When it rains several inches in a very short time, rivers in southern California begin to flood. In 1891, thirty inches of rain were measured over two days where the San Diego Wild Animal Park now exists. Floods can also occur when there is little or no rain. Much of San Diego's water is the result of rainfall in the mountains. If it rains too much there, the water rushes down the mountains and into the ocean causing the rivers of San Diego to overflow.
Large flooding is very hard to predict.
Sometimes it will flood every year and other times it won't flood for forty years. This irregularity is caused by San Diego's climate. Normally, high-pressure areas over the ocean cause northerly coastal winds that keep the California Current cold. This stabilizes the high-pressure region off the coast of San Diego. High pressure means falling air, which is a condition minimizing cloud formation.