Hurricane Hilary was a large and powerful Pacific hurricane in August 2023 that brought torrential rainfall and gusty winds to the Pacific Coast of Mexico, the Baja California Peninsula, and the Southwestern United States, resulting in widespread flooding and mudslides. The cyclone was the eighth named storm, sixth hurricane, and fourth major hurricane of the 2023 Pacific hurricane season.[nb 1] Hilary originated from a tropical wave south of Mexico on August 16, and strengthened into a hurricane a day later while paralleling the southwest coast of Mexico. The hurricane underwent rapid intensification, reaching maximum sustained winds of 140 mph (220 km/h) and a central pressure of 940 mbar (27.76 inHg) on August 18, making it a Category 4 on the Saffir-Simpson scale. After environmental conditions became unfavorable, Hilary weakened as it approached land, making landfall as a tropical storm in San Quintín along the western Baja California peninsula, becoming post-tropical before being absorbed into a new system shortly after.
In anticipation of "catastrophic and life-threatening flooding", the National Hurricane Center (NHC) issued its first-ever tropical storm warning for Southern California, extending from the Mexico–United States border to just north of Los Angeles.[1] Persistent heavy rainfall, potentially "more than a year’s worth of rain" in some areas, was forecasted. Due to this, flood watches were issued, affecting about 26 million people across four states (Arizona, California, Nevada, and Utah), and the entire region was placed under a high risk threat for flash flooding by the Weather Prediction Center (WPC).[2][3][4]
In Mexico, the hurricane killed two people due to floodwaters, and left 250 million pesos (US$14.7 million) in damage. The storm left behind flooded roads, mudslides and downed trees in the Baja California peninsula and in Southern California. Some areas of the latter region received up to 600% of their annual rainfall averages for the month of August.[5] The floods killed one person in San Bernardino County, California. Damage in the United States were estimated at US$900 million, much of it in Inyo County, California, where most of the roads in Death Valley National Park were damaged by floods. Even so, its overall impact on the region was less severe than initially feared.[6][7][8] Hilary broke records in four U.S. states for wettest tropical cyclone or its remnants.
The origins of Hilary was from a tropical wave that exited the western coast of Africa around August 3. The wave moved westward across the Atlantic Ocean, reaching the Lesser Antilles by August 9. On that day, the National Hurricane Center (NHC) first identified the potential for the eventual development, 156 hours before Hilary's formation.[9] The tropical wave crossed Central America into the far eastern Pacific Ocean on August 12, producing a large area of disorganized convection, or thunderstorms.[9][10] As early as August 13, hurricane prediction models anticipated a tropical cyclone would affect Mexico and the southwestern United States.[11] A low pressure area developed on August 14 south of the Gulf of Tehuantepec, as the thunderstorms gradually organized. After a low-level circulation formed early on August 16, the system developed into a tropical depression by 06:00 UTC, located about 345 mi (555 km) south of Acapulco. Six hours later, the depression intensified into a tropical storm, and the NHC named it Hilary.[9]