So at the last autox it was cold about 50F (it's cold by CA standards ).

One of my autox "mentors" suggested I lower my tire pressure when the weather is cold and the surface is cold like that. I lowered front by 1 psi and rear by 3 psi from my normal (36,40).

he was right for the most part. but why? I also noticed that after the first run the pressure went up significantly. (37 front and 40 rear, from 35 front and 37 rear).

usually at the other place I run which is in the valley the air temp is 90-95F and the tire pressure only goes up by 1psi after the first run.