As Hurricane Irene bears down on the region, the National Weather Service forecasts sustained winds of 30 to 40 mph in the District and the suburbs Saturday night into Sunday, with gusts up to 50 mph. Ocean City can expect much more bluster — sustained wind up to 90 mph and gusts even faster.

Just how much damage will all this blowing cause?

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.

If buildings break apart, airborne debris becomes a problem. Shingles, glass and nail-riddled wall studs can all start flying around. And the wind will whip even faster through tight areas, such as the tunnel-like alleys between two houses built within spitting distance.

Wind damage is caused not only by air pushing on structures but also by the suction that is created when big bursts of air flow around a structure. This negative pressure on the backside of a building can pull apart walls and roofs.

And unlike a typical summer thunderstorm, tropical storm blasts are often sustained for several hours, Elsner said. “That can compromise a building that would otherwise be resistant to a brief gust.”

As for power lines, Pepco spokesman Bob Hainey said, “it’s really hard to tell” at what wind speeds power lines will start failing. But, Hainey said, most outages are caused by tree limbs falling across the lines, which Pepco certainly expects — and in potentially large numbers — across the region this weekend.