Heavy rain in Maryland prompted the evacuation of at least one elementary school and dozens of water rescues on Tuesday, as a slow-moving storm system threatened to bring the risk of flash floods to the Mid-Atlantic, the authorities said.

The Georges Creek region, which surrounds a tributary stream of the North Branch Potomac River in western Maryland, was most affected, approaching record water levels for the area. Several major roads in Allegany County were impassable by midafternoon, the sheriff’s office there reported. Parked cars began to float on the street in Westernport, Md., a town along the creek, as floodwaters inundated the ground floors of homes and businesses in the downtown area.

The National Weather Service had issued flash flood warnings for parts of western Maryland and West Virginia on Tuesday afternoon.

The Weather Prediction Center has issued a Level 2 out of 4 risk for excessive rainfall, potentially leading to flash flooding across eastern North Carolina, eastern Virginia, Maryland, eastern West Virginia and extending into central and southern Pennsylvania through Wednesday.

In Allegany County, in northwestern Maryland, the heavy rain forced officials to dismiss classes early, to cancel all after-school activities and suspend busing for the afternoon.