery of a large, deep-layer subtropical ridge that
extends from Mexico westward across the eastern and central Pacific.
By 72 hours, a broad mid- to upper-level trough is forecast to drop
southward and southeastward over the southwestern U.S. and the
northeastern Pacific, producing a break in the ridge which will
allow Sergio to move toward the northwest at a slower forward
speed through 120 hours. The new NHC track forecast is similar to
but slightly south of the previous advisory track, and lies close
to an average of the consensus models TVCE, HCCA, and FSSE.

Sergio is forecast to remain in a low-shear, warm-water, and
high-moisture environment for the next 36 hours or so, during which
time rapid intensification is expected. In the 48- to 72-hour
period, the vertical shear is forecast by the GFS and ECMWF global
models to increase from the northeast at around 25 kt, which should
act to cap the strengthening process, and possibly even induce some
weakening. By 96 and 120 hours, the shear is expected to decrease
to 5 kt or less, which would favor re-strengthening. However, due
to the uncertainty in how much Sergio's inner-core wind field will
be disrupted by the aforementioned strong shear, the intensity
forecast is simply leveled off at 95 kt at 72 hours and beyond.
The official intensity forecast is slightly higher than the previous
advisory and closely follows the IVCN consensus model, which is
lower than the more robust HCCA and FSSE models, which bring Sergio
to near category 4 strength in 36-48 hours.

FORECAST POSITIONS AND MAX WINDS

INIT  30/0900Z 12.0N 104.3W   45 KT  50 MPH
 12H  30/1800Z 12.0N 105.9W   60 KT  70 MPH
 24H  01/0600Z 11.9N 108.2W   75 KT  85 MPH
 36H  01/1800Z 11.6N 110.4W   90 KT 105 MPH
 48H  02/0600Z 11.7N 112.4W  100 KT 115 MPH
 72H  03/0600Z 13.0N 115.9W   95 KT 110 MPH
 96H  04/0600Z 15.2N 118.6W   95 KT 110 MPH
120H  05/0600Z 17.1N 120.8W   95 KT 110 MPH

$$
Forecaster Stewart

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