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Hurricane Rosa Discussion Number  20
NWS National Hurricane Center Miami FL       EP202018
800 PM PDT Sat Sep 29 2018

Recent microwave images indicate that Rosa's structure has begun to
deteriorate as a result of increasing southerly shear.  Despite the
cold cloud tops noted in infrared imagery, the hurricane's eyewall
is open on the south side, and there is very little convective
banding within the southern half of the circulation.  The various
subjective and objective intensity estimates have either held
steady or fallen a bit from six hours ago, and given the microwave
signature, the initial intensity is lowered to 85 kt.

Rosa is moving northward, or 360 degrees at 10 kt, along the
western edge of a subtropical ridge which extends across northern
Mexico.  The cyclone is forecast to turn north-northeastward and
accelerate slightly during the next 48-72 hours as it is steered
between the ridge and a large mid- to upper-level trough located
off the west coast of the United States.  The track guidance is
clustered fairly tightly, although the ECMWF is notably slower than
the rest of the models, taking a little longer to bring Rosa's
center to the Baja California peninsula.  The updated NHC track
forecast was nudged southeastward beyond 36 hours to account for
the latest model guidance, and it shows Rosa reaching the coast in
about 48 hours.  A 72-hour forecast is provided for continuity, but
Rosa's surface circulation is likely to dissipate before that time
over northwestern Mexico or southern Arizona, with the mid-level
remnants continuing northward across the Desert Southwest and
Intermountain West.

Rosa is now over waters colder than 26 degrees Celsius, and
combined with increasing vertical shear, the cyclone's intensity is
expected to decrease quickly, with rapid weakening even a
possibility beginning on Sunday.  The official forecast follows the
sharp weakening trend noted in the guidance, and it's actually
slightly above the intensity consensus at a few forecast times.
Rosa is likely to devolve into an exposed low-level center with the
associated deep convection being sheared off to its north and
northeast by the time it is nearing the Baja California coast on
Monday.  However, it will take some time for the circulation to spin
down, and Rosa is still expected to bring tropical-storm-force winds
to portions of Baja California in 36-48 hours.  Based on the new
track and intensity forecast, a Tropical Storm Warning and Watch has
been issued for portions of the west and east coast of the Baja
California peninsula, respectively.


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 on Monday, possibly
spreading to the northern Gulf of California Monday night.
Interests in those locations should monitor the progress of Rosa.


FORECAST POSITIONS AND MAX WINDS

INIT  30/0300Z 22.4N 118.9W   85 KT 100 MPH
 12H  30/1200Z 23.9N 118.7W   75 KT  85 MPH
 24H  01/0000Z 25.7N 117.7W   60 KT  70 MPH
 36H  01/1200Z 27.5N 116.5W   45 KT  50 MPH
 48H  02/0000Z 29.6N 115.3W   35 KT  40 MPH...OVER BAJA CALIFORNIA
 72H  03/0000Z 35.7N 111.4W   20 KT  25 MPH...POST-TROP/INLAND
 96H  04/0000Z...DISSIPATED

$$
Forecaster Berg

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