A cross-country storm, a blast of Arctic air and an outbreak of showers and thunderstorms are threatening to snarl traffic and pile up delays at airports over the Thanksgiving holiday weekend, one of the nation's busiest travel periods.

One of the strongest storms on the West Coast in decades knocked out power for thousands of people, unleashed strong winds that toppled trees and left two dead in Washington before making its way through Oregon to Northern California where on Thursday it dropped heavy snow and record amounts of rain.