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ZCZC MIATCDEP5 ALL TTAA00 KNHC DDHHMM 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 NNNN