Tropical cyclones that form in the eastern Pacific Ocean rarely reach the California coastline with their full strength. California’s unique geography and oceanic conditions act as a natural defense, forcing these powerful systems to weaken significantly as they track northward. The state’s primary risk comes not from the hurricane-force winds but from the tremendous moisture carried by the remnants of these systems.
The Historical Record of Landfalls
The state has experienced only one confirmed tropical storm landfall in the 20th century. This occurred in September 1939 when a system known as “El Cordonazo” made landfall near Long Beach with tropical storm-force winds. The storm caused widespread flooding and tragically led to the deaths of dozens of people, prompting the establishment of a dedicated weather forecast office for Southern California shortly thereafter.
Since that 1939 event, no tropical storm had made landfall in California until the arrival of Hilary in August 2023. Operationally, Hilary was classified as a tropical storm when it moved into Southern California. However, a post-analysis by the National Hurricane Center determined it had degenerated into a post-tropical low shortly after crossing the Baja California Peninsula. While many cyclones have tracked close, they typically weaken to non-tropical systems before their centers reach the state.
Factors That Typically Weaken Cyclones
The consistently cool sea surface temperatures along the California coast are a primary defense against tropical systems. Tropical cyclones require sea surface temperatures of at least 80 degrees Fahrenheit (about 26.5 degrees Celsius) to maintain their structure and intensity. The southward-flowing California Current brings cold water from the Gulf of Alaska, which keeps the coastal waters well below this threshold.
This cold water acts as a form of natural “refrigeration,” rapidly stripping the storm of the heat energy required to power its central heat engine. Furthermore, the prevailing northwesterly winds along the coast cause extensive upwelling. This process draws cold water from deep beneath the surface upward to replace surface water pushed offshore. This upwelling further cools the environment, making it nearly impossible for a storm to sustain hurricane strength as it moves north.
Atmospheric conditions also play a significant role in weakening and redirecting these storms. Vertical wind shear, which is the change in wind speed or direction with increasing altitude, is often high near the California coast. Strong shear tears the storm’s structure apart, tilting the central vortex and disrupting the mechanism that pulls warm, moist air into the core. These upper-level winds tend to steer storms westward, directing them away from the mainland and out over the open Pacific Ocean.
Distinguishing Tropical Systems
A tropical cyclone is a general term for a rotating, organized system of clouds and thunderstorms that originates over tropical or subtropical waters. Within this classification, storms are categorized by their maximum sustained wind speed using the Saffir-Simpson Hurricane Wind Scale.
A system is classified as a tropical storm when its sustained winds reach 39 miles per hour (mph). It becomes a hurricane when sustained winds reach 74 mph or greater. Most systems that affect California have weakened further into a post-tropical cyclone, meaning the storm has lost its core tropical characteristics, such as a symmetrical structure and a warm center. A post-tropical cyclone can still be a powerful storm, often retaining significant moisture and a broad wind field, but its internal mechanics are no longer purely tropical.
Primary Hazards Associated with Remnants
The primary danger to California from these remnant systems is extreme rainfall and the resulting flash flooding. The moisture-laden air from a tropical system, even after being downgraded, can interact with California’s mountainous terrain to produce extraordinary amounts of precipitation. This is particularly dangerous in steep canyons and desert areas, where the dry ground cannot absorb the sudden deluge of water.
The rapid runoff can trigger life-threatening flash floods, mudslides, and debris flows, especially in areas recently affected by wildfires, which leave the soil unstable. While a strong storm surge is rare due to the weakened nature of the incoming systems, the combination of high surf and elevated water levels can still cause localized coastal erosion and damage to infrastructure along the immediate shoreline. The heavy rainfall can also cause disruption far inland, affecting desert regions and the interior mountain ranges.