The 1997-98 El Niño was the strongest such event recorded in the 20th century, and California experienced it at full force. The Bay Area was hit by a series of storms between January and March 1998 that were among the most intense in the region's modern weather history. Atmospheric rivers arrived in waves. Hillsides failed. Rivers flooded. Tens of thousands of people were evacuated. Infrastructure across the region was damaged or destroyed. The El Niño of 1997-98 was not merely a wet winter; it was a stress test of the Bay Area's flood control capacity, its hillside development patterns, and its emergency response systems, and it revealed weaknesses that the region has been working to address in the decades since.
How El Niño Affects Bay Area Weather
El Niño is a periodic warming of the central and eastern Pacific Ocean that shifts the pattern of the jet stream and storm tracks. In normal years, the jet stream across the Pacific tends to track toward the Pacific Northwest, delivering storms primarily to Oregon and Washington. During El Niño events, the jet stream shifts south, directing storms toward California and, particularly, toward the Bay Area and Southern California. The storms themselves become more intense: a southward-shifted jet stream has more energy in the subtropical position, and the warmer Pacific during El Niño provides more moisture for the storms to carry.
Strong El Niño winters in the Bay Area are characterized by above-average rainfall concentrated in very intense events rather than distributed moderate rainfall. The 1997-98 event produced roughly twice the normal annual rainfall in some Bay Area locations, but most of that rainfall came in a few catastrophic storms rather than an extended drizzly wet season.
The 1998 Storms: Damage and Flooding
The worst storms of the 1997-98 El Niño hit the Bay Area in January and February of 1998. The Russian River in Sonoma County, which routinely floods in wet years, rose to record levels that inundated the town of Guerneville and required the evacuation of thousands of residents. The storm total at Guerneville, which sits at a bend in the river where water concentrates, reached levels not seen since the 1982-83 El Niño event.
Hillside failures were extensive across the region. The combination of saturated soils from weeks of above-normal rainfall and then intense atmospheric river events produced landslides and debris flows throughout the Berkeley Hills, the Marin Headlands, the Santa Cruz Mountains, and the Marin Coast. Highway 1 along the Marin and San Mateo coasts was closed multiple times by slide debris. Portions of Highway 17 through the Santa Cruz Mountains were blocked. The cumulative infrastructure damage across the Bay Area from the 1997-98 El Niño was estimated in the billions of dollars.
Lessons from 1997-98 and the Next El Niño
The 1997-98 El Niño prompted significant investment in Bay Area flood control and emergency preparedness. Creek and channel improvements were made throughout the region. Hillside development regulations were reviewed. The National Weather Service expanded its warning systems for atmospheric river events and major storm sequences. When strong El Niño events were forecast in subsequent years, particularly the 2015-16 event, which arrived with predictions of a repeat of 1997-98, the Bay Area mobilized in ways that reflected the 1998 lessons. The 2015-16 event ultimately underperformed its forecasts, but the 2022-23 winter provided a reminder: an active atmospheric river year can still overwhelm Bay Area flood control, as flooding at places like the Pajaro Valley demonstrated.
El Niño events are not uniformly wet for the Bay Area. Some strong El Niños, including 2015-16, produce less rainfall than expected at the latitude of the Bay Area while delivering intense storms further south. The relationship between El Niño strength and Bay Area rainfall is real but imperfect, and the variability within El Niño winters is wide enough that any individual El Niño season should be approached with both preparation and humility about specific predictions.