A major winter storm moving across the nation Wednesday is threatening to disrupt travel plans for millions of Americans heading home for Christmas. The weather system even has package delivery companies nervously checking out the forecast, with the timely delivery of precious gifts on the line.

"We're closely monitoring the storm," FedEx spokesman Scott Fiedler told NBC News. "We have a team of 15 meteorologists who track the weather around the world every day."

FedEx is seeing only minor delays so far but has contingency plans in place to help mitigate any effects of the weather, Fiedler said.

UPS, which projects that Thursday will be its busiest day of the holiday season, also has its staff meteorologists tracking the storm.

"We're keeping our eyes on airport gateways in the Midwest, but we don't expect any issues," said UPS spokeswoman Susan Rosenberg. "We're used to marshaling all our resources for this our busiest week. Of course, snow at this time of year is not unusual."

Still, the size of the storm is impressing forecasters.

"This is a massive system," said TODAY meteorologist Al Roker. "We've got watches and warnings that spread across 16 states, from New Mexico all the way to Wisconsin and Michigan."

Blizzard warnings are in effect for Colorado, Kansas, and Iowa, Roker said, and up to a foot of snow is expected to fall in the Rockies.

On Wednesday afternoon, flights at Denver International Airport were delayed about 30 minutes because of de-icing operations, said Laura Coale, an airport spokeswoman.

"Currently, there's only a handful of flight cancellations - mostly commuter flights," she added.

United Airlines is allowing passengers to make same-day flight changes because of the weather. The airline has also issued a fee waiver for passengers traveling through eight other cities, including Chicago, Milwaukee and Omaha, through tomorrow.

Meanwhile, American Airlines is allowing travelers flying to or from 17 Midwest cities tomorrow and Friday to change their plans without penalty.

US Airways is also relaxing its fee change policies in Denver, Chicago, Des Moines, Omaha and Milwaukee for travel on Wednesday.

Air traffic was moving smoothly in other parts of the country, though wind at Newark International Airport was causing delays of about 50 minutes, according to the FAA flight delay information map.

Meanwhile, the Pacific Northwest is digging out after a strong storm that left drivers dealing with slick roads and icy conditions. An approaching Pacific system should dump more than a foot of snow over the Washington Cascades on Wednesday, the National Weather Service said.

The messy weather arrives just as much of the country will be on the move for Christmas and New Year's.

AAA projects more than 93 million Americans will travel 50 miles or more from home during the year-end holidays, a period that begins this Saturday and ends on Jan. 1. Ninety percent of those on the move are expected to drive.

It's a good bet some people are already heading out and, as the storm moves east, many may find problematic conditions in the Midwest and on the East Coast. Snow in the Chicago area on Thursday may be heavy and lead to roads quickly becoming snow covered, making travel treacherous, the National Weather Service warned.

"The worst of the weather Friday looks to be centered from the Great Lakes to New England," said Weather Channel meteorologist Brian Fortier.