A flash flood warning is in effect for east central Orange County until 5:45 p.m.
The National Weather Service issued the alert at 2:44 p.m. Sunday for east central Orange County and west central Riverside County.
At midafternoon, a large thunderstorm developed east of Irvine, prompting a flash flood warning for the Orange County mountains and the Lake Elsinore area.
Excessive runoff from heavy rainfall could cause flooding of small creeks and streams, on country roads and on farmland along the banks of creeks and streams, according to the weather service.
Forecasters warn drivers to avoid areas where the water covers the roadway as the water depth may be too great to allow vehicles to cross safely.
—City News Service contributed to this report.