I agree with the October Date
I agree too with the october date. Especially in the passes of Washington, Montana, and Idaho, snow occationaly falls on the higher passes mid to late October (sometimes earlier if they get a good cold snap with moisture), but is almost expected around November.
Just remember, October through April (I'm pretty sure on the beginning month), it is a REQUIREMENT to carry chains over ALL Washington State mountain passes, and snow tires are heavily suggested during this time of year (I think 4 wheel drive vehicles are exempt from chains, but I wouldn't be sure).
You can check with the Washington State Department of Transportation WSDOT at www.wsdot.wa.gov. It's not fun flagging down a WASP (Washington State Patrol) officer because your car is stuck in a early winter snowbank on Snoqualmie Pass.
Through the Fall and Winter months in the Northwest and through the northern tier great plain states, I would very much so suggest tucking a Winter Survival Kit into your car's cab. Blankets, food items (even candy), and Water will all be essential, even if you are just stuck at a truckstop.
Another suggestion would be to get and take with you some sort of CB Radio (Palm CB/Mini Mobile, Mobile, or Perminant Mounted) with a WX All Hazards Radio function. I think they should be required for all motorists. WX Weather/All Hazards radio tunes to National Weather Service broadcasts and can get you the latest information on weather, and Washington State Law requires that they run a Mountain Pass Conditions report every cycle or two, so you will always have the latest Pass Conditions.
Have a good trip!
From a former Washington State resident,
Brad M.