What are best conditions to burn slash?
You want the piles of slash to be dry enough to burn up. But you also don't want the surrounding area too dry so that the fire may escape. A light rainfall event can be good, it will dampen the surrounding vegetation, but not make the slash too wet. In your area you may want to wait for a light snow cover before you burn the piles, so they can be done safely. Be sure to watch over the piles until they are extinguished or not going to be a problem.