Westmoreland, Allegheny, Armstrong, Butler and surrounding counties are under a flash flood watch, according to the National Weather Service.
Showers and thunderstorms are expected to bring over 2 inches of rain per hour.
Soil that is already wet from recent rainfall could create flash flooding around streams, creeks and roads, officials said.
According to the National Weather Service, flash flooding can lead to dangerous situations. Those under the watch should monitor forecasts and be prepared to take action.
The watch is in effect until 10 p.m. Saturday.
Other counties under the watch include Mercer, Venango, Forest, Lawrence, Clarion, Jefferson, Beaver, Indiana and Washington.
