Patience is the hardest part when wintersowing. But you'll have to wait until the nights are no longer below freezing and the days are consistantly warmer. I've ws'd & indoor sowed tomatoes and now only indoor sow (which btw I just did this weekend) because I get plants much sooner then if winter sown. When my indoor sown seedlings are large enough I put them in a cold frame so they are hardened off and ready to plant by mid May (with protection when needed)

Normal for me in zone 6 that the tomatos will germinate around the nights that are 50 degrees, but after they do make sure they dont get hit by another 32 degree night.

ML