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Millions at risk for severe thunderstorms, tornadoes and flash floods

The Midwest and California are bracing for a messy winter storm, and record highs are forecast for the Midwest, the Southeast and the mid-Atlantic.

Millions of people in the South and the Southeast are at risk of severe thunderstorms, tornadoes and flash floods Tuesday and Wednesday, while a messy winter storm is expected to hit the Midwest and a separate storm slams California later this week. Meanwhile, record highs are expected across the Midwest, the Southeast and the mid-Atlantic.

Threats of severe thunderstorms, flash floods

As many as 20 million people from southeast Louisiana to south-central Alabama are at risk for severe thunderstorms Tuesday that could bring damaging wind gusts, large hail and some strong tornadoes, classified as having winds of more than 111 mph.

The thunderstorms are expected to continue through the day and last into the overnight hours. The cities of New Orleans, Montgomery, Alabama, and Gulfport, Mississippi, are in the enhanced risk area.

In addition to the severe thunderstorms, flash flooding will also be a threat. As of Tuesday morning, 24 million people were under flood alerts in parts of the Southeast, Tennessee and Mississippi river valleys, which are expected to receive 2 to 4 inches of rainfall. The greatest risk for flash flooding is in south-central Alabama, including Montgomery.

In northern Alabama, schools were closed, and in central Alabama shelters opened in preparation for possible severe weather, NBC affiliates WAFF of Huntsville and WSFA of Montgomery reported.

At least 4 million people in northern Louisiana up through central Kentucky were under tornado watches until 11 a.m. CT Tuesday as storms charge through those regions, bringing what could be up to 2 to 3 inches of rain an hour and an associated flash flood risk.

In Louisville, Kentucky, multiple roads were blocked by flash flooding and crews were conducting water rescues as of Tuesday morning, NBC affiliate WAVE Of Louisville reported.

On Wednesday, 5 million people are at risk for severe thunderstorms that could stretch across portions of Georgia and northern Florida — including the cities of Macon and Tallahassee — and could also bring damaging wind, hail and tornadoes, though the threat is not expected to be as high as it is Tuesday.

Messy winter storm moving through the northern Plains and the upper Midwest

A total of 11 million people are under winter alerts from northeast Colorado into the upper peninsula of Michigan for snow and freezing rain. As much as 3 to 9 inches of snowfall — and isolated areas that could receive up to a foot of snow — are possible from northeast Nebraska into northwest Wisconsin through Thursday.

Minneapolis will see a myriad precipitation types with this event, likely switching between freezing rain and snow for most of the duration. The heaviest snow will be Tuesday through 3 p.m., when snow could fall at 1 to 2 inches an hour at times. There will be a break in the snow midday before another round Tuesday night. As of Tuesday morning, 5 to 9 inches of snow are expected in the metro Minneapolis area.

Road conditions started to deteriorate across southwest Minnesota on Tuesday morning, with the Minnesota Department of Transportation warning that people should avoid driving in Rock and Nobles counties due to reduced visibility, heavy snow and strong winds, NBC affiliate KARE of Minneapolis reported, adding that further travel impacts are expected throughout the state Tuesday and Wednesday.

Short break for California ... before a huge storm

One state forecast to stay dry Tuesday is California, which saw record rainfall this holiday weekend — an increasing occurrence as climate change heightens the odds of extreme precipitation. Oakland saw its wettest day on record since 1970 over the weekend with nearly 5 inches of rain, and downtown San Francisco got nearly 5.5 inches of rain Dec. 31, making it the second-wettest day in the more than 170 years of records at that location.

But the break won't last long. Another strong storm system is expected to move into the West Coast between Wednesday and Friday, and it is forecast to reach bomb cyclone status by the time it slams into the California coast early Wednesday morning.

That storm will put 12 million people in north-central California under flood watches Wednesday and Thursday. There could be 2 to 4 inches of rainfall in the valleys, 3 to 5 inches in the foothills and 7 inches in the mountains below 6,000 feet.

The Bay Area, Sacramento and Los Angeles are the most populated areas to watch for flooding, but flooding will be possible across much of the state Wednesday and Thursday. 

NBC Bay Area noted that a National Weather Service forecast warned that the Wednesday storm will be “truly a brutal system that we are looking at and needs to be taken seriously" and that it will bring a "likely loss of human life."

Record high temperatures forecast elsewhere

Numerous record highs are likely Tuesday across the Midwest and the Ohio Valley. St. Louis could see a high in the early 70s Tuesday before expected highs plunge to the high-30s and the mid-40s the rest of the week.

In addition to those areas, the warmth will also expand south and east.

Temperatures soaring 15-25 degrees above average will lead to highs in the 50s and the 60s across the northern tier areas and the 70s and the 80s across the Southeast and the mid-Atlantic. Atlanta and Washington, D.C., are expected to see highs in the high-60s and the low-70s Tuesday and Wednesday, while New York is expected to see highs nearing 60.

Cooler temperatures are slated to arrive in the Midwest on Wednesday and the Northeast by Friday, but will still remain much above average for this time of the year.