Flash flood watch for Antelope Valley, mountains, as deep marine layer covers Southern California

A low-pressure system drew a deep marine layer over Southern California Sunday, with a flash flood watch for the Antelope Valley and some Los Angeles County mountains in force until the evening, the National Weather Service.

Cold air aloft was contributing to the unstable conditions, the NWS Los Angeles/Oxnard office said.

The flash flood watch excludes the Santa Monica Mountains. Some slow-moving thunderstorms could develop with the system moving through the watch areas, creating flooding danger, especially in higher terrain and the back country of the mountains, the NWS said.

The chance of afternoon and evening thunderstorms for interior areas of Los Angeles County will last through Monday.

Rainfall with lightning and small hail also was possible Sunday afternoon for San Bernardino County mountains and the High Desert, with rainfall totals between a quarter or half inch possible, along with lightning and small hail, the NWS said.

Rain, or at least drizzle, came down Sunday morning in several areas of Southern California with NWS reporting small or trace amounts at Orange County John Wayne Airport and Lake Forest in Orange County, while precipitation was observed in Costa Mesa, Anaheim, and Santa Ana.

There was overnight rain or drizzle in Riverside, and in the San Bernardino County Mountains there were measurable amounts at Big Bear Lake, with .12 of an inch; Baldwin Lake, .10 of an inch; and Angelus Oaks at .01 of an inch.

For the Los Angeles area, trend will be warmer and drier starting later Monday and through the middle of the week, with another cooling trend for later in the week, the NWS said.

In Orange, Riverside and San Bernardino counties, the low pressure system will move on by Tuesday, with a warming trend expected to continue through next week.