Issued at 900 AM MDT Fri Aug 18 2023
638 WTPZ44 KNHC 181451 TCDEP4 Hurricane Hilary Discussion Number 9 NWS National Hurricane Center Miami FL EP092023 900 AM MDT Fri Aug 18 2023 After an impressive 70-kt increase in winds over the past 24 hours, Hilary's intensity has leveled off. Satellite data indicate that Hilary remains an impressive major hurricane with a 15-20 n mi diameter eye and symmetric convective cloud pattern surrounding that feature. A blend of the latest satellite intensity estimates support holding the initial intensity steady at 125 kt. Microwave data is showing some signs of a double eyewall structure, which is quite common in strong hurricanes like Hilary. Hilary is quite large with its tropical-storm-force winds extending up to 250 n mi from the center. The Air Force Hurricane Hunters are scheduled to investigate the hurricane later today, and the data they collect will provide a better assessment of the system's strength and structure. Hilary has turned to the northwest with the initial motion estimated to be 305/9 kt. A general north-northwest to north motion with a steady increase in forward speed is expected later today and through the weekend as the system is steered by the flow between a strong subtropical ridge over the south-central U.S. and a mid- to upper-level low off the central California coast. This pattern should take the core of Hilary near the central Baja California Peninsula coast Saturday night and then inland over southern California Sunday night. The models are in fairly good agreement, and little change was made to the previous track forecast. It should be noted that strong winds and heavy rains will occur well ahead of the center. Fluctuations in the hurricane's strength are expected today, but Hilary is expected to begin weakening late tonight as it moves over progressively cooler SSTs and into an environment of increasing shear and drier air. The NHC intensity forecast follows the HCCA and IVCN models and is largely an update of the previous one. Based on the forecast, a Tropical Storm Watch has been issued for portions of southern California. This is the first time NHC has issued a watch for that region. Supplemental soundings (weather balloons) are being launched by much of the National Weather Service offices across the western U.S. This effort is highly appreciated as the data should help provide a better assessment of the environment and steering pattern ahead of Hilary. KEY MESSAGES: 1. Heavy rainfall associated with Hilary could produce areas of flash flooding and result in landslides over portions of the Baja California Peninsula late tonight through late Sunday. Rainfall impacts from Hilary within the Southwestern United States are expected to peak this weekend into Monday. Flash, urban, and arroyo flooding is expected, with the potential for rare and dangerous impacts. 2. Hurricane conditions are expected within portions of the hurricane warning area along the west-central coast of the Baja California Peninsula on Sunday and are possible in the Hurricane Watch area. 3. Tropical storm conditions are possible on Sunday in portions of southern California where a Tropical Storm Watch has been issued. 4. Large swells from Hilary will spread northward along the coast of southwestern Mexico and the Baja California Peninsula. These swells will reach the Gulf of California and northern portions of the Baja California Peninsula later this weekend. FORECAST POSITIONS AND MAX WINDS INIT 18/1500Z 17.9N 111.4W 125 KT 145 MPH 12H 19/0000Z 19.2N 112.5W 130 KT 150 MPH 24H 19/1200Z 21.3N 113.6W 115 KT 130 MPH 36H 20/0000Z 23.7N 114.4W 100 KT 115 MPH 48H 20/1200Z 26.8N 115.2W 85 KT 100 MPH 60H 21/0000Z 30.4N 116.4W 65 KT 75 MPH 72H 21/1200Z 35.2N 117.4W 45 KT 50 MPH...INLAND 96H 22/1200Z...DISSIPATED $$ Forecaster Cangialosi
