New York (CNN) -- The National Hurricane Center on Saturday warned residents of tall structures in the path of Hurricane Irene that the wind hitting upper floors will be stronger than those in most storms.
"As Irene moves through areas with high-rise structures, these structures will experience winds significantly stronger than indicated by the advisory intensity," forecasters said. "Winds at the 30-story level will likely be 20% higher than at the surface. And winds 80-100 stories up could be about 30% higher than the surface."
Irene was expected to reach the New York metropolitan area Sunday at hurricane strength. Winds were at 80 mph Saturday evening.
The center's analysis buttressed Mayor Michael Bloomberg's call for high-rise residents to stay away from windows if they live on the 10th floor or above. At the city's request, building owners were shutting down elevators Saturday. Reaching trapped residents would be a drain on police and fire who need to respond to "real emergencies," Bloomberg added.
In a hurricane the higher you go, the stronger the wind gusts. CNN meteorologist Sean Morris said the higher friction at the ground surface slows wind speeds.
According to the National Hurricane Center, forecasters started seeing the significant upper wind increase when the eye-wall collapsed over the Bahamas.
They also noted that there was little thunderstorm activity associated with Irene. Thunderstorms tend to bring the higher winds down to the surface, and with the lack of such activity the stronger winds are staying at higher levels.
As the storm re-emerges over the Atlantic late Saturday and moves northward, the colder waters also will aid in intensifying this effect.
Construction workers had wind dynamics in mind while they secured One World Trade Center ahead of Irene's arrival.
While the windows being installed on the soon-to-be tallest building in the U.S. are designed to withstand 125 mph winds, they're only in place up to the 52nd floor. Steel beams taking the place of the Twin Towers have risen to the 80th floor. The remaining upper floors are open to the elements.
That's a serious concern because anything blown off a building from that height, even something as small as a bolt, can become a deadly projectile in a hurricane.
"They are very big on safety here," said Kelly Potts, an electrician working on One World Trade Center. "If anything is not tied down they will address it and fix it."
The construction cranes high atop the World Trade Center site also were to be secured. They are designed to withstand strong winds.
Construction workers on the site are well aware how serious high winds are at a skyscraper under construction. While the Time Warner Center was being built a few years ago at Columbus Circle in New York a plywood board was blown off the building. It struck a man below, killing him.
Bloomberg also urged people not to have a generator inside their home or apartment because "carbon monoxide fumes kill."
"Homeowners and residents: If you haven't done so already, please bring outdoor furniture inside -- plywood, trash cans, any loose items that can blow around," he said.
CNN Radio's Steve Kastenbaum contributed to this report.