At least 18 people, including four children, are dead in four states after suspected tornadoes struck the central United States overnight and as millions face continued severe weather threats through the remainder of the Memorial Day weekend.

Just under 109 million people across broad swaths of the US were under threat of large hail, damaging winds and a fierce twisters Sunday, mainly throughout the mid-Mississippi, Ohio and Tennessee River valleys. As the storms move east, the Storm Prediction Center warned of “violent tornadoes, extreme hail and corridors of widespread wind damage.”

President Joe Biden has been briefed on the storms.

A heightened tornado watch designated as a rare “particularly dangerous situation” calling for “several tornadoes and few intense likely” went into effect on Sunday, according to the Storm Prediction Center.

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The watch covers parts of Arkansas, Illinois, Kentucky, Missouri and Tennessee – including in St. Louis and Jefferson City, Missouri; Paducah, Kentucky; and Carbondale, Illinois – and impacts 4.7 million people.

This is a special kind of tornado watch issued only when there’s unusually high confidence in the potential of multiple at least EF2-strength and long-lived tornadoes in the area, according to the National Weather Service.

The supercell thunderstorms firing up in the watch area will also be capable of large hail bigger than the size of baseballs and damaging wind gusts to 75 mph.

“Intense supercell thunderstorms will continue to develop across the watch area through this evening. Several tornadoes are likely, some of which are expected to be intense. Very large hail is also likely, along with the risk for potentially significant damaging wind gusts,” says the Storm Prediction Center.

Severe storms kill over a dozen people

At least seven people in Cooke County, Texas, were killed when severe storms hit the area Saturday night, Sheriff Ray Sappington told CNN. Two victims were 2 and 5 years old.

At least eight people died in Arkansas, officials say.

Gov. Sarah Huckabee Sanders declared a state of emergency Sunday afternoon to support the severe weather and tornado-ravaged parts of the state. 

One person died in Kentucky, according to Louisville Mayor Craig Greenberg.

In northeast Oklahoma, two people were killed and at least 23 were injured as a result of severe storms overnight, local officials told CNN.

Survey teams from the National Weather Service found severe damage Sunday in Claremore, Oklahoma, giving the tornado a preliminary rating of EF3 – which pack winds between 136 and 165 mph.

Inside a Shell gas station in northeast Texas, 60 to 80 people were trapped until the storm blew over, Sappington said. Multiple injuries were reported at the station, but none were life-threatening, he added.

In north Denton County, a possible tornado injured an unknown number of people, damaged several homes, overturned 18-wheelers, downed trees and knocked out power lines on Saturday night, authorities said early Sunday.

“Multiple victims” were reported in Ray Roberts after severe weather struck the area overnight, city of Denton fire officials said.

Damage to several homes was also reported in the neighboring city of Celina, where officials said the city was affected by “apparent tornadic activity” on Saturday.

Lake Ray Roberts Marina in the city of Sanger in Denton County sustained damage to boats, boat houses and the fuel dock by the severe weather Saturday night. Despite reports of people rescued after being trapped in overturned RVs, there are no reports of serious injuries.

“There is so much damage, we don’t even know where to start,” the marina said in a Facebook post Sunday morning.

“We know the boat houses are heavily damaged, all have lost walkways, and most boats are damaged,” the post said. “We lost our fuel dock and offices along with our dock cat, Ginger.”

A survey team with the National Weather Service in Dallas found that a tornado of at least a preliminary rating of EF2 strength struck Valley View in northern Texas with estimated maximum winds of 135 mph.

The team also said damage found in south Montague County, just south of Forestburg, was consistent with an EF2 tornado and estimated maximum winds of 125 mph.

Gov. Greg Abbott on Sunday signed a disaster declaration for four additional counties – Denton, Collin, Cooke and Montague – bringing the total to 106 counties.

“I saw the harrowing video inside the Valley View gas station as the tornado passed through with the people stockpiled in their clinging to safety,” Abbott said at a news conference, adding that it’s a “miracle” everyone made it out alive.

At least 100 people have been injured in Texas, said Abbott, who referred to the deaths as a “heart-wrenching loss of life.”

As damaged areas recover, storms move east

More than 600,000 customers across 12 states were without power by Sunday evening, including more than 135,000 outages in Kentucky alone, according to the tracking website poweroutage.us.

There is a Level 3 of 5 risk of severe thunderstorms over parts of the Mississippi and Ohio valleys into Monday morning, and EF2 to EF5 tornadoes and wind gusts of 74 mph are possible, according to the Storm Prediction Center.

Storms will move toward the East Coast on Monday, bringing disruptive high winds and large hail from DC through the Southeast.

The travel hubs of Chicago, Indianapolis, St. Louis and Nashville could have to contend with damaging storms, leading to delayed or canceled flights.

The potential for heavy rain over parts of the Ohio and Tennessee Valleys, middle Mississippi Valley, and Central Appalachians prompted the slight risk, Level 2 of 4, of excessive rainfall in those regions through Monday morning.

Louisville, Kentucky; Cincinnati; St. Louis; Nashville and Indianapolis are among the cities included in the Level 3 threat.

Record-breaking heat possible for some

The unofficial start of summer is also ushering in sweltering heat, reaching potentially record high temperatures for parts of the US on Monday.

Houston; New Orleans; Miami; Mobile, Alabama, Tampa, Florida; and Charleston, South Carolina, are among locations where warm weather will feel more like July than late May through the holiday weekend.

A subtropical upper-level high over Mexico will help spawn excessive heat warnings and heat advisories over southern Texas through Monday, according to the Storm Prediction Center. “The ridging will create a dangerous early-season heat wave over south Texas and southern Florida,” according to the center.

Daily high temperatures and heat index reading exceeding 115 are possible in some areas, the Storm Prediction Center noted.

Those spending time outdoors or who are dealing with a loss of power should drink enough water, dress in lightweight clothing and locate cooling centers if necessary.

CNN meteorologist Elliana Hebert and CNN’s Ed Lavandera, Jillian Sykes, Chris Boyette and Cindy Von Quednow contributed to this report.