Forsythia are very hardy, and there should be no problems with pruning at this point. General rule: as long as the plant/bush is quiet, i.e., not actively budding or flowering it should be OK. Better to do it on a day when the weather is not projected to be extremely hot/humid. Next thing to figure out (if you haven’t already) is what kind of appearance you want next spring when they’re in bloom. Do you want a boxy look, or more of a flowy natural look? Keep in mind that closely pruned forsythia will not produce very dense flowering patterns for the first couple of springs.