Seattle here!
Yes Rain (in between dodging Mt. St Helen's ash!)!
Here are a couple of quick rain ideas (since I'm guessing, you're bypassing the store that sells rain gear).
* A 40 Gallon Glad Leaf bag works well for a temporary solution
* Galoshes or Totes over the shoes (weird but so is the bag)...it works!
* Put your stuff in bags. 2Gallon Ziplocks work the best but smaller ones or a grocery bag is OK too
* A zip off hood from another jacket under the helmet -OR-
* A shower cap OVER the helmet
* Spare pair of dry socks.
* Cycle in your sandels
Also you may want to....
allow extra braking time. Take it easy on the turns and signal longer like a full 5 seconds before going into a turn.
Hope it goes well!