The summer abundance of Culex tarsalis in Kern County, California, during 1990-98 was related quantitatively to rainfall, snow depth and water content, and runoff of the Kern River. Total monthly rain that fell during winter, lagged by 4-6 months, explained only 13% of the variability in the number of host-seeking females collected per trap night per month during summer. In contrast, regression analysis showed that river runoff 1 month earlier explained 67% of the variability in mosquito abundance. The water content of snowpack measured within the Kern River watershed during winter explained 70% of the variation in average mosquito abundance during the following summer. After being absent from Kern County since 1983, western equine encephalomyelitis virus (WEE) returned during the wet years of 1996-98 after the flow of the Kern River exceeded 150,000 acre-ft (450 hectare-meters) per month. Water content of snow in the Sierra Nevada during winter provided an excellent early warning of vernal river runoff, mosquito abundance, and enzootic WEE activity levels on the floor of the San Joaquin Valley.