Having lived in Southern CA for many years my blood thinned too.
When headed to Montana a few years ago in October, I had to get cold weather gear.
Learned a lot.
All I can tell you is:
Carry extra boots, gloves and hats.
Dress in layers.
As the day wears on you will warm up but once that sun goes down so does the thermometer.
I keep a wool sweater in a Zip Loc bag in either the saddle bag, back pack, or truck.
Same with the gloves, Filson hunting hat, balaclava, and a pair of socks.
I bought a set of the cheaper Herters 3 in 1 coats and a pair of bibs. I think set was less than $200.
They are water proof and warm. Here is a picture of me in them with the Filson hat. Good hat! I removed the hood but can snap it ack on any time. They make the same set up in Orange and Duck patterns.
This is in Idaho and I lost my Orange vest on the way up. A tree snagged on it and the horse kept going, it was torn off like stripper clothes.
I also have some Woolrich bibs as well as Filson gear. If it was warm or wet the day before I have different clothes to change out of and keep dry.
I am not big on the Poly or polar fleece stuff as I am kind of furry and I build a static charge.