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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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