One of the busiest travel days in many years could be a nightmare for Americans racing to their Thanksgiving gatherings, forecasters said Tuesday.

Thanksgiving Eve is historically the busiest travel day of the year, and AAA said more than 55 million people were expected to pack roads, planes, trains and buses on Wednesday — the most since 2005.

Most will find it tough sledding.

Heavy snow was forecast from Colorado to Michigan and from the upper Mississippi Valley, while a second storm was causing blizzard conditions and closing hundreds of miles of interstate highway in the Northwest.

"It's going to be bad," said Todd Krause, a warning coordination meteorologist for the National Weather Service in Chanhassen, Minnesota.

"The snow is going to come down hard. It's going to come down fast," Krause said. "Visibility is going to be very, very poor during the height of the snowstorm."

The weather service forecast widespread accumulations of 6 to 12 inches of snow along the path of the Midwestern storm, issuing winter storm warnings along a wide path from central Nebraska through much of Iowa, South Dakota, Minnesota and Wisconsin to the Upper Peninsula of Michigan.

"Strong winds will contribute to more blowing and drifting snow in these areas, resulting in dangerous travel conditions," the agency said.

The storm system started by blasting Colorado, covering many parts of the state in 20 inches or more of snow Tuesday. The 22.2 inches that blanketed Boulder, 25 miles northwest of Denver, was a record for Nov. 26 and the third-snowiest day ever.

Denver recorded 11 inches of snow. Almost 500 arrivals and departures, about a quarter of all scheduled traffic, were canceled at Denver International Airport, where about 9½ inches of snow was recorded.

"It's a little gnarly," said Ryan Glitch of Fort Collins, who spent the day driving around the Denver area in his 1993 Ramcharger offering assistance to anyone who asked.

"I've pushed three through intersections where it wasn't plowed real good — by hand, not bumper," Glitch told NBC affiliate KUSA of Denver.

"I've taken one lady to surgery, another lady to get her prescription filled. One lady needed groceries. I'm bringing firewood to someone tonight," he said.

"It doesn't hurt to be nice," he said. "What would Mr. Rogers do?"

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In Wyoming, more than 200 miles of Interstate 80 were closed Tuesday, the state Transportation Department said.

In Columbia, Missouri, where winds up to 60 mph were forecast for early Wednesday, Sean Beller urged drivers to be extra careful.

"People really don't pay attention to trucks anymore like they should," Beller, a truck driver who was passing through Columbia, told NBC affiliate KOMU. "Especially when there is high winds, my truck will just blow right off the road."

The same applies for everyday cars, he said: "Just because it's much smaller doesn't mean the wind won't pick it up."

Leschia Molzahn drove to Central Nebraska Regional Airport in Grand Island on Tuesday to put her mother on a plane to Arizona, where she planned to visit relatives for the holiday.

"She's been on the plane for three hours, sitting on the tarmac," Molzahn told NBC affiliate KSNB of Hastings as airport crews applied green de-icing gel to aircraft.

Numerous flights were delayed and some were canceled as several inches of snow coated the runways. One of the canceled flights was the one Molzahn was on.

"It's no different than driving a car out on a highway," Mike Olson, the airport's executive director, told KSNB. "Conditions are slippery. You don't have a lot of control. Airplanes are no different."

U.S. airlines had posted few cancellations for Wednesday as they held out hope that flying conditions would hold up. But all major airlines instituted contingency plans waiving fees for some rebookings.

As the storm system moves east, it could push elevated winds into the New York area, potentially affecting the Macy's Thanksgiving Day Parade.

Conditions were forecast to be dry with temperatures in the 50s on Thursday, but forecasters said winds could reach 25 mph, with occasional gusts up to 40 mph.

Sustained winds of 23 mph and gusts of 34 mph would ground balloons, said Susan Tercero, the executive producer of the parade, which will be televised by NBC.

"We refer to it as a game-day decision," Tercero said. "We get there right before the parade begins."

In the West, a different storm system called a bomb cyclone smashed into mountain areas of Oregon and Northern California overnight, with heavy snow and wind gusts as high as 100 mph accompanied by severe rain in lower-lying areas.

A bomb cyclone is caused when cold air collides violently with warm air, often over warm ocean waters, causing the system's internal pressure to plummet.

It's called "bombogenesis," and the National Weather Service said the bombogenesis cycle washing over the northwest Pacific coast on Tuesday night and Wednesday could create historically powerful storms for the next day or so.

San Francisco, San Rafael, San Jose and Livermore in California were expected to get 1 to 1½ inches of rain by midday Wednesday, while as much as 2 feet of snow was forecast for parts of the Sierra Nevadas and the Cascades mountain range bridging Oregon and California.

The weather service forecast blizzard conditions above 3,500 feet in the region, while southwest Oregon could get wind gusts up to 90 mph — at another time of the year, that would be a category 1 hurricane.

"This particular system has the low center very well clustered right off the southwest Oregon and the northwest California coasts," said Tom Wright, a meteorologist for the weather service's office in Medford, Oregon.

"Heavy snow and near-blizzard conditions at times will make travel very difficult to nearly impossible over the mountains," Wright said. "Travel through southwest Oregon and Northern California is strongly discouraged."