Denver ? Holiday travelers scrambled to adjust their plans Tuesday as a fast-moving snowstorm threatened to bring long delays along with a white Christmas for millions of people throughout the West and Midwest.

The storm system was already snarling traffic in Arizona on Tuesday, with blizzard-like conditions closing roads and causing a pileup involving 20 vehicles. South of Phoenix, a dust storm set off a series of collisions that killed at least three people.

The blustery weather in Arizona is part of a storm system that promised to bring more than a foot of snow in parts of Colorado and southern Utah by noon today. Blizzard warnings were likely on Christmas Eve in Kansas and other Plains states as the storm moves east.