These dates conventionally delimit the period of each year when most tropical cyclones form in the northeastern Pacific Ocean.

In addition, Tropical Storm Tillie produced severe flooding to much of the Southwestern United States, in particular in Arizona in early to mid-September.

[3][6][7] During the season, tropical cyclone advisories were issued by the Naval Fleet Warning Central (NFWC) in Alameda, which held responsibility for the basin until 1970.

[5] Moving generally west-northwest, Prudence initially failed to intensify, and maintained winds of 50 mph (85 km/h) for a few days.

After briefly turning west on July 22, Prudence resumed a west-northwest course[3] while attaining its peak wind speed of 70 mph (115 km/h).

[3] Even though Prudence never made landfall, the cyclone produced high waves along the California coast, especially along Newport Beach.

At the time of the upgrade, Tropical Storm Roslyn was located roughly 500 mi (805 km) south-southwest of Cabo San Lucas.

[3] Tropical Depression Silvia was first noted by the NFWC on August 23 about 60 mi (95 km) south of Mazatlan, with winds of 50 mph (80 km/h).

Simultaneously, Silvia moved westward, passing about 60 mi (95 km) south of Cabo San Lucas.

At first, the cyclone moved north-northwest;[3] Tillie reached its peak intensity of 60 mph (95 km/h) at that time of its formation approximately 150 mi (240 km) west of Mazatlan.

[5] Early on September 8, while located about 100 mi (160 km) southwest of Cabo San Lucas, Tillie began a gradual turn towards the west-northwest.

[10] The storm's residual moisture was adverted over southern Arizona, allowing a passing cold front to trigger widespread showers and thunderstorms on the evening of September 9.

Coupled with rain during the previous week, the Santa Cruz River produced heavy runoff, with peak flows of 15,900 cu ft/s (450 m3/s) recorded near Cortaro.