This is taken from a Washington Post article during Irene, may help explain what to expect. "At 20 mph, trees will sway and leaves may strip off.
At 30 mph, dead branches can fall. If you’re daring — or dumb — enough to walk outside, prepare to grab one of those swaying trees to keep upright. Lawn furniture will blow around unless it’s heavy or well-secured.
But stuff doesn’t really start to fly until the wind blows even harder.
At 40 mph, branches start to rip off trees, even “well-secured branches,” said James B. Elsner, a geography professor at Florida State University who studies hurricanes.
Heading toward 50 mph, shingles can tear off roofs, Elsner said, and eaves can lift and blow away, “especially on weaker structures like sunrooms.”
Trouble really starts at 60 mph, or Level 10 on the 200-year-old Beaufort wind scale, developed by a British admiral to estimate wind speeds before the invention of instruments to measure it. “You can get windows blown out,” Elsner said, and parts of roofs may fly off.
Above 75 mph, the situation is dire. “You can get some full-scale compromises of buildings,” Elsner said."
