This is what actually happened in the last major El Nino in 1998:
http://www.cbsnews.com/news/el-nino-brings-death-to-california/
There were full-sized HOUSES floating down Laguna Canyon in Laguna Beach one night and 4 people died. This is not a laughing matter, but it's what we need to kill the drought.
But for people like the BAF sprint group that frequently trains in LA, sprinting in the rain in LA is probably still safer the sprinting indoors all the time in London. For the people in the Bay Area, an El Nino in LA is basically what happens every year (except 2015) in Napa.