Tropical Storm Nora entered the lower Colorado River region and brought an unusual combination of tropical-force winds, intense rainfall, flooding, and evacuations to western Arizona. The event stands among the most significant twentieth-century tropical cyclone episodes in the state. Arizona lies far from the principal hurricane landfall zones of the United States, but eastern Pacific tropical cyclones periodically carry moisture and organized circulation northward across Mexico and the Baja California Peninsula. Nora was especially important because it retained a well-defined circulation as it approached the Arizona border and passed through the Yuma region.
Nora originated over the eastern North Pacific on September 16, 1997. It intensified rapidly and eventually became a major hurricane, reaching maximum sustained winds of approximately 115 knots. Its development occurred during the strong 1997–1998 El Niño episode, when unusually warm sea-surface temperatures extended northward along portions of the eastern Pacific. Those warm waters helped sustain several powerful tropical cyclones during the 1997 season. Nora moved generally northwestward before turning north and northeast toward the Baja California Peninsula.
Early on September 25, Nora made landfall on Baja California near Punta Eugenia. It crossed the peninsula and subsequently moved across the northern Gulf of California region before approaching the southwestern United States. By the afternoon, the weakening cyclone entered the United States near the California-Arizona border. National Weather Service historical records describe the storm center as passing directly over Yuma as Nora moved northward along the lower Colorado River corridor. Wind gusts in Yuma reached approximately 54 miles per hour.
Rainfall proved more historically significant than the wind. The National Weather Service recorded 3.59 inches of rain at Yuma Airport during the storm. That amount was extraordinary for one of the driest cities in the United States. At the time, Yuma's average annual precipitation was approximately 3.17 inches, meaning the storm delivered more than a typical year's rainfall within a short period. Other measurements differ slightly according to observation site and reporting interval, but all confirm unusually heavy precipitation across southwestern Arizona.
Conditions became still more extreme in the mountains northeast of Yuma. A rain gauge on Harquahala Mountain recorded nearly twelve inches of precipitation within twenty-four hours. National Weather Service records list 11.97 inches, while a later technical study reported an automated measurement of 12.01 inches. The measurement exceeded the previous Arizona twenty-four-hour rainfall record of 11.40 inches established at Workman Creek during the destructive Labor Day storm of 1970, although the Harquahala observation was initially treated as unofficial because of questions associated with the automated gauge and verification standards.
Flooding spread across western Arizona as Nora's tropical moisture crossed desert watersheds that normally receive limited precipitation. The U.S. Geological Survey reported that most areas affected by the storm received approximately one to three inches of rain, with locally higher amounts. The storm moved rapidly, which reduced the duration of heavy rainfall over individual locations, but flooding still affected roads, washes, low-lying areas, and communities along the lower Colorado River region. Approximately 240 people were evacuated in the Yuma area.
Nora also demonstrated the forecasting challenges associated with tropical rainfall in desert environments. The National Weather Service office in Phoenix altered the rainfall-estimation settings used by the Doppler radar south of Yuma before the storm arrived because standard radar calculations were underestimating precipitation in tropical rain bands. Later analysis found substantial differences between radar estimates and measured rainfall, especially close to the Yuma radar. The event therefore contributed to meteorological study of radar performance during tropical precipitation in the Southwest.
By September 26, Nora had weakened to a tropical depression while continuing rapidly northward through western Arizona and toward Utah. Its circulation dissipated later that day. The storm nevertheless left a clear place in Arizona's climatological record. National Weather Service records identify Nora as one of several tropical cyclones since the mid-twentieth century that reached southern Arizona while retaining an organized tropical depression or tropical storm circulation.
The historical significance of Nora rests in the convergence of several exceptional conditions. A major Pacific hurricane survived its journey across Baja California, reached the lower Colorado River region with organized circulation, produced tropical-storm-force conditions around Yuma, generated rainfall comparable to or greater than annual desert totals, forced hundreds of evacuations, and produced extraordinary precipitation on Harquahala Mountain. The September 25, 1997, event remains an important example of Arizona's vulnerability to eastern Pacific tropical cyclones and the severe flooding they can produce far inland from the Pacific Ocean.
References / More Knowledge:
National Weather Service, Phoenix. “Historical Arizona Storms of the 20th Century.” https://www.weather.gov/psr/Arizona_historic_storms
National Weather Service, Tucson. “Southeast Arizona Tropical Weather Page.” https://www.weather.gov/twc/tropical
Cushmeer, Naeemah. “KYUX WSR-88D Rainfall Estimates During Tropical Storm Nora.” Western Region Technical Attachment No. 99-22, National Weather Service, 1999. https://www.weather.gov/media/wrh/online_publications/TAs/ta9922.pdf
National Oceanic and Atmospheric Administration, Western Region. “Hurricane Nora, September 22–26, 1997.” https://www.weather.gov/media/wrh/online_publications/TMs/TM-258.pdf
U.S. Geological Survey. “National Water Conditions: September 1997.” https://water.usgs.gov/nwc/back_issues/sep97_cov.html
National Weather Service. Summary of Natural Hazard Statistics for 1997. https://www.weather.gov/media/hazstat/sum97.pdf