SAN DIEGO -- A flash-flood watch was in effect Tuesday in San Diego County's valleys, mountains and deserts because of the threat of heavy rainfall and thunderstorms, forecasters said.

The warning was issued by the National Weather Service about 3:30 a.m. and was to remain in force through this evening.

Areas below areas recently denuded by wildfire are especially at risk of life-threatening debris flows and flooding, forecasters said.

Other areas at high risk include washes that drain steep slopes and narrow canyons in the mountains and foothills, forecasters said, adding that severe flooding can occur suddenly and unexpectedly miles from storm activity.

On Monday, heavy rain and lightning caused scattered flooding and forced a brief closure of state Route 78 east of Yaqui Pass Road in Anza-Borrego State Park, according to the California Highway Patrol.