For me, it's hit and miss, but the guys above are much more knowledgeable. The thing about Western WA is there are ponds and bays and fields with feed virtually the entire way down and they can short stop you anywhere, or go right by. I was scratching my head one year and we went up to BC around Christmas for a vacation and rode Amtrak. A short distance above where I hunt the bays had tens of thousands of ducks as far as you could see just loafing around. For whatever reason they didn't come down any further. Three days later on the way back, those bays were empty and my area had some, but the majority apparently just kept going. The weather didn't change at all or get even close to freezing for that time period.