ROSA EXPECTED TO PRODUCE FLOODING RAINS ACROSS NORTHWESTERN MEXICO AND THE SOUTHWESTERN UNITED STATES

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