It turns out that Seattle has a 7% chance of having 1" of snow on the ground on Christmas Day. That works out to about once every 14 years. Bellingham is a bit better at 10%, while Olympia is at 3%.
Absolutely need a White Christmas? Head to Marquette, Michigan or International Falls, Minnesota (or Stampede Pass, closer to home). Odds there are 100%.
To find out the odds for any major city, check out www.usatoday.com