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.

Travel should be avoided in the Colorado high country for a roughly 36-hour window beginning Tuesday morning and lasting until Wednesday afternoon.

The snowstorm bearing down on Colorado has already produced a winter storm warning and avalanche warning for much of the mountains.