By Christina Maxouris, Jason Hanna and Dave Hennen, CNN
(CNN) -- Brutal weather is making pre-Thanksgiving travel unpleasant or downright dangerous across the US, and leaving tens of thousands of people without power.
Two massive storm systems are bringing a combination of snow, rain and high winds as millions of people take to the roads and skies the day before Thanksgiving:
• One system is hammering Western states with rain along the coast and valleys and heavy snow in the mountains.
• Another has already dumped snow and rain over the Rockies and central US and is bringing snow or high winds to the Midwest while pushing colder weather to the Northeast.
High winds -- with gusts above 35 mph in several places, including San Francisco, the Sierra Nevada range, Chicago and Cleveland -- are causing widespread power outages. Wind advisories across the country cover nearly 90 million people.
The storm in the Midwest is "kind of now fizzling out as far as the rainfall and snowfall ... but it's the wind that's still going to be a problem," CNN meteorologist Ivan Cabrera said Wednesday afternoon.
Nearly 300,000 homes and businesses were without power across the country Wednesday. As of 5 p.m. ET, most of the outages were in Michigan, Ohio and California, according to utility tracking site poweroutage.us.
And snow, whether already on the ground or falling now, is making highway travel difficult in the central US and Midwest. Officials in several states have reported slick conditions including in South Dakota, where roads along and south of Interstate 90 were paved with ice or snow.
"Winter driving skills are needed in northern Iowa today. Buckle up, slow down and give all your attention to the task of driving," Iowa's Department of Transportation tweeted.
Powerful winds and heavy rain in the West
A bomb cyclone -- a type of rapidly strengthening storm -- began dumping rain over parts of the West on Tuesday night, and more rainfall is expected through Thanksgiving. Winter storm watches, warnings and advisories touch parts of every western state from Arizona to Montana.
The storm was "historic" and "unprecedented" because it set November low-pressure records in parts of Oregon and Northern California, the National Weather Service said.
Lower pressure typically will yield strong winds -- as this storm did. Hurricane-strength winds -- sustained winds of 85 mph and gusts to 106 mph -- hit Cape Blanco, Oregon, on Tuesday afternoon, the weather service said.
It's also begun to dump heavy snow -- with some mountain ranges in eastern California measuring snow piles in feet Wednesday, the weather service said.
In Oregon, Department of Transportation officials reported whiteout and blizzard conditions, urging drivers in affected roadways to pull over. In Arizona, the weather service warned of "impossible travel conditions" Thursday and early Friday with heavy and drifting snow.
Significant travel delays and road closures because of dangerous travel conditions were also expected across California, the service said.
Heavy rains will also pose a flash-flooding threat in Southern California through Thanksgiving, and in the Southwest by early Friday, the weather service said.
In Santa Barbara County, officials warned that heavy rain could cause dangerous debris flows at and below the areas burned this week by the Cave Fire. "If you live or are near creeks and streams be aware that they may experience high flows and can rise quickly. If a debris flow occurs, and it is unsafe to leave, or you are unable to leave, go to high ground," the county said.
Heavy snow and high winds stretch to Northeast
Another major storm has Midwest residents digging out from snow-covered cars and sidewalks and struggling with slick roads.
The city of Minneapolis declared a snow emergency due to the blizzard conditions. More than 7 inches of snow have fallen since midnight, making it the biggest Thanksgiving eve storm in decades, the weather service said.
Parts of Michigan have seen 15 inches of snow and nearly 14 inches were reported in Wisconsin.
The storm had unleashed powerful winds, snow and rain over the Plains before moving over the upper Midwest on Wednesday, dumping snow from Nebraska to Wisconsin, CNN meteorologist Michael Guy said.
It will continue moving north and east, not only bringing snowfall but fierce winds to the Great Lakes and into the Northeast before exiting into the Atlantic by Thanksgiving morning.
In New York, the fierce winds could ground the balloons in Macy's Thanksgiving Day Parade. 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.
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