Butterflies fill the San Fernando Valley
University of California, Irvine has this page with pictures and more information.
Migration: Painted Lady butterflies are often seen migrating northward through Orange County (and elsewhere in the western United States) in February, March and April. A notable migration occurred in 1973. The butterfly was unbelievably abundant in Orange County on February 18 and 19, 1973 and it continued to fly in great numbers through April. The butterflies were always seen flying north to northwest. Larvae were so common in the city of Orange that in late April the city sprayed a vacant lot literally infested with the larvae. The spraying was in response to complaints by homeowners that the larvae were turning swimming pools black with their bodies, in addition to feeding on numerous ornamentals.
There is lots more information to be found using this Google Search on Painted Lady
As well as this story, Butterflies invading the Valley, from the LA Daily News.