The National Hurricane Center monitored the disturbance for several days as the system organized into a tropical storm on September 29.

The storm then turned towards the northwest as it underwent rapid intensification and an eyewall replacement cycle, before peaking as a Category 4 hurricane on October 4, with maximum sustained winds of 140 mph (230 km/h).

Sergio's approach warranted the issuance of tropical storm watches and warnings along the western and eastern coasts of Baja California from October 10–11.

[1] The National Hurricane Center (NHC) forecast on September 25 that a low-pressure area would form a few hundred miles south of the Gulf of Tehuantepec in about two days.

[4] The NHC continued to track the disturbance for a few more days as the system traveled west-northwest,[5] before it developed into Tropical Storm Sergio on September 29 at 12:00 UTC, while approximately 385 mi (620 km) south of Zihuatanejo, Mexico.

[7] Despite being located within a favorable environment of warm sea surface temperatures and low wind shear, Sergio only gradually intensified over the next couple of days, the system becoming a Category 1 hurricane on October 2 at 00:00 UTC.

[9] Sergio then began another period of intensification, becoming the eighth Category 4 hurricane of the season on October 4 at 00:00 UTC, breaking the old record of seven set in 2015.

[1][10] Six hours later, the system peaked with maximum sustained winds of 140 mph (230 km/h) and a minimum central pressure of 942 mbar (27.8 inHg), while located about 825 mi (1,328 km) southwest of Cabo San Lucas.

[1] After maintaining its intensity for 18 hours, the hurricane began to weaken around 00:00 UTC on October 7 due to upwelling and a third eyewall replacement cycle.

[13] The system gradually weakened over the next two days, falling to tropical storm status on October 9 at 18:00 UTC, due to cooling sea surface temperatures.

Sergio made landfall near Los Castros, Baja California Sur, around 12:00 UTC on October 12, as a 50 mph (80 km/h) tropical storm.

[17] A blue alert, meaning minimal danger, was issued for Sinaloa and emergency shelters were opened due to rainfall and Sergio's proximity.

[19] Sergio made landfall in western Baja California Sur and later in Sonora,[1] bringing heavy rains and strong winds to northern Mexico.

Hermosillo experienced street flooding which impeded traffic and overflowed sewers in several areas; multiple power outages also occurred.

[30] High waves from Sergio affected Hawaii and its remnants moved into Arizona, California and Texas, generating heavy rains and spawning multiple tornadoes.

[14][31] Sergio produced 10–15 feet (3.0–4.6 m) waves and rip currents along the shores of Hawaii's Big Island, Maui, Molokai, and Oahu on October 10.

[35] A portion of Old U.S. Route 80 was closed between Gila Bend and Buckeye after rainfall exceeding 1 in (25 mm) per hour caused flooding in the area.

[37] Runoff from heavy rainfall in the Maricopa Mountains flowed across State Route 238, resulting in multiple closures from Mobile to Bosque and US$2,000 in damage.

Numerous other road closures occurred throughout the Phoenix Metropolitan Area as a result of flash floods, damage in the region totaling approximately US$73,000.

[39] Widespread storms caused flash floods throughout the Coachella Valley, with a 12-hour rainfall total of 0.72 in (18 mm) observed at Palm Desert.