Upper Pines, and only the front part of it, is the only campground the NPS keeps open in the winter. Normal February temps in Yosemite Valley are usually highs in the mid 50’s and low’s in the mid to low 30’s. Although getting heavy snow is always a possibility in the valley, it usually stays on the mild side compared to higher elevations. Thus far it has been a fairly mild winter there. Accuweather can give you a 15 day forecast. And of course carry chains and extra blankets. Winter is really nice there for avoiding the crowds of summer. As already mentioned, the natural “firewall” in February is a bonus. Should be a great trip!