A storm that was pounding California's Sierra Nevada with snow Tuesday took aim at the Colorado Rockies and could roll east and slam the Appalachians on Thanksgiving Day, bringing unsettled travel weather to millions of Americans hitting the skies and roads for the holiday weekend.

With Thanksgiving holiday travel well underway, many have reached their destinations — but for everyone else, a storm tracking from the Rockies to the Midwest and Northeast into tomorrow will bring rain and snow,

The Transportation Security Administration expects to screen 18.3 million travelers over the next week, potentially making this the busiest Thanksgiving travel period ever.