(Clarifies Wichita is the second-largest metro area of Kansas after Kansas City)
* Oklahoma, Kansas, Nebraska and Iowa are hit by twisters
* At least five dead in Oklahoma town as storm siren fails to sound
By Steve Olafson
OKLAHOMA CITY, April 15 (Reuters) - Dozens of tornadoes tore through parts of Oklahoma, Kansas, Nebraska and Iowa overnight and one twister killed at least five people early on Sunday as storm sirens failed to sound in an Oklahoma town and caught people unaware.
Storms skipped across what is often called "Tornado Alley" in the U.S. Central and Southern Plains and more were forecast. But casualties appeared limited because many of the twisters hit sparsely populated areas, and during daylight hours or evening when people were still awake.
In Oklahoma, a twister struck the northwest city of Woodward early on Sunday morning after lightning apparently disabled its storm warning system, Mayor Roscoe Hill said.
Two children died at the Hide A Way mobile home park on the west side of Woodward, a town of 12,000 people, while two adults were killed in a small community just outside the city limits, Hill said. Details of the fifth death were not immediately known, according to Keli Cain, spokeswoman for Oklahoma Emergency Management.
"This thing took us by surprise," Hill said, adding storm sirens had not sounded. "It's kind of overwhelming."
Hill said he was told the tornado hit the west and north sides of the city, badly damaging an apartment complex where residents were trapped and awaiting rescue.
A tornado that struck Woodward in April 1947 still ranks as the deadliest in Oklahoma history, with 116 people killed, according to the National Weather Service.
In Kansas, a twister also churned through parts of Wichita, the state's second-largest metropolitan area after the Kansas City metro area. Storm chaser Brandon Redmond, a meteorologist with the Severe Weather Alert Team, said the twister passed over his vehicle and lifted it two feet (60 cm) off the ground in an industrial area south of the city.
"The tornado literally formed over our vehicle," he told Reuters. "I've never been that scared in my life. ... We had power flashes all around us and debris circulating all around the vehicle, sheet metal, parts of a roof, plywood."
Damage was reported to a mobile home park, and Redmond said there was significant damage in the industrial area on the city's south side. There were no immediate reports of injuries.
Residents in the affected Plains states and in a wide area of the middle of the United States stretching from Minnesota to Texas hunkered down for more severe weather. The National Weather Service said the worst conditions were expected in Oklahoma, Nebraska and Kansas, while other areas could see baseball-sized hail and strong winds.
"Conditions will remain very favorable ... for very strong and potentially long-lived tornadoes," the National Weather Service said in an advisory.
It warned that nighttime tornadoes could be particularly dangerous because they are usually fast-moving and often obscured by rain and darkness.
EARLY TORNADO SEASON
The U.S. tornado season started early this year, with twisters already blamed for 62 deaths in 2012 in the Midwest and South, raising concerns that this year would be a repeat of 2011, the deadliest tornado year in nearly a century.
Some 550 people died in tornadoes last year, including 316 killed in an April outbreak in five Southern states, and 161 people in Joplin, Missouri, the following month.
As of early Sunday morning, the National Weather Service website said it had received preliminary reports of 121 tornadoes across four states over the previous 24 hours. Some could be duplicate reports of the same tornado and it usually takes experts at least a day or so to confirm if they were tornadoes.
In Iowa, The Greater Regional Medical Center hospital in Creston was damaged by a possible tornado, said a woman who answered the phone there but declined to give her name.
An Iowa emergency management spokesman said two people were injured, but the National Weather Service could not immediately confirm the storm was a tornado.
Creston City Councilman Randy White said patients were being moved to hospitals in surrounding communities after the tornado passed north and west of downtown, knocking out power to all but a small part of the town of about 7,500 people.
The tiny Iowa town of Thurman, population around 250, was also hit by a storm that caused structural damage to some homes and ripped shingles off the roofs of others while downing power poles and trees, officials said.
"Some kind of a storm went through, whether it was a straight wind or tornado hasn't been determined," said Randy Chapman, a deputy at the Fremont County Sheriff's Office. "I would estimate a fourth of the houses have been made unlivable."
Tornadoes also raced through north-central Kansas in the early evening. Five homes in rural Saline County were damaged, but the tornado avoided towns and no one was hurt, said Joe Koch, county director of emergency management.
An apparent tornado near Oxford, Nebraska, on Saturday evening took a roof off a farm house and toppled a grain bin but no injuries or other serious damage in the area were reported, said Bridget Timmerman, a dispatcher for the Harlan County sheriff's office.
Tornadoes briefly touched down earlier in Nebraska's Nuckolls County and Thayer County. (Reporting by Corrie MacLaggan, Steve Olafson and Tim Gaynor; Writing by Cynthia Johnston; Editing by Greg McCune)