No idea really, as I only had time for one run in custom match yesterday and probably won't have free time again until tomorrow. I'd say you are approaching it right but maybe the dynamic aspect needs tweaking to 'more frequent' - remember when weather was added to Driveclub how the dynamic setting would rain all of the time?
It also might be set on lower frequency at least until they know the servers can cope - there have been a few server crashes reported last night - or maybe it's something we unlock? Sorry.
It seems bugged to me. I've still not seen any hints of winter - not even missing leaves on the trees - in any of the tracks I've tried in supposed winter season in custom mode.