841 
 WTPZ45 KNHC 300840
 TCDEP5
 
 Hurricane Rosa Discussion Number  21
 NWS National Hurricane Center Miami FL       EP202018
 200 AM PDT Sun Sep 30 2018
 
 Infrared and microwave satellite images indicate a continued erosion
 of Rosa's inner-core structure since the previous advisory due to
 southwesterly vertical wind shear near 20 kt along with colder water
 beneath the hurricane and entrainment of drier mid-level air in the
 southwestern quadrant. The intensity was assessed as 80 kt at 0600
 UTC based on the high-end estimates from TAFB and UW-CIMSS ADT and
 SATCON, but since then the rapid erosion of the eye feature and
 strong northeastward tilt to the vortex column noted in microwave
 imagery suggests a lower estimate of 75 kt for the advisory
 intensity.
 
 This initial motion estimate is 005/10 kt. Rosa is forecast to
 continue moving northward around the western edge of a deep-layer
 ridge for the next 24 h or so, followed by a turn toward the
 north-northeast on Tuesday as a mid-/upper-level trough approaches
 from the west. As the low- and upper-level circulations continue
 to decouple, Rosa should essentially maintain its current forward
 speed until landfall occurs in 36-48 hours due to the cyclone not
 being influenced by the faster deep-layer steering flow. The new NHC
 track forecast is a little slower than the previous advisory track,
 and closely follows the consensus models HCCA and IVCN. A 72-hour
 forecast position continues to be provided for continuity purposes,
 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 moving over waters colder than 25 deg C, with colder
 water near 22 deg C ahead of the cyclone just prior to landfall.
 The combination of increasing wind shear, cooler waters and drier
 and more stable air being entrained from the west should result in
 steady or even rapid weakening of the cyclone until landfall occurs.
 The official forecast follows the sharp weakening trend indciated in
 the previous advisory, which is supported by the latest intensity
 guidance. Rosa is expected 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.
 
 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  30/0900Z 23.3N 118.8W   75 KT  85 MPH
  12H  30/1800Z 24.8N 118.3W   65 KT  75 MPH
  24H  01/0600Z 26.5N 117.2W   50 KT  60 MPH
  36H  01/1800Z 28.3N 116.0W   45 KT  50 MPH
  48H  02/0600Z 30.6N 114.5W   35 KT  40 MPH
  72H  03/0600Z 37.2N 110.8W   20 KT  25 MPH...POST-TROP/INLAND
  96H  04/0600Z...DISSIPATED INLAND
 
 $$
 Forecaster Stewart
 
 

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