The National Weather Service issued a flash flood warning after its radar indicated flash flooding from a storm over the lower San Diego County Mountains.
The storm producing the flash flooding was occurring near Live Oak and Boulevard areas around 12:15 p.m. PT.
The warning was expected to last until approximately 3 p.m. PT Monday.
After a weekend with hundreds of lightning strikes in the mountains east of San Diego, the region was primed for another hot, humid day with a chance for thunderstorms Monday.
There is an excessive heat warning in effect now through Wednesday for areas east of San Diego including Yuma, El Centro and Imperial County.
Temperatures will range between 110 to 115 over the hottest deserts. Residents won't get a break once the sun goes down as overnight lows will range from mid-80s to around 90 according to the National Weather Service.
A flash flood watch was in effect until Monday night for the mountains and deserts.
"Don't be surprised if we see some more activity out there this afternoon," said meteorologist Greg Bledsoe. " There is also an excessive heat warning for the mountains with temperatures expected to reach around 112."
Indeed, dark storm clouds gathered at about noon on Monday over the Cuyamaca Rancho State Park, just south of Julian. Lightning appeared to touch down about every few minutes. Strong winds and rain followed, drenching the already soggy soil.
Mud on the side of State Route 79 showed evidence of soil being carried by rushing water.
Aucke-Jan Drenth was visiting from the Netherlands. He was told by an East County resident that there were lightning storms in the mountains so he made the drive.
"We saw the clouds getting bigger and decided to stop to watch the lightning," he said.
In Descanso, outdoor gift shop owner Pam Martin was protecting her stand from the wind and rain. With wind chimes clanging and signs blown off the shelves, she was prepared for the harsh weather.
"Yesterday was really bad. It just poured and flooded for about 45 minutes. It's done that every single day this last week," said Martin.
She said water at her shop was ankle deep on Sunday.
Elsewhere around the county, temperatures will range from the upper 70s at the coast to mid 90s inland.
For those looking to escape the hot, humid weather, there are cool zones open throughout San Diego County. Click here for hours and locations