A storm system will produce moderate to heavy rain in the Southern California region through midday Wednesday, the National Weather Service said. There is a chance of thunderstorms, and the heaviest rain is likely to fall between 4 a.m. and 2 p.m.
A flash flood watch is in effect for Orange County and parts of Riverside, San Bernardino and San Diego counties from 6 a.m. to 6 p.m. Wednesday. The watch means that conditions may lead to flash flooding, the weather service said.
The storm is fed by a weak atmospheric river, which is a plume of subtropical water vapor at the lower and middle levels of the atmosphere. Atmospheric rivers dump moisture when they lift over higher terrain, which causes the moisture to cool and condense, falling as rain.
Rain is predicted to fall at a rate of about a half-inch per hour, but could be three-quarters of an inch per hour or higher in thunderstorms.
Because of the warm nature of this system, snow levels will be at about 8,000 feet, then lower to about 7,000 feet in the afternoon. Accumulations will be about 3 to 6 inches. Although there may not be any snow on Interstate 5 over the Grapevine, roads will still be wet and winds will be gusty.
Precipitation will taper off Wednesday evening.