Better to be safe than sorry, we guess.  National Weather Service has posted this for tonight:

…FLASH FLOOD WATCH IN EFFECT THROUGH 11 PM PDT THIS EVENING…

THE NATIONAL WEATHER SERVICE IN LOS ANGELES/OXNARD HAS ISSUED A

* FLASH FLOOD WATCH FOR THE FOLLOWING AREAS…THE ANTELOPE VALLEY…LOS ANGELES COUNTY MOUNTAINS EXCLUDING THE SANTA MONICA RANGE…LOS ANGELES COUNTY VALLEYS…AND THE SANTA CLARITA VALLEY.

* THROUGH 11 PM PDT THIS EVENING

* SLOW MOVING THUNDERSTORMS WITH HEAVY DOWNPOURS WILL BRING THE POTENTIAL FOR FLASH FLOODING AND MUD AND DEBRIS FLOWS TO THE REGION.

* THERE IS THE POTENTIAL FOR FLASH FLOODING OF CANYONS…ARROYOS AND LOW LYING AREAS ALONG WITH MUD AND DEBRIS FLOWS IN AND BELOW RECENT BURN AREAS WITHIN THE WATCH AREA.

A FLASH FLOOD WATCH MEANS THAT CONDITIONS MAY DEVELOP THAT LEAD TO FLASH FLOODING. FLASH FLOODING IS A VERY DANGEROUS SITUATION.

YOU SHOULD MONITOR LATER FORECASTS AND BE PREPARED TO TAKE ACTION SHOULD FLASH FLOOD WARNINGS BE ISSUED.