A fierce thunderstorm walloped metro Denver Thursday afternoon, flooding streets, uprooting trees and knocking out power to 18,000 customers, officials said. Xcel Energy spokesman Mark Stutz said crews were expected to restore power to most customers Thursday night. But some residents might not have electricity restored until Friday, he added.Off and on Thursday, the National Weather Service issued flash flood warnings and advisories for communities across the metro region.The Boulder Office of Emergency Management issued a flash flood warning until 9:15 p.m. for Fourmile Canyon west of Boulder, where a September fire burned 6,181 acres, stripping the canyon of trees and other vegetation.The creek reached peak flow during the height of the storm, but its waters were receding Thursday night, OEM officials said.An automated telephone alert was sent to about 300 homes, telling residents telling to remain on alert and be prepared to seek higher ground, emergency management officials said.In Denver, floodwaters were above car hoods at East Seventh Avenue and Lincoln Street.Denver and Aurora officials warned that surging floodwaters had forced several manhole covers out of place.While city workers were replacing the manhole covers, Denver spokeswoman Ann Williams warned, "Drivers should be aware that manhole covers can float off during heavy rainfall and should approach manholes with caution during and following heavy rainfall.""I've never seen it flood like this before," a Five Points resident told 7NEWS.A lightning bolt toppled a towering maple tree that had stood more than a century at East Sixth Avenue and Ogden Street in Denver. It crashed against a business building."It was happy hour and we were about to mix a cocktail," said Dave Elger, who was in the building. "All of a sudden the rain started pouring down and lightning struck and this tree came falling into the house. The whole house shook. I thought the roof was caving in." Aurora police responded to several motorists who called 911 because their vehicles stalled in high water, police spokesman Detective Bob Friel said late Thursday afternoon. Lightning struck a home in the 19500 block of East Batavia Drive and power lines were reported down at Victor Way and Center Avenue, he said. Several officers stayed beyond their shifts to assist with the heavy 911 call volume generated by the storm, he said. A severe thunderstorm warning created flight delays of up to 90 minutes at Denver International Airport Thursday evening, said DIA spokeswoman Laura Coale. She did not know the number of flights delayed. Coale said travelers should check their flight status with the airline or at FlyDenver.com.