521 
 WTPZ45 KNHC 291449
 TCDEP5
 
 Hurricane Rosa Discussion Number  18
 NWS National Hurricane Center Miami FL       EP202018
 800 AM PDT Sat Sep 29 2018
 
 Rosa has become significantly better organized during the last
 several hours, likely due to the completion of an eyewall
 replacement cycle before the center moved over cold water.  The
 storm now has a 30-40 n mi wide eye, and the eyewall convective tops
 have become much colder and more symmetric.  The initial intensity
 has been increased to 90 kt based on a blend of various satellite
 intensity estimates, and it is possible that this intensity is
 conservative.  The hurricane has good to excellent cirrus outflow in
 the northeastern semicircle.
 
 The current re-intensification was poorly anticipated, and it is
 unclear how much more strengthening will occur before Rosa reaches
 the 26C isotherm in about 12 h.  Even if the hurricane strengthens
 a little more, the combination of decreasing sea surface
 temperatures and increasing shear should cause steady to rapid
 weakening after 12 h, and the new intensity forecast still calls for
 the cyclone to weaken below hurricane strength before the center
 reaches the Baja California peninsula between 48-72 h.  After
 landfall, Rosa is expected to weaken even faster, and the surface
 circulation is forecast to dissipate just after 72 h in agreement
 with all of the dynamical models.  However, the mid-level
 circulation and the associated rainfall will continue moving across
 the southwestern United States after the surface circulation
 dissipates.
 
 The initial motion is now 355/10.  During the forecast period, the
 hurricane will recurve into the westerlies between a deep-layer
 ridge over northern Mexico and a large mid- to upper-level trough
 over California and the adjacent Pacific.  This should result in a
 continued northward motion through tonight, followed by a turn
 toward the north-northeast on Sunday.  The new forecast track is
 similar to, but a little faster than, the previous track and lies
 near the consensus models.  On the forecast track, the center of
 Rosa will move near or over the central and northern portions the
 Baja California peninsula on Monday and Monday night, and then move
 into the southwestern United States on Tuesday.
 
 
 Key Messages:
 
 1. The main hazard expected from Rosa or its remnants is very heavy
 rainfall in Baja California, northwestern Mexico, and the 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. Rosa could also bring tropical storm conditions to portions of
 the central and northern Baja California peninsula starting on
 Monday.  Interests in those locations should monitor the progress of
 Rosa.
 
 
 FORECAST POSITIONS AND MAX WINDS
 
 INIT  29/1500Z 20.5N 118.4W   90 KT 105 MPH
  12H  30/0000Z 22.0N 118.6W   90 KT 105 MPH
  24H  30/1200Z 23.9N 118.4W   75 KT  85 MPH
  36H  01/0000Z 25.9N 117.6W   60 KT  70 MPH
  48H  01/1200Z 27.7N 116.7W   45 KT  50 MPH
  72H  02/1200Z 33.0N 114.0W   25 KT  30 MPH...INLAND
  96H  03/1200Z...DISSIPATED
 
 $$
 Forecaster Beven
 
 

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