What to know about flash flooding

The notices from the National Weather Service come across social media accompanied by maps with sections highlighted in alarming red.

They are flash flood warnings, meant to alert the unsuspecting to the danger that rain can bring. The warnings were in abundance Sunday as forecasts for on-and-off downpours for most of eastern Pennsylvania and all of New Jersey, in some already sodden areas, proved accurate.

But what exactly constitutes a flash flood?

They can occur suddenly within six hours of heavy rainfall, usually three, according to National Weather Service meteorologist Amanda Lee.

This type of flooding can take place in low-lying rural and urban environments, said Lee, overwhelming soil that can naturally absorb the rainfall.

Lee noted paved surfaces can’t act as a sponge either, and man-made drainage can become overwhelmed by sudden heavy precipitation.

”The water does not have an easy way of running off,” she said.

What’s more, Lee said if there’s enough rain in a short period of time, rushing water can knock people off their feet.

Upper Makefield Fire Chief Tim Brewer said up to 7 inches of rain hit the area in less than an hour Saturday, causing 11 vehicles to become trapped on a local road. Three of the cars were swept away. Five people were killed in the flood and two remained missing Sunday.

As heavy rain continued to fall and rescuers searched for the missing, a flash flood warning was in effect for the area, as authorities continued to trumpet the perils.

The Pennsylvania Emergency Management Agency says to never drive in standing water, noting “mere inches” can carry a vehicle away.