ROSA CONTINUES TO RAPIDLY WEAKEN… …HEAVY RAINFALL EXPECTED ACROSS BAJA CALIFORNIA AND THE SOUTHWESTERN UNITED STATES…

Hurricane Rosa Discussion Number 17
NWS National Hurricane Center Miami FL EP202018
200 AM PDT Sat Sep 29 2018

Rosa’s satellite appearance has degraded significantly since the
previous advisory with an eye no longer evident in infrared imagery.
However, a well-defined low-level eye is evident in recent passive
microwave imagery. A comparison of the two data sources indicate
that the upper-level circulation is tilted to the east of the
low-level eye by 12-18 nmi due to westerly shear of about 15 kt as
assessed by UW-CIMSS. The initial intensity of 85 kt is based on a
blend of satellite intensity estimates form TAFB, SAB, and UW-CIMSS
ADT and SATCON.

Rosa is moving northward now and the initial motion estimate is
350/08 kt. During the next 72 hours, the hurricane is forecast to
move around the western periphery of a deep-layer ridge that extends
from the Gulf of Mexico westward to Baja California, resulting in a
northward motion today, and a turn toward the north-northeast at a
faster forward speed on Monday and Tuesday as a mid-latitude trough
approaches from the west. The latest NHC model guidance is tightly
packed about the previous forecast track, so no significant changes
were required. The new NHC track forecast lies close to a blend of
the consensus models HCCA, FSSE, and IVCN.

Rosa’s intensity has decreased 40 kt during the past 24 hours, with
the bulk of the weakening having occurred during the past 18 hours,
and additional weakening is expected due to the cyclone moving over
cooler water and into a regime of increasing southwesterly vertical
wind shear of more than 30 kt by 48 hours. Although Rosa is
currently located over 28 deg C SSTs, the depth of the warm water is
quite shallow, so cold upwelling occurring beneath the hurricane
will act to hasten the weakening process today. Rosa is forecast to
approach Baja California as a tropical storm, and then quickly
degenerate into a tropical and a remnant low as the cyclone moves
across the mountainous terrain of northern Baja California and
northwestern mainland Mexico. The official intensity forecast
closely follows the intensity consensus models IVCN and HCCA.

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 in a couple of
days. Interests in those locations should monitor the progress of
Rosa.

FORECAST POSITIONS AND MAX WINDS

INIT 29/0900Z 19.2N 118.2W 85 KT 100 MPH
12H 29/1800Z 20.5N 118.4W 75 KT 85 MPH
24H 30/0600Z 22.5N 118.5W 65 KT 75 MPH
36H 30/1800Z 24.4N 118.1W 55 KT 65 MPH
48H 01/0600Z 26.2N 117.3W 45 KT 50 MPH
72H 02/0600Z 30.9N 115.0W 30 KT 35 MPH…INLAND NRN BAJA CALIF
96H 03/0600Z 38.0N 111.0W 15 KT 15 MPH…POST-TROP/REMNT LOW
120H 04/0600Z…DISSIPATED INLAND

$$
Forecaster Stewart