431 
 WTPZ45 KNHC 010839
 TCDEP5
 
 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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