Flash flooding hit parts of Pennsylvania late Sunday morning, but other parts of the state remained completely dry.

From midnight to 4 p.m. on Sunday, more than 3 inches of rain fell northwest of York, causing flash flooding around Dover, the National Weather Service (NWS) said. There was also a 5- to 8-mile-wide swath of storms that dropped almost 2 inches of rain through northern York County and on the south side of Harrisburg, running up to New Cumberland and near Camp Hill.

Most of the rainfall occurred late Sunday morning.

Practically no rain fell over the Delaware Valley and Philadelphia areas, or across southern York and Lancaster counties.

On Saturday evening, storms northwest of Chambersburg caused more than 3 inches of rain, and around the same amount fell in the middle of Somerset County in southwest Pennsylvania.

Other isolated storms left about one-half to three-quarters of an inch of rain.

Storms on Sunday morning were heavy but generally brief.

The National Weather Service said that if the storms had lasted an hour or more, they could have left 4 to 5 inches.

Still, an inch or two of rain in 20 to 30 minutes can cause flooding in low-lying areas. Those short, heavy rainfalls are what the National Weather Service is anticipating during the rest of the day.

The most prone areas on Sunday afternoon are to the west of the Susquehanna Valley and between Route 15 and Route 219 in western Pennsylvania: essentially, the middle third of the commonwealth.

A flash flood watch, in effect until 11 p.m. Sunday or 1 a.m. Monday—depending on where you live—covers all of central Pennsylvania and most of the eastern portion of the state.

Clouds are forming due to a higher dew point, a measure of moisture. The dew point will be in the low to mid-70s in eastern Pennsylvania, but in the mid- to upper-60s in the mountains and higher-altitude regions.

A higher dew point can cause clouds to form, leading to rain. a NWS forecaster explained.