Whuch MEC baselayer for BIG SEKI in August

My vote is for the T3.
I like mine under a shell in case of rain. It's also remarkably comfortable across a wide range of temps.
Ex Light wets out easy…so only good for when it's dry.

wow! a base layer that heavy? I've only used them in 0f type weather for day hikes snowshoeing (not overnight).

Haha yeah that might just be my weird opinion.
Since the invention of powerdry HE, I've been dual purposing base layers as a light insulation layer.
I'm actually second questioning that logic…so others here might have better suggestions.

I tend to get cold easy too…guessing that's not an issue with you.

What exactly are you using the base layer for? As your main hiking shirt? As a second layer to add warmth? Or as something to change into at night in camp?

Sorry didn't really specify that, I normally wear a thin arcteryx phase something or other or if bugs are really bad a MEC nylon rail riders type knock off. I never really wear base layers when hiking just doing chores or lounging around.

I guess what I'm really wondering from the California experts is how bad can the weather get. I'm fine with the phase shirt down to about 0 Celsius, if wind is a factor I just throw on my rain jacket.

My experience in the mountains to this point has always included beautiful weather with some light rain. I guess I'm wondering what people are wearing in windy afternoon thundershowers on the jmt and places like that.

Some people can get by with just a rain jacket over a thin shirt, then change into their dry down jacket when they stop for the night. I can sometimes get cold easily so I am more likely to bring some kind of mid layer now. my patagonia cap 4 hoody is perfect for that.
If you are fine down to 0 degrees C with just your phase shirt, I think you could get by with just a rain jacket + base layer during storms.

I'm not really sure what you mean when you say "base layer" as you refer to your phase not being a base layer. I usually associate the word base layer with whatever I'm wearing next to my skin. Probably a better way to describe these things would be first layer + second layer + outer shell.