The winds in Montgomery and Chester Counties were fierce Friday night, but not from tornadoes — as some residents believed when they saw houses that had been destroyed by fallen trees — the National Weather Service said Saturday.

The phenomenon that caused winds to reach an estimated 70 to 80 miles per hour — as fast as a weak tornado — is called a microburst, said Sarah Johnson, a meteorologist in the Mount Laurel office of the National Weather Service.

It happens when a thunderstorm forces winds at a high speed down to the surface of the earth. "When they get down to the surface they have nowhere to go but out, so it creates a very localized area of severe wind damage," Johnson said.

According to radar images, that happened at 10:39 p.m. in Pottstown, where part of the roof of the Yorkshire Apartments, at 593 N. York St., was torn off, Johnson said.

About a half hour later, the winds hit East Whiteland Township, where nine structures where damaged, mainly from trees going down, she said.

No injuries were reported.

Heavy rain hit the region from Berks County through the Lehigh Valley into Sussex County, N.J., Johnson said. Rain gauge readings from Berks showed 3 to 5 inches, which meant the Schuylkill in Philadelphia was full to the brim Saturday evening.

The storm caused several thousand Peco Energy customers to lose power. By 7 p.m. Saturday, the utility's outage map showed 1,568 people without power.