Hilary — 2023

A tropical depression formed by 0600 UTC on 16 August 2023 about 300 nautical miles south of Acapulco, Mexico, from a tropical wave that crossed into the eastern Pacific on 12 August. The system became Tropical Storm Hilary six hours later and moved generally west‑northwest. Favorable ocean and atmospheric conditions led to extremely fast strengthening: Hilary rapidly intensified from a 30‑kt depression to a 120‑kt Category 4 hurricane between 16–18 August. After reaching peak intensity offshore, Hilary underwent eyewall changes and began weakening while turning north‑northwest toward the Baja California peninsula; it weakened to a tropical storm just before landfall and its remnants were absorbed off southern California by 21 August. Hilary made landfall on the Baja California peninsula near the community of San Fernando at about 1700 UTC on 20 August 2023 as a tropical storm with estimated sustained winds of 50 kt (about 58 mph). The center passed near or over Isla Cedros earlier that day. Although the tropical cyclone center did not move into the continental United States, the large wind field and the non‑tropical low that absorbed Hilary produced strong winds and heavy rains across southern California, Arizona, Nevada, and parts of the interior western U.S. on 20–21 August. The storm’s maximum sustained winds at peak were estimated at 120 kt (about 138 mph) with a minimum central pressure of 940 mb, corresponding to a Category 4 hurricane at peak intensity on 18 August. A secondary peak of about 115 kt was observed early on 19 August by reconnaissance aircraft. By landfall on 20 August the central pressure had risen to about 990 mb and winds had fallen to roughly 50 kt. Hilary produced locally large coastal inundation and very heavy rainfall. In Mexico, storm totals reached 12.83 inches (326.0 mm) at San Lucas Norte and 10.85 inches at Santa Águeda in Baja California Sur; Isla Cedros reported a lowest sea‑level pressure of 980.7 mb while near the center. In the United States, mountain and foothill totals in southern California exceeded 10 inches in several spots: Upper Mission Creek recorded 13.07 inches and Mount San Jacinto 11.75 inches. Downtown Los Angeles recorded 2.48 inches on 20 August (a daily record) and 2.99 inches for the month (a monthly record); San Diego set a daily record of 1.82 inches. Death Valley recorded 2.20 inches on 20 August, its highest daily total on record. Hilary was directly responsible for three confirmed deaths: two in Mexico (one in Santa Rosalía, Baja California Sur, on 19 August and one near Cataviña, Baja California, on 20 August) and one in California (a resident of Angelus Oaks presumed drowned when a mobile home was swept away on 20 August). Damage estimates total at least about $900 million (USD) in the United States, and Baja California Sur estimated roughly 250 million pesos (≈ $14.5 million USD) in damage. Notable impacts included widespread flash flooding, mudslides, damaged and washed‑out roads (including long closures in Death Valley National Park), destroyed or flooded homes, grounded fishing boats, downed power lines leaving hundreds of thousands without electricity in Mexico, and numerous swiftwater rescues across southern California and Nevada. Noteworthy aspects of Hilary include its extremely rapid intensification from a weak depression to a Category 4 hurricane in 48 hours and the historic rainfall records set across southern California and Death Valley. Forecasts of Hilary’s formation and track were generally strong, with large lead times and better‑than‑average track accuracy at shorter ranges, but the rapid intensity changes (both strengthening and later rapid weakening) were harder to predict and produced larger intensity forecast errors than the recent average.