Wait, you guys really aren't aware of this bug? It's currently (Qohor) impossible to clear a Cold Winds winter. The global flag won't be removed if there are still winter wastelands, winter wastelands won't be removed if the flag still exists. Get rid of the flag trigger in Cold_Winds.7 to fix it.
Incidentally, while exploring the limits of that bug, I also encountered one where clearing out walkers was not removing holdings that they didn't control the counties of, leaving a smattering of walkers after formally ending winter. Unfortunately I didn't look very far into the code that clears those out, since that case is rare when the walkers can be beaten back normally.