He said he ran to a day care center where his grandson was. No children were hurt, and they were herded into a vault at a nearby bank until the storm system cleared.
"It passed right over us like a big, white monster," said Thomas Coupe, 87, of Windsor.
The tornado overturned 15 railroad cars and destroyed a lumber car on the Great Western Railway of Colorado, said Mike Ogburn, managing director of Denver-based Omnitrax Inc., which manages the railroad. Fourteen of the overturned cars were tankers, but they were empty.
All of northeastern Colorado was under a tornado watch through Thursday night, the National Weather Service said.
About 130 people waited at a downtown Red Cross shelter for friends to pick them up or for evacuation orders to expire, but the shelter was moving to nearby fairgrounds because of a lack of power. A second shelter opened at an events center in Loveland.
Gov. Bill Ritter declared a state of emergency for Weld County and toured the area. He said he talked to the Federal Emergency Management Agency about possible assistance.
Area police departments sent officers to patrol affected neighborhoods and deter looters, said Windsor Police Chief John Michaels.
Some 60,000 customers lost power in the area, but power was later restored to all but 15,000 of them, according to Xcel Energy. The company said it lost two large transmission lines and about 200 utility poles, and that it responding to several gas leaks at homes near Windsor.
In Southern California, area residents flooded TV stations with pictures and video of funnel clouds and at least one tornado on the ground. The storm unleashed mudslides in wildfire-scarred canyons and dusted mountains and even low-lying communities with snow and hail.
California Highway Patrol Officer Alex Santos was watching the wild weather from a highway overpass in Moreno Valley, about 55 miles east of Los Angeles, when he saw two tornados closing in.
"There was so much dust you couldn't see. Next thing I know I see this big rig getting toppled over," he said. One twister "was going straight up into the clouds. I was kind of in disbelief and I realized I had to get down there."
Santos said the driver of the tractor-trailer had to be cut free from the cab and suffered lacerations to the head and injuries to the back.
The National Weather Service in Cheyenne, Wyo., was trying to verify whether a tornado touched down in Laramie, where a storm packing strong winds damaged several buildings and overturned vehicles Thursday afternoon. There were no reported injuries from the storm.