ZCZC MIATCDEP4 ALL TTAA00 KNHC DDHHMM Hurricane Hilary Discussion Number 12 NWS National Hurricane Center Miami FL EP092023 300 AM MDT Sat Aug 19 2023 Hilary continues to display a large and well-defined eye, surrounded by very cold convective cloud tops of -75 degrees Celsius. There has been some noticeable thinning of the convection on the western side of the circulation, which could indicate that the hurricane is beginning to pull down cooler, more stable air from the north. That said, an Air Force Reserve Hurricane Hunter aircraft measured peak 700-mb flight-level winds of 128 kt, which still supports an initial intensity of 115 kt. The plane also found that the central pressure has dropped slightly to 943 mb. Hilary continues to move toward the north-northwest, or 335/11 kt. Southerly flow between a large mid-level high over the Central Plains and a cut-off low near the central California coast is expected to accelerate Hilary north-northwestward and then northward during the next 48 hours, with its forward motion likely reaching 25-30 kt by the time the system reaches southern California on Sunday. In fact, the track guidance has sped up again on this cycle, and the updated NHC track forecast is a little faster than the previous forecast after 24 hours. Users are reminded that the exact details of the track forecast, including where Hilary might make landfall, are of little overall importance since strong winds and heavy rainfall will extend far from the center. These hazards will also begin well in advance of the arrival of the center. The degradation of the convection on Hilary's western side probably suggests that weakening will begin very soon. Hilary's circulation will begin moving over sea surface temperatures colder than 26 degrees Celsius later this morning, and the cooler waters and corresponding more stable air mass are likely to cause fast weakening over the next couple of days. Hilary is expected to pass near the west-central coast of the Baja California peninsula still as a hurricane tonight, but then weaken to a tropical storm by Sunday while it's moving near the northwestern coast of the peninsula. Hilary will still be producing tropical-storm-force winds when it reaches southern California on Sunday. The circulation is likely to dissipate around or soon after 48 hours, but a 60-hour forecast point is still provided for track continuity. KEY MESSAGES: 1. Heavy rainfall associated with Hilary will begin well in advance of the center, from the Baja California Peninsula into the Southwestern United States. Preparations for the impacts of flooding from rainfall should be completed as soon as possible, as heavy rain will increase ahead of the center today. In the Southwestern United States, flash, urban, and arroyo flooding is expected, with dangerous and locally catastrophic impacts likely from tonight into Monday. 2. Hurricane conditions are expected along the west-central coast of the Baja California Peninsula within the hurricane warning area tonight and Sunday morning, and are possible in the Hurricane Watch area on Sunday. 3. Tropical storm conditions are expected to begin on Sunday in southern California within the Tropical Storm Warning area. Winds could be particularly strong and gusty in and near areas of higher terrain. 4. Large swells generated by Hilary will affect portions of the coast of southwestern Mexico, the Baja California peninsula, and southern California over the next few days. These swells are likely to cause life-threatening surf and rip current conditions. FORECAST POSITIONS AND MAX WINDS INIT 19/0900Z 20.9N 113.3W 115 KT 130 MPH 12H 19/1800Z 22.8N 113.9W 105 KT 120 MPH 24H 20/0600Z 26.2N 114.8W 85 KT 100 MPH 36H 20/1800Z 30.6N 116.0W 60 KT 70 MPH...W COAST OF BAJA CALIF 48H 21/0600Z 36.4N 117.4W 40 KT 45 MPH...INLAND 60H 21/1800Z 42.6N 117.5W 25 KT 30 MPH...POST-TROP/INLAND 72H 22/0600Z...DISSIPATED $$ Forecaster Berg NNNN