688 
 WTPZ45 KNHC 300238
 TCDEP5
 
 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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