Fullerton gets about 14 inches of rain a year. That number makes people relax, and it should not, because almost all of it falls between November and March and it tends to arrive in a small number of intense storms.
Fullerton sits in the Coyote Creek watershed. Brea Creek drains south through the city below Brea Dam and joins Coyote Creek, which runs along the county line north and west of Fullerton. The system works, but properties near the creek corridor — the Coyote Hills area, parts of West Fullerton and streets backing onto a channel — are the ones that feel it first when a storm outruns the design.
The second pattern is grade. South Fullerton below Commonwealth is flat, and flat means water sits. Streets with old kerb-line drainage back up, and once water is standing at the door threshold it takes very little to get inside.
The third is slope. Homes below hillsides in Rolling Hills, Bastanchury and along the Coyote Hills ridge take sheet flow off the slope during heavy rain, especially where landscaping or hardscape has redirected it toward the house.