Transit agencies in the New York City area were promising a fairly smooth commute with some lingering delays on Tuesday, the morning after heavy rain pummeled the region, flooding parts of the subway system, inundating major roads and causing long flight delays.

Slow-moving summer storms capable of extreme rain are poised to drench the East Coast Monday, threatening millions from North Carolina to New England with even more dangerous flash flooding. Rainfall totals could reach 2 to 4 inches in Monday afternoon and evening’s storms,

Forecasters warned more rain and flash flood risk is expected on July 15 after storms flooded out roads the day before.

Catastrophic floods often bring with them loss of power, cuts to key emergency communications and sudden hazardous situations that leave victims with only minutes or seconds to seek higher ground and stay safe.

Parts of Lancaster and York experienced significant flood damage after storms hit the region. See photos and videos of the aftermath.

Heavy rain caused flash flooding and road closures around the Lehigh Valley on Monday evening as storms made their way through. More rain is expected this week.

As 27 million Americans are under severe thunderstorm and flood watch on Wednesday, a line of strong storms packing intense rainfall is threatening the Mid-Atlantic.

The National Water Center warned several rivers would face serious flooding by Friday afternoon, including the Colorado River near San Saba, Texas; the Guadalupe River near Comfort, Texas; the Guadalupe River near Kerrville, Texas; and the Guadalupe River near Hunt, Texas.

Residents south of the San Saba River in west-central Texas have been ordered to evacuate because of surging waters.