Weather in northern Thailand?

Going to be traveling to northern Thailand in late January/early February. We're going to be doing the Mae Hong Son loop and then will also probably head north to Chiang Rai as well. What sort of weather can we expect? Was reading the wiki travel page and they said it can get pretty cold at night there? I'd imagine because of the elevation? Is this accurate? We're going to be in southern Thailand prior to this so we were just planning on bringing shorts and shirts but I'm wondering if I should bring some warmer clothes as well.