(CNN) Millions of Thanksgiving travelers in the US will get walloped this week by several storms, including a dangerous system threatening parts of the West Coast.

From California to Michigan, more than 20 million people are under winter weather advisories, watches or warnings, CNN meteorologist Dave Hennen said. And we're still 26 days from the official start of winter.

A "historic" storm is heading towards southwest Oregon and northwest California Tuesday and Wednesday, the National Weather Service said. The storm is forecast to rival the strength of a Category 1 hurricane and pack wind gusts up to 74 mph, CNN meteorologist Judson Jones said.

Here's what else to expect from coast to coast:

California: Get ready for flash floods and snow

A severe storm will pummel California this week, bringing heavy snow in the mountains and heavy rain to the coast and valleys, Hennen said.

"Winter storm warnings cover the Sierra, where snow will be measured in feet starting Tuesday and lasting into Friday," he said.

Up to 2 feet of snow could cover the mountains outside Los Angeles.

Elsewhere in Southern California, more than 10 million people are under flash flood watches from San Diego to Anaheim, where 1 to 3 inches of rain could make this Thanksgiving very soggy.

⚠️ A Winter Storm Watch has been issued for late Tue thru Fri eve for the mountains



⚠️ A Flash Flood Watch has been issued for late Tue thru Thu eve for all areas west of the mountains



Check out the updated rainfall and snowfall estimates for the storm below 👇 #cawx pic.twitter.com/smuYrhQ1uq — NWS San Diego (@NWSSanDiego) November 24, 2019

The Rockies and the Midwest: Get your shovels out

Weather warnings are in effect for Denver, where a winter storm will likely dump 10 to 15 inches of snow over the next few days, Hennen said Monday.

Some parts of the Rockies could see 2 to 3 feet of snow.

The snowstorm has already snarled traffic across the state, the Colorado Department of Transportation said.

Part of Interstate 76 in Colorado to the Nebraska border was closed in both directions, the department said. And more road closures are expected.

"The same storm will spread into the Midwest and likely drop 8 to 12 inches of snow in Minneapolis," Hennen said. Iowa and Michigan will also get hammered with snow starting Wednesday.

"Winds and rain will slow airports from the Midwest all day (and) into the major hubs in the Northeast during the afternoon and evening," Hennen said.

Ahead of the winter storm, Delta said it is issuing waivers that will allow passengers traveling to, from or through Denver on Tuesday and Minneapolis on Wednesday to change their travel itinerary without incurring a change fee.

Further south in the Plains and Mississippi Valley, gusty winds and strong storms will develop Tuesday into Wednesday.

The Northeast: Planes and Charlie Brown could get grounded

Ferocious winds and rain could wreak havoc Wednesday for airline passengers in the Northeast.

The balloons can't be flown when sustained winds exceed 23 mph and gusts exceed 34 mph, per New York City regulations

There's good reason for that. In 1997, the Cat in the Hat balloon injured four people after intense winds forced it astray.

Once the Northeast recovers from the brutal winds Wednesday and Thursday, travelers headed home Sunday will face another round of tumultuous wind and rain.

Everywhere: Bring your patience on the road

If you're driving to see loved ones this Thanksgiving, expect more company on the road.

AAA predicts the second-highest number of travelers in over a decade this Thanksgiving week.

The automobile associations expects 1.6 million more travelers on the road compared to last year, thanks in large part to lower gas prices.

Wednesday is expected to be the most congested day of the week, and cities such as Atlanta, Los Angeles, New York, Boston, Houston and San Francisco could see more than three times their normal levels of traffic.