What causes flash floods and why are they so dangerous?

An entire summer's worth of rain hit parts of Pennsylvania, New Jersey, New York and New England over the course of just a few hours this week. Some of the flooding was so powerful it damaged roads, bridges and homes. One woman in New York was killed and heavy rains in the region continue to fuel dangerous floods. 

Nationally, floods are the most common natural disaster and flash flooding is the most dangerous type. So far this year, flooding has killed 34 people in the U.S. — 24 of those people died while driving. Since 1980, flood damage in the U.S. has on average cost more than $4 billion every year.