I live in socal. If the high temperature is around 80F max, then the BPL Merino Wool Hoody is your dual purpose upper choice. Above 80F, then you can go with some of Mark's recommendations.
If temps range from around low of 40F to a high of maybe 65F-70F then I wear a pair of REI Mistral Schoeller pants as a dual use item. If colder than I need a base layer in addition to the Mistral pants.
If high temps are above 80F then I just wear Rail Riders Adventure Shirt and EcoMesh Pants, base layer not required.
For me it gets tricky when low temps get below 40F and warm during the day above 80F, which often happens in the desert. Not uncommon to get temperature swings of over 50F and then the single use base layer is impossible for me.
Very little rain down here, so a poncho-tarp usually handles the rain gear and shelter duty. If no rain, then I don't need a shelter.
I always carry a Montbell UL Windshirt @ 2.7 oz.
For insulation either a Montbell Ex Light down jacket (6.1 oz) or Patagonia Down Sweater (11.7 oz) handle most the colder trips. If really cold, I bring my Montbell UL Down Inner pants (6.7 oz).
If cold heavy rain and/or snow is a sure thing, then I bring a pair of Dri-Ducks for rain protection, as long as I am not in areas where they can get caught on stuff (weight for both ~ 9.6 oz). If I anticipate really nasty cold stuff and/or bushwacking then it is a Mtn Hardware Typhoon Jacket and North Face Venture Side Zip Rain Pants. These situations happen maybe once a year. For shelter I will sometimes bring a Wild Oasis Tarp Tent instead of the poncho-tarp; or maybe use a MLD side zip Soul Bivy with the poncho tarp.
That pretty much covers how I handle socal.