Hurricane Hilary was a large and intense Pacific hurricane in August 2023 that brought torrential rainfall and gusty winds to the Pacific Coast of Mexico along the Baja California peninsula, and the Southwestern United States, resulting in widespread flooding and mudslides to the region. The hurricane was the eighth named storm, sixth hurricane, and fourth major hurricane of the active and highly destructive 2023 Pacific hurricane season. 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 system underwent rapid intensification, reaching maximum sustained winds of 140 mph (220 km/h) and a central pressure of 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 on August 20 as a tropical storm in San Quintín along the western Baja California peninsula. Hilary became a post-tropical cyclone over land, before being absorbed into a new non-tropical low-pressure area early on August 21.
The threat from Hurricane Hilary prompted widespread and varied preparations. In Mexico, thousands of people evacuated to shelters as ports closed along the coast. 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. The Weather Prediction Center and various National Weather Service offices issued forecasts related to the rainfall potential, affecting about 26 million people across Arizona, California, Nevada, and Utah.
In Mexico, the hurricane killed three people and left at least 854 million pesos (US$48 million) in damage. Power outages affected 315,929 people in the country, although most service was restored within four days. 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. The floods killed one person in San Bernardino County. The estimated damage total in the United States was US$900 million, much of it in Inyo County, where most of the roads in Death Valley National Park were damaged by floodwaters. The park was closed for two months, its longest closure ever. Hilary broke records in four U.S. states for wettest tropical cyclone or its remnants.
Meteorological history
The origins of Hurricane Hilary were 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. The tropical wave crossed Central America into the far eastern Pacific Ocean on August 12, producing a large area of disorganized convection, or thunderstorms. As early as August 13, hurricane prediction models anticipated a tropical cyclone would affect Mexico and the southwestern United States. 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.
Upon its formation, Hilary was moving northwestward, steered by a ridge over Mexico. Atmospheric and environmental conditions were conducive for further strengthening, including warm sea surface temperatures of near , ample moisture, and low wind shear. Accordingly, the NHC warned that Hilary had a high likelihood of rapid intensification. Convection around the storm increased and organized into a central dense overcast, with an eye developing in the center. On August 17, Hilary intensified into a hurricane, by which time it had expanded into a large tropical cyclone, with tropical storm-force winds extending 230 mi (370 km) from the center. Symmetric outflow and pronounced rainbands developed around the hurricane. Early on August 18, Hilary intensified into a Category 3 hurricane on the Saffir-Simpson scale, thus becoming a major hurricane. The convection around the center organized into a thick ring around the eye. At 06:00 UTC that day, the NHC estimated that Hilary attained a peak intensity of 140 mph (220 km/h), making it a Category 4 hurricane, while its barometric pressure dropped to . Operationally, the NHC estimated slightly higher winds of 145 mph (230 km/h), and anticipated some slight further strengthening. The revised intensity estimate was based on satellite imagery using the Dvorak technique. By the time of its peak, Hilary had been rapidly intensifying for a continuous 48-hour period following its formation.
After reaching its peak intensity, Hilary weakened slightly due to an eyewall replacement cycle, in which the inner eye collapsed as the larger, outer eyewall contracted. The Hurricane Hunters first investigated the storm late on August 18, by which time Hilary fell to Category 3 intensity. Around that time, the hurricane began moving more to the northwest, reaching the western periphery of the ridge over Mexico. Another Hurricane Hunters flight on August 19 observed that Hilary re-intensified into a Category 4 hurricane, with a secondary peak of 130 mph (215 km/h), and a pressure of . Thereafter, drier and stable air began affecting the hurricane, along with cooler water temperatures, causing the eye to fall apart. Hilary quickly weakened as it accelerated north-northwestward, responding to the steering influences of a trough of low pressure near the California coast and a ridge of high pressure over the central United States. The hurricane weakened into a tropical storm on August 20, as wind shear from the trough displaced the thunderstorms far north of the center. At around 17:00 UTC that day, Hilary made landfall in a sparsely populated part of the municipality of San Quintín, Baja California, with sustained winds estimated at 60 mph (95 km/h). This was about south-southeast of San Diego, California.
After moving ashore, Hilary's circulation became elongated and disrupted over the mountainous terrain of Baja California. Devoid of convection, Hilary degenerated into a post-tropical cyclone, and early on August 21, it was absorbed by a new non-tropical low-pressure area, which was developing near the southern coast of California outside of Hilary's wind radius. Operationally, the NHC had tracked the two systems as the same, assessing that Hilary continued into southern California as a tropical storm, which would have had made it the first tropical storm to cross into the state since Nora in 1997. The combined system, which included the remnants of Hilary, continued moving through the western United States and eventually crossed into Canada.
Preparations
Mexico The Mexican government first issued tropical storm watches for southern Baja California Sur on August 17, upgrading them to warnings later that day. Hurricane watches were issued for western parts of the Baja California peninsula at 03:00 UTC on August 18, and by 15:00 UTC the same day, they were upgraded to hurricane warnings as hurricane watches were issued for the west coast of Baja California.
Nearly 1,900 people evacuated along the Baja California peninsula, utilizing at least 90 shelters. This included about 850 people evacuated by the Mexican Navy from five offshore islands. In Tijuana, at least 51 people rode out the storm in shelters. Officials in the city also moved homeless people to shelters. In Los Cabos at the southern tip of Baja California, dozens rode out the hurricane in storm shelters. Several schools in Cabo San Lucas were used as temporary shelters, and 18,000 soldiers were on alert. Officials closed ports from Oaxaca to Colima on August 17, and later extended closures to Baja California Sur. School classes and public activities were canceled in Baja California. Around 20 flights were canceled at Los Cabos International Airport. Beaches were closed in La Paz, Tijuana, and Ensenada.
United States On August 15, five days before Hilary's landfall and a day before the system was named, local National Weather Service (NWS) offices in California first released videos and briefings related to the potential impacts. Two days later, the agency first issued a flood watch on August 17 for the mountain and desert regions of Southern California. On August 18, the NHC issued its first-ever tropical storm watch for Southern California, later upgraded to a tropical storm warning. The Weather Prediction Center (WPC) also issued a level 4/high risk for excessive rainfall, which was the first ever high risk of flash flooding in the desert valleys of Southern California east of the Peninsular Ranges. The NWS issued a flood watch for portions of California, Nevada, Utah, and Arizona, affecting about 26 million people. WPC forecaster David Roth described the rainfall potential as "exceedingly rare, bordering on unique, for the region from a tropical cyclone and unique for Nevada, which had its first ever High Risk WPC Day issued for the state". Roth expected a broad area would experience 1-in-100-year rainfall totals. The NWS also issued flash flood warnings for San Bernardino and Kern counties.
On August 20, the Storm Prediction Center issued a level 2/slight risk in Southern California, which included 5% risk of tornadoes. That night, a tornado warning was issued for Alpine and Descanso in San Diego County. The WPC later forecast the chance for of rainfall in the mountains of Idaho from the system. President Joe Biden urged people in the storm's path to take precautions. The Federal Emergency Management Agency (FEMA) prepared emergency supplies and coordinated with state agencies. Emergency operations centers were opened in San Diego and Los Angeles.