3 feet of snow possible in the Sierra.

Tornadoes possible in the central U.S. by the weekend.

Gorgeous weather expected for the Preakness in Baltimore on Saturday.

Severe thunderstorms and tornadoes will threaten almost 1 million square miles of 18 states in coming days as tumultuous spring weather sweeps the nation.

"Tornado Alley is certainly about to wake up," AccuWeather extreme meteorologist Reed Timmer said.

Tornado Alley references a swath of Plains states, including portions of Texas, Oklahoma, Kansas, Nebraska, Colorado and South Dakota.

Southern and Midwestern states such as Arkansas, Louisiana, Mississippi, Iowa, Tennessee, Kentucky, Wisconsin and Minnesota are also sometimes included, with the southern states referred to as "Dixie Alley."

The wild weather sets in late Thursday with storms across parts of South Dakota and Nebraska to Michigan and Indiana before sprawling into a wider area Friday and the weekend. More than 40 million people live in the storm zone, which will roll as far south as New Mexico and Texas.

But there is no end in sight – the National Weather Service's Storm Prediction Center (SPC) highlighted a risk area for severe weather for eight straight days.

"Pretty sure it's the first time that all days on the day 4-8 have had contours drawn" since the forecasting tool became operational in 2007, climatologist Harold Brooks tweeted.

Alabama-based TV meteorologist James Spann also chimed in on Twitter: "Storm chasers are licking their chops… significant severe weather possible on every SPC outlook days 4-8."

On Saturday, parts of Texas and Arkansas have the greatest chance of severe weather, the Weather Channel warned. On Sunday, key targets might be Illinois, Indiana and Michigan.

Damaging winds, large hail and a few tornadoes will all be threats from these storms.

A "storm train" roaring in from the Pacific Ocean will move across the Rockies and clash with warm moisture rolling up from the Gulf of Mexico, triggering the weather chaos, AccuWeather senior meteorologist Paul Walker said. This is normally a dry season for California, he said, but this year the rain has been unrelenting.

"It's hard to nail down where the most severe weather and tornadoes will hit over the next few days," Walker said. "Pay attention to the weather, pay attention to weather alerts, pay attention to local warnings. Just pay attention!"

Damaging storms have already flared up this week, with homes and businesses in Orlando, Florida, battered by wind gusts above 55 mph on Tuesday.

In the West, a storm more typical of winter will bring everything from rain and thunderstorms to wind and mountain snow, AccuWeather said. The storm will make for slower-than-normal travel, ruin outdoor plans and increase the risk of flooding and damage in some communities.

Up to 5 inches of rain is possible in some areas of the West. That amount of rain is "extremely out of the ordinary" for this time of year, AccuWeather said.

In the Sierra, "total snow accumulations of 12 to 18 inches, with localized amounts up to 35 inches, are expected," the National Weather Service in Sacramento said. AccuWeather warned that "travelers through the high terrain should use extreme caution as roadways will be slippery and snow covered with lane restrictions possible."

The storm weather follows a lengthy pattern of wet weather that, along with serious flooding in the Midwest, has brought "sizable drought relief" across the nation, the National Oceanic and Atmospheric Administration reported last week. Drought now covers a "near-record low" of 2.4% of the nation, Thursday's U.S. Drought Monitor said.