A storm system tore into the U.S. Mid-Atlantic Thursday after spawning Midwest tornadoes that killed at least 12 people and damaged country music's Branson, Mo.
The violent weather threatened to persist through Friday in Illinois, Indiana, Ohio, Arkansas and Mississippi, AccuWeather.com said.
WLKY, Louisville, confirmed four twisters damaged roofs and trees Wednesday with wind speeds approaching 100 mph.
The governors of Illinois, Missouri and Kansas declared emergencies in areas hit by the storms.
At least 20 tornadoes were reported in Illinois, Missouri, Kansas, Nebraska, Kentucky, Indiana and Tennessee.
Four women and two men were killed in Harrisburg, Ill., a 10,000-population city about 55 miles southwest of Evansville, Ind., when a house lifted up by a 170 mph tornado fell on them and crushed them, authorities said.
The tornado's Enhanced Fujita Scale rating of EF-4 -- one notch below the strongest category -- put it on par with the devastating tornado that killed 64 people in Tuscaloosa, Ala., April 27 and a notch below the multiple-vortex EF-5 tornado that killed at least 161 people in Joplin, Mo., May 22, the National Weather Service said.
More than 100 people in Harrisburg were injured and about 300 buildings, including the Harrisburg Medical Center, were damaged or destroyed, the Saline County Sheriff's Office said.
The medical center lost an 80-foot section of exterior wall to the tornado, exposing patient rooms, and part of the roof, but no one was injured because patients and staff had moved to the hospital's better-protected center, hospital officials told CNN.
Blocks of houses and businesses were reduced to rubble. Trees and power lines were tangled along the streets.
"We're going to be swamped with limbs alone," Fire Chief Bill Summers told the Harrisburg Daily Register.
An EF-2 tornado, with winds of 111 mph to 135 mph, caused significant damage and three dozen injuries in Branson, but no deaths.
The city's waterfront area, 15 hotels and at least six signature theaters -- including the Americana Theater, Branson Variety Theater and Dick Clark's American Bandstand Theater -- were extensively damaged as the twister moved up the main drag through the heart of the tourist district, officials said.
Three people died elsewhere in Missouri and three were killed in eastern Tennessee.
The Midwest outbreak produced more than 250 reports of large hail and damaging winds in addition to the 20 tornadoes, the weather service's Storm Prediction Center in Norman, Okla., said.
Already this year there have more tornadoes than usual, with two people killed in Alabama in January, the weather service said. Last year, 550 people were killed by tornadoes, the deadliest season since 1925, when 794 people died, the service said.
Thunderstorms were forecast to return to the nation's midsection Friday as a storm moving from the West travels through the southern Great Lakes to the Lower Mississippi, Ohio and Tennessee valleys, AccuWeather.com reported.
The storm was forecast to be less intense than Wednesday's -- with the greatest risks damaging wind gusts and hail -- but has the potential to produce tornadoes, AccuWeather.com said.
In the Northeast, snow was expected to blanket New England and New York's Hudson Valley region north of New York City with 3 to 8 inches Thursday after dropping several inches Wednesday.
Early spring flowers had already begun to bloom in the Hudson Valley due to unseasonably mild temperatures before the snow.
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