Arizona has been affected by hurricanes on numerous occasions. Usually, these storms originate in the eastern Pacific Ocean, make landfall in the Mexican states of Baja California or Sonora, and dissipate before crossing into the United States. Thus, in most cases, it is only the tropical cyclones' remnant moisture that produces heavy rainfall—and in some occasions, flooding—in portions of Arizona. However, approximately every five years, a tropical cyclone retains sufficient strength to enter the state as a tropical storm or a tropical depression. Arizonans can expect indirect flash floods caused by the remnants of tropical cyclones to occur about every two years.
Tropical cyclones in Arizona are not common, since the predominant wind pattern steers most storms that form in the Eastern Pacific either parallel or away from the Pacific coast of northwestern Mexico. As a result, most storms that could affect Arizona are carried away from the United States, with only 6% of all Pacific hurricanes entering US territory. Not all Arizona hurricanes originate from the Pacific Ocean, however; in July 2008, an Atlantic hurricane named Hurricane Dolly produced rainfall in the eastern portion of the state, and another Atlantic storm reached Arizona as a tropical depression. Many, but not all, of these systems also impacted California.
Despite their rarity, tropical cyclones are among Arizona's most significant weather makers. In years when Arizona is affected by a tropical cyclone, these can be responsible for up to 25% of the rainfall in areas along the Colorado River. Arizona hurricanes are also responsible for torrential rains in localized areas, with the state's 24-hour rainfall record— of precipitation—occurring during Hurricane Nora's landfall in 1997. The heavy rainfall can trigger extensive flash floods, such as the ones produced by the remnants of Tropical Storm Octave in 1983, or the lingering moisture from Tropical Storm Emilia in 2006.
Climatology
Tropical cyclones are not common over Arizona, but on average, a tropical storm or a tropical depression affects the state approximately every five years. However, indirect flash floods caused by the remnants of tropical cyclones are more common, as they tend to occur about every two years.
Storms that approach the southwestern United States, specifically Arizona, generally track closer to the Mexican coast than average, making them more likely to recurve northwards under the influence of an approaching trough. These troughs tend to extend farther to the south during the latter part of the Pacific hurricane season, in the period between late August and early October. These pronounced troughs thus produce a synoptic-scale flow that is conducive to steering hurricanes towards the southwestern United States. All of the systems that have impacted Arizona have formed in the Pacific hurricane season, and only storm remnants have affected the state before August.
The infusions of tropical moisture from tropical cyclones can be a significant portion of the annual rainfall in the region. In years when storms approach Arizona, eastern and northern portions of the state receive on average 6–8% of the monsoon-season precipitation from tropical systems and their remnants. This percentage rises towards the southwestern corner of the state, which can receive up to a quarter of its monsoon-season rainfall from tropical cyclones.
Storm systems
+ Chronology of tropical cyclones in Arizona Storm Peak intensity Season Intensity Date Unnamed Unknown 1921 Remnant low Unnamed Unknown 1921 Tropical depression Unnamed Unknown 1926 Remnant low Unnamed Unknown 1927 Remnant low One Category 1 1929 Tropical depression Unnamed Unknown 1935 Tropical storm Unnamed Tropical storm 1951 Tropical storm Unnamed Category 1 1958 Tropical storm Claudia Tropical storm 1962 Tropical storm Tillie Tropical storm 1964 Remnant low Emily Category 1 1965 Remnant low Kirsten Tropical storm 1966 Remnant low Katrina Category 1 1967 Tropical storm Hyacinth Tropical storm 1968 Tropical depression Pauline Category 1 1968 Remnant low Norma Tropical storm 1970 Remnant low Irene-Olivia Category 3 1971 Remnant low Joanne Category 2 1972 Tropical storm Kathleen Category 1 1976 Tropical storm Liza Category 4 1976 Remnant low Doreen Category 1 1977 Tropical storm Heather Category 1 1977 Tropical depression Octave Tropical storm 1983 Tropical storm Norbert Category 4 1984 Tropical depression Polo Category 3 1984 Remnant low Raymond Category 3 1989 Tropical depression Boris Category 1 1990 Remnant low Lester Category 1 1992 Tropical storm Hilary Category 3 1993 Remnant low Flossie Category 1 1995 Remnant low Ismael Category 1 1995 Remnant low Nora Category 4 1997 Tropical storm Frank Tropical storm 1998 Remnant low Isis Category 1 1998 Remnant low Olivia Tropical storm 2000 Remnant low Juliette Category 4 2001 Remnant low Ignacio Category 2 2003 Remnant low Marty Category 2 2003 Remnant low Javier Category 4 2004 Remnant low Emilia Tropical storm 2006 Remnant low John Category 4 2006 Remnant low Henriette Category 1 2007 Remnant low Dolly Category 2 2008 Remnant low Julio Tropical storm 2008 Remnant low Jimena Category 4 2009 Remnant low Norbert Category 3 2014 Remnant low Odile Category 4 2014 Remnant low Newton Category 1 2016 Remnant low Rosa Category 4 2018 Tropical depression Sergio Category 4 2018 Tropical depression Hilary Category 42023 Remnant lowAugust 20, 2023
Before 1960 Records of tropical cyclones in the East Pacific before 1950 are sparse, but there were still several storms that produced rainfall over Arizona in this period. August 1921: The first known tropical disturbance to affect the state occurred when a remnant low of a tropical cyclone moved into the western portions of the state. September 1921: A tropical depression that had tracked parallel to the Mexican coastline moved into Arizona, causing heavy rainfall on September 30. This tropical storm caused more than three inches of rainfall along the Colorado River valley, with of rain reported in Yuma. Throughout the state, Flagstaff saw of rainfall, while of precipitation fell in Prescott, in Tucson, and in Phoenix. September 1926: Five years later, the remnants of another September storm hit the state, but this time the precipitation was heaviest on the southeastern portion of the state. The 1926 storm caused over of rain in the vicinity of Douglas. September 1927: The remnants of another tropical system caused of rainfall throughout the state. June 1929: The Atlantic hurricane reanalysis project discovered that a rare Atlantic hurricane reached the eastern part of the state as a tropical depression. Damage from this storm, if any, is unknown. August 1935: The remnants of an unnamed tropical storm that landed on Southern California caused torrential rain and flooding across Arizona, especially along the Santa Cruz River and Rillito Creek on Southern Arizona. The rainfall from the storm contributed to an extremely wet month of August, which still holds the monthly rainfall record at the National Weather Service office in Tucson, as of rainfall fell during the month. September 1939: Two tropical systems entered the state during the month. On September 4, the remnants of a former hurricane entered southwest Arizona, near Yuma. More than of precipitation fell in northwest Arizona, with many parts of the state collecting more than an inch of rain. This same system produced more than twice the average annual rainfall in Imperial Valley, California. On the 11th, the remnants of a separate system also passed over southwest Arizona. August 1951: No tropical cyclones are known to have affected Arizona in the 1940s. However, in the 1950s, the remnants of two more storms affected the state. On August 24, 1951, the moisture from a hurricane that made landfall in Baja California moved over the state, producing more than of precipitation over southwestern Arizona. Flagstaff saw of rain, with similar totals measured at Prescott () and Phoenix (). The storm also washed out several roadways near Gila Bend, isolating the city from motorists. Overall, the storm caused $750,000 (1951 USD) in property damage. July 1954: Three years later, the remnants of another hurricane moved over Arizona from the south during the month of July. Damage from this storm is unknown.
1960s September 1962: Remnant moisture from Tropical Storm Claudia caused severe flash floods in the vicinity of Tucson, with of precipitation falling over the headwaters of the washes of Santa Rosa, Jackrabbit, and Brawley during a 14- to 15-hour period. Over of rainfall also fell near the Arizona-Sonora Desert Museum. The ensuing flood of the Santa Cruz River and its tributaries produced a path of destruction about long and up to wide. Santa Rosa Wash conveyed at its peak; Los Robles Wash carried up to , while the Santa Cruz River proper peaked at . The washes and rivers reached depths of up to , and overflowed its banks in places by . Flooding from the storm inundated the towns of Marana and Sells, both in Pima County. Total damage in Pima and Pinal Counties exceeded $11 million (1962 USD). September 1964: The next storm to affect the state was Tropical Storm Tillie in 1964. Although the storm remained at sea, its residual moisture was advected over southern Arizona, allowing a passing cold front to trigger widespread showers and thunderstorms on the evening of September 9. Tucson received of rainfall in a 24-hour period between September 9 and 10, and two locations—one in the Catalina Mountain foothills and one near Sahuarita—recorded of precipitation. Coupled with rain during the previous week, the Santa Cruz River produced heavy runoff, with peak flows of recorded near Cortaro. September 1965: The following year, the remnants of Hurricane Emily crossed into Arizona from Baja California. Any damage from the storm is not known. September 1966: The remnants of Kirsten caused of rainfall in Nogales. August 1967: Hurricane Katrina brought heavy rainfall into the southern portion of the state as a tropical depression. The decaying storm produced about of rainfall across southern Arizona. The peak recorded rainfall occurred at Wellton, where were measured between September 1 and 2. Yuma recorded within a 24-hour period; that was the heaviest rainfall recorded in a four-year period, and was more than the normal rainfall that the city receives during the entire fall season. August 1968: Two storms approached Arizona in 1968. The first was Tropical Storm Hyacinth in August. It reached the southeastern corner of the state as a tropical depression, and produced showers and thunderstorms over the eastern portion of the state. October 1968: The last storm to impact Arizona during the decade was Hurricane Pauline, which added high amounts of moisture ahead of a cold front in early October. The added instability in the atmosphere allowed the cold front to produce severe thunderstorms, including an F2 tornado that wrecked several homes and caused $250,000 (1968 USD) in damage when it touched down in Glendale.
1970s
September 1970: The remnants of Tropical Storm Norma became Arizona's deadliest storm when they contributed to the disaster known as the "Labor Day storm of 1970". As Norma dissipated, moisture from the cyclone was entrapped in a large extratropical low. Much of the southern and central parts of the state saw of rainfall, and mountainous locations saw between . Much of the region saw extensive flash flooding that killed 23 people and caused significant damage.