CNN
—
The threat of severe weather is spreading to millions across the east on Memorial Day after powerful and destructive storms battered a wide swath of the central United States over the weekend.
Destructive thunderstorms and tornadoes killed at least 21 people, including four children, over the weekend in parts of Kentucky, Arkansas, Texas and Oklahoma. The storms killed 15 people in Arkansas and Texas alone. Four more people were killed in Kentucky.
LIVE UPDATES: At least 21 killed in tornado storms that hit central US
Sunday was the most severe weather day so far this year, with more than 600 reports of tornadoes, wind gusts exceeding 75 mph, and softball-sized hail in at least 20 states. The winds reduced buildings to rubble, damaged cars, and downed power lines.
Charlie Cajo/Northwest Arkansas Democrat-Gazette/AP
The Hometown Flea Market in Rogers, Arkansas, sustained extensive damage.
The weekend storms left more than 600,000 homes and businesses without power across the central, southern and eastern U.S. by Monday morning, according to PowerOutage.us.
Severe storms pounded parts of Kentucky Sunday night, causing damage in some areas and prompting Gov. Andy Beshear to declare a state of emergency early Monday. Phone lines in the Bowling Green area were down and police set up an alternative emergency number for people needing help.
The National Weather Service in Paducah, Kentucky, said it would send at least two storm survey teams out Monday to assess the damage.
Four deaths from the storm have been reported statewide: one in Hardin County, one in Hopkins County, one in Mercer County and one in Louisville, about 70 miles away. Another person is “in critical condition,” Gov. Beshear said at a press conference on Monday.
More than 70 million people across the South and Northeast are at risk of severe thunderstorms, rated Level 2 out of 5, on Monday.
Powerful storms continued to form across parts of the Southeast, including metro Atlanta, on Monday morning. These storms will gradually weaken throughout the morning, with more storms expected to develop later on Monday.
Severe thunderstorms are expected to develop across the Northeast and Mid-Atlantic on Monday afternoon, including a large section of the I-95 corridor. Storms in the area could produce damaging wind gusts, hail and tornadoes.
View this interactive content on CNN.com
Tornadoes are possible in the severe storms that developed in the East on Monday, with a slightly increased risk from eastern North Carolina through eastern Pennsylvania and western New Jersey. Richmond, Virginia, Washington DC, Baltimore and Philadelphia are just a few of the cities where tornadoes may occur.
The storm will also dump heavy rainfall that could cause flooding, especially in low-lying areas and urban areas. Parts of New York, New Jersey, Pennsylvania, Delaware and Maryland are under a Level 2 out of 4 risk of excessive rainfall.
Severe thunderstorms in the east will move offshore and into the Atlantic Ocean before calming down by Monday night.
Locally severe thunderstorms are possible across parts of the Southern Plains late Monday afternoon and evening.
North Central Texas and the Dallas-Fort Worth metropolitan area are at Level 2 of 5 for severe thunderstorm danger. The main threat from storms in the area is large hail and damaging wind gusts, but tornadoes are also possible.
High temperatures and an unseasonable heat wave are expected to make the Memorial Day holiday sweltering in parts of South Texas, the central Gulf Coast and South Florida.
Extreme heat warnings remain in effect for parts of Texas until Monday night, and extreme heat watches stretch from Texas into Mississippi.
Houston, New Orleans, Miami, Mobile, Alabama, Tampa, Florida, and Charleston, South Carolina are some of the areas that could experience extreme heat on Monday.
Temperatures will be at or near record highs in some places, with the heat index potentially reaching more than 115 degrees Fahrenheit. The heat index — a measure of how hot the body actually feels — is expected to reach about 110 degrees Fahrenheit in Houston and 119 degrees Fahrenheit in Laredo on Monday, according to the National Weather Service in Houston.
View this interactive content on CNN.com
Temperatures may not cool off overnight in some areas. Overnight lows are expected to be 10 to 15 degrees above normal, with record heat possible in dozens of locations.
Such extreme and prolonged heat increases the risk of heat illnesses, including heatstroke, especially for vulnerable groups such as children, adults with underlying health conditions, pregnant women and outdoor workers.
The risk of extreme heat, the deadliest form of extreme weather, is becoming more prevalent as the planet warms due to human-induced climate change: During last year's warm season, the rate of emergency room visits due to heatstroke increased by 20% compared to the previous five seasons.
CNN's Eliana Hebert, Chris Boyette and Joe Sutton contributed to this report.
