Winter lift is among the strongest I've seen in RC and full scale. Although conditions must be right. Thermals are on temp difference and clouds work great too. On cold day if you have huge snow filed next to the pine woods - there could be great temp difference and with lift.
I did not fly triangles on a full scale in a WInter, but stayed up for hours.
As for the lakes - I'd imagine if snow got blown by the wind and cleaned-up some ice surface while leaving snow covered portions of the lake - it could produce some temp difference. Although I have never seen thermals over the lakes myself, just great sinks in a Summer time.