* Residents of Norman take cover as twister touches down
* More powerful storms expected on Saturday afternoon
* Forecasters say Kansas and Oklahoma at highest risk (Adds damage, reports of another tornado, details)
By Mary Wisniewski
CHICAGO, April 13 (Reuters) - Forecasters are warning of a major tornado outbreak in Kansas and Oklahoma this weekend, and Oklahoma got a first taste of it on Friday as a twister touched down near the National Weather Service office in Norman.
A tornado was seen near the University of Oklahoma campus in Norman just after 4 p.m. local time - the same town that holds the National Storm Prediction Center, according to a Twitter post from Rick Smith, a National Weather Service meteorologist.
The Norman tornado ripped roofs from buildings, downed power lines and uprooted trees across Norman, a town of 110,000 people 20 miles (32 km) south of Oklahoma City, television images showed. City Hall was among the structures damaged, said Keli Cain, a spokeswoman for the Oklahoma Department of Emergency Management
An operator at the University of Oklahoma said people had been warned to get to a basement or low floor.
“I was watching this tornado on TV, which was neat until I realized it was right here in Norman,” said the operator, who did not give a name.
Atmospheric conditions for the weekend will be similar to those that caused severe storms in parts of the Midwest and Southeast in early March that killed more than 50 people, said Steve Weiss, science support branch chief for the National Storm Prediction Center.
“We see potentially some ... very damaging tornadoes,” Weiss said.
Oklahoma is already having severe weather Friday, and a twister was also reported in Copperton in the state’s southwest corner. Tornado warnings were issued for two counties, Comanche and Kiowa.
But the biggest storms were expected on Saturday, said Weiss, who was watching heavy rain out of his office window in Norman on Friday afternoon. “It’s not unusual to have successive days,” he said.
Conditions favor strong thunderstorms in Kansas and Oklahoma on Saturday, with a few “supercell” storms with rotating updrafts, Weiss said.
“The potential is that some of the supercells could be long-lived, so if they produce tornadoes they could be on the ground for a while,” he said.
Forecasters said the storms could start in earnest Saturday afternoon into the early evening and continue after dark, and Oklahoma activated its emergency operations center in anticipation of the storms.
“The really dangerous part is that it looks like it’s going to be overnight,” said Kurt Van Speybroeck, emergency response meteorologist for the National Weather Service. “It’s a really bad combination to get tornadoes at night because they’re harder to see. It could be a really bad evening.”
Storms could strike heavily populated areas such as Oklahoma City, and Wichita and Topeka, Kansas, Weiss said.
The high-risk area is from about the I-40 highway in Oklahoma City going north along I-35 to I-70 in central Kansas, said Van Speybroeck.
Northwest Texas into Nebraska and parts of Iowa and Missouri are also at risk for thunderstorms and tornadoes this weekend. Southwest Wisconsin has a slight risk of tornadoes for Saturday into Sunday, with a chance for thunderstorms, hail and tornadoes for the whole state Sunday, Van Speybroeck said.
The storm warnings have led to the rescheduling of the Rotary Club “Beer Sprocket” fundraiser Saturday in Choctaw, near Oklahoma City. The outdoor event attracts about 600 people.
“The last thing I want to do is put the revelers for this event in harm’s way, as well as the staffers and vendors,” organizer Mike Turek said.
Van Speybroeck said the large number of tornadoes was linked to warm, moist air off the Gulf of Mexico, which brings energy for severe storms.
“When the cooler air starts to move out of the west and across the region that causes an increase in instability,” he said. “It has been a pretty vigorous spring.”
He predicted a gradual slowdown in these storm systems as summer approaches.
The U.S. tornado season started early this year. Tornadoes have been blamed for 57 deaths so far 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.
In 2011 there were 550 tornado deaths, including 316 people on April 27 in five Southern states, and 161 people in Joplin, Missouri, on May 22.
Insurers have already lost as much as $2.5 billion during the 2012 tornado season, mostly from a record March 2 outbreak. That follows record-breaking losses of $26 billion during the 2011 tornado season. (Additional reporting by Ben Berkowitz, Andrew Stern, and Steve Olafson in Oklahoma City; Editing by Xavier Briand and Eric Beech)