Arizona cannot be hit by a hurricane in its fully organized form because it is a landlocked state separated from the Pacific Ocean by hundreds of miles of terrain and the Baja California Peninsula. However, the state is frequently affected by the remnants of powerful Eastern Pacific tropical cyclones. These remnants bring significant, destructive rainfall. Understanding how these storms lose their identity but retain their moisture is key to comprehending Arizona’s true risk from tropical weather.
Why Hurricanes Do Not Reach Arizona
A hurricane, by definition, is a robust tropical cyclone with sustained winds of at least 74 miles per hour. These immense storm systems are powered exclusively by the heat and moisture drawn from warm ocean water, specifically sea surface temperatures that must be approximately 80 degrees Fahrenheit or higher. This continuous input of energy fuels the storm’s powerful circulation and allows it to maintain its structure and intensity over the open water.
When a hurricane makes landfall or moves over cooler waters, it is immediately cut off from this necessary warm, moist air supply. The process of dissipation begins rapidly, as the storm can no longer sustain its internal heat engine. Increased friction from the landmass, including mountains and other terrain, quickly reduces wind speeds and disrupts the storm’s low-level circulation.
The intrusion of dry continental air drawn into the system over land effectively chokes the storm of the moisture needed for deep convection, causing the cyclone to weaken rapidly. By the time a tropical cyclone has traveled the distance from the Pacific coast of Mexico, across the Baja Peninsula or Northern Mexico, and into Arizona, it has been downgraded significantly. The cyclone is typically reduced from a hurricane to a tropical storm, a tropical depression, or simply a post-tropical remnant low-pressure area.
How Tropical Moisture Affects the State
While the wind threat dissipates over land, the tremendous volume of moisture collected by the storm system remains and is channeled into the southwestern United States. Tropical cyclones that form in the Eastern Pacific Ocean are often steered by specific atmospheric patterns toward the Arizona region. A common steering mechanism involves a large-scale trough, or area of low pressure, moving down the West Coast of the United States.
This trough creates a favorable path, pulling the remnant moisture plume northeastward from the Pacific, often through the Gulf of California, and directly into Arizona. The warm waters of the Gulf of California can sometimes provide a brief secondary moisture boost to the dissipating system just before it enters the state. This influx of tropical air interacts with the existing atmospheric patterns, sometimes coinciding with the North American Monsoon season, which enhances the potential for heavy rain.
The primary danger from these tropical remnants is the massive, concentrated rainfall they deliver to the arid and semi-arid landscape, not the wind. Desert environments are particularly susceptible to flash flooding because the dry, hard-packed ground cannot absorb water quickly. The sudden, intense rainfall rapidly fills dry creek beds and washes, overwhelming urban drainage systems and leading to widespread, destructive flooding.
Identifying the Most Recent Significant Tropical Remnant
The remnants of Hurricane Rosa in 2018 represent the most recent, widely reported, and significant tropical moisture event to impact Arizona. Rosa was a powerful Category 4 hurricane over the Pacific before weakening and making landfall as a tropical depression along the coast of Baja California in late September 2018. The storm’s moisture field then surged northward into the southwestern United States.
The widespread rainfall began impacting southern Arizona on September 30, with the main event unfolding across central Arizona on October 1 and 2, 2018. Widespread rainfall totals of one to three inches were measured across central Arizona, with localized amounts much higher. Phoenix Sky Harbor International Airport, the official gauge for the city, recorded 2.36 inches of rain on October 2, which was enough to make it the wettest October day on record for the city.
The heaviest rain fell in the mountainous regions, with a peak total of 6.89 inches recorded at Towers Mountain, located northwest of Phoenix. The resulting flash floods closed schools, snarled traffic, and caused numerous road closures throughout the Phoenix metropolitan area. The rainfall from Rosa’s remnants caused an estimated $50 million in flood damage across the Southwestern United States, illustrating the ongoing threat posed by these weakened tropical systems.