"Unfortunately, we're really not talking about a lot, at most a tenth or a quarter of an inch ... maybe more in the San Gabriel Mountains," said meteorologist Dave Bruno with the National Weather Service in Oxnard.
Morning commuters can expect sprinkles, he said.
The rain is expected to end by midnight, with the "slightest chance" of more precipitation over the weekend, Bruno said.
That's bad news for a parched region in desperate need of precipitation. Gov. Jerry Brown has declared a statewide drought emergency, and officials are scrambling to mitigate the effects of rainfall totals that are far below normal.
Overnight Wednesday there was just light drizzle in parts of L.A. County, with the only measurable rain in Santa Barbara County, Bruno said.
It will be a bit chilly as the rain falls Thursday, Bruno said, with temperatures expected to be in the mid-50s to lower 60s throughout the day.
In downtown L.A., the high is expected to be 62 degrees with a low overnight of 51 degrees. The normal high temperature for this date is 68 degrees, Bruno said.