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Tropical Storm Rosa Discussion Number  25
NWS National Hurricane Center Miami FL       EP202018
200 AM PDT Mon Oct 01 2018

Convection has been steadily decreasing due to the combination
of southwesterly shear of about 30 kt and cool SSTs of less than
24 deg C. As a result, only a small patch of thunderstorms
remains in the northeastern quadrant of Rosa's circulation. ASCAT
passes around 0414Z and 0516Z only indicated wind speeds of 41-42 kt
in the eastern semicircle. Assuming that some undersampling is
occurring, the intensity has only been decreased to 45 kt for this
advisory. Additional weakening is expected due to a further increase
in the vertical wind shear and decreasing ocean temperatures beneath
the cyclone. The official intensity forecast calls for Rosa to be
near 35 kt at the time of landfall later today, but it is possible
that more weakening could occur than currently indicated.
Regardless, strong tropical-storm-force wind gusts will be
widespread across the higher elevations of the northern Baja
California peninsula. Rosa is expected to become remnant low before
the cyclone reaches Arizona due to the rugged terrain of Baja and
northwestern Mexico, and the continued strong wind shear.

Rosa is moving northeastward now, or 035/10 kt. A northeastward to
north-northeastward motion is expected until and after landfall,
with some acceleration beginning later tonight. The latest NHC
model guidance remains tightly clustered, and the official forecast
track lies close to the previous track and an average of the TVCE,
HCCA, and FSSE multi-model consensus aids.

Key Messages:

1. The main hazard expected from Rosa or its remnants is very heavy
rainfall in Baja California, northwestern Sonora, and the U.S.
Desert Southwest.  These rains are expected to produce
life-threatening flash flooding and debris flows in the deserts, and
landslides in mountainous terrain.  For more information about
potential rainfall in that area, please see products from the
Weather Prediction Center and your local NWS forecast office.

2. Tropical storm conditions are expected over portions of the
central and northern Baja California peninsula later today,
especially over higher elevations. These conditions could spread
into the northern Gulf of California later tonight. Interests in
those locations should monitor the progress of Rosa.

FORECAST POSITIONS AND MAX WINDS

INIT  01/0900Z 26.7N 117.0W   45 KT  50 MPH
 12H  01/1800Z 28.0N 116.1W   35 KT  40 MPH
 24H  02/0600Z 30.1N 114.8W   30 KT  35 MPH...INLAND NRN BAJA CALIF
 36H  02/1800Z 32.9N 113.5W   20 KT  25 MPH...POST-TROP/INLAND
 48H  03/0600Z 35.9N 112.2W   15 KT  15 MPH...POST-TROP/INLAND
 72H  04/0600Z...DISSIPATED INLAND

$$
Forecaster Stewart

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