The clouds were associated with a front that stalled to the north of Los Angeles, resulting in some surprising and substantial rainfall totals in Santa Barbara and San Luis Obispo Counties. No measurable rain was reported in Los Angeles County by the NWS.
From an early morning run at Ahmanson Ranch (Upper Las Virgenes Canyon Open Space Preserve) on September 20th.
Related post: Monsoon Clouds and Valley Oak at Sunrise on Lasky Mesa