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Hurricane Rosa Forecast Discussion Number 20
2018-09-30 04:38:56| National Hurricane Center (East Pacific)
Issued at 800 PM PDT Sat Sep 29 2018 688 WTPZ45 KNHC 300238 TCDEP5 Hurricane Rosa Discussion Number 20 NWS National Hurricane Center Miami FL EP202018 800 PM PDT Sat Sep 29 2018 Recent microwave images indicate that Rosa's structure has begun to deteriorate as a result of increasing southerly shear. Despite the cold cloud tops noted in infrared imagery, the hurricane's eyewall is open on the south side, and there is very little convective banding within the southern half of the circulation. The various subjective and objective intensity estimates have either held steady or fallen a bit from six hours ago, and given the microwave signature, the initial intensity is lowered to 85 kt. Rosa is moving northward, or 360 degrees at 10 kt, along the western edge of a subtropical ridge which extends across northern Mexico. The cyclone is forecast to turn north-northeastward and accelerate slightly during the next 48-72 hours as it is steered between the ridge and a large mid- to upper-level trough located off the west coast of the United States. The track guidance is clustered fairly tightly, although the ECMWF is notably slower than the rest of the models, taking a little longer to bring Rosa's center to the Baja California peninsula. The updated NHC track forecast was nudged southeastward beyond 36 hours to account for the latest model guidance, and it shows Rosa reaching the coast in about 48 hours. A 72-hour forecast is provided for continuity, 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 over waters colder than 26 degrees Celsius, and combined with increasing vertical shear, the cyclone's intensity is expected to decrease quickly, with rapid weakening even a possibility beginning on Sunday. The official forecast follows the sharp weakening trend noted in the guidance, and it's actually slightly above the intensity consensus at a few forecast times. Rosa is likely 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. Based on the new track and intensity forecast, a Tropical Storm Warning and Watch has been issued for portions of the west and east coast of the Baja California peninsula, respectively. 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 30/0300Z 22.4N 118.9W 85 KT 100 MPH 12H 30/1200Z 23.9N 118.7W 75 KT 85 MPH 24H 01/0000Z 25.7N 117.7W 60 KT 70 MPH 36H 01/1200Z 27.5N 116.5W 45 KT 50 MPH 48H 02/0000Z 29.6N 115.3W 35 KT 40 MPH...OVER BAJA CALIFORNIA 72H 03/0000Z 35.7N 111.4W 20 KT 25 MPH...POST-TROP/INLAND 96H 04/0000Z...DISSIPATED $$ Forecaster Berg
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