Fall will assert itself this week as a cut-off low pressure system (so named because it is "cut off" from the jet-stream) meanders its way towards the California coast. As it approaches, we can expect an increase in clouds and eventually a chance for showers late Tuesday night. Add a thunderstorm chance on Thursday with colder air aloft moving into the region.
The latest National Weather Service forecast discussion didn't really lend any insight into their thinking on rainfall amounts, but a quick glance at the model runs myself has me thinking we could see anywhere from a tenth to a half inch of rain by the time things are all said and done. That amount could be higher if the thunderstorm chance on Thursday pans out.
In the higher elevations, forecasters are calling for the first snow of the season, with as much as one to three inches possible on the peaks of the San Gabriel Mountains.
Temperatures will be mild, with highs only reaching the low 70s on Wednesday and Thursday, with temperatures recovering to the 80s towards the weekend as the low pushes east and is replaced by high pressure.
Weather should quiet down on Friday and return to a typical Southern California dry pattern through the weekend. Temperatures should return to normal with mostly sunny skies.