Urban flooding is swamping city streets more often and more severely as urban heat islands supercharge heavy rainfall, concrete and asphalt send stormwater rushing into overwhelmed drains, and coastal cities drown from both ends — rain piling up above and rising tides backing up below.

How cities make rainfall worse

An effect known as the urban heat island is making rain more intense and more frequent. Cities heat up faster than the land around them, and that extra warmth fuels thunderstorm development directly overhead.

The result is measurably more rainfall over the city than over the countryside just miles away —leading to worse flash flooding on city streets.

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(MORE: How cities can cool their streets)

Stormwater drainage systems can’t keep up

Impervious surfaces (concrete, asphalt, rooftops) are the hidden driver of urban flooding.

Unlike soil, they absorb nothing. Every drop of rain that falls on them becomes immediate runoff, moving fast across the surface with nowhere to go but the nearest drain.

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The expansion of these surfaces in cities generates more runoff. And the drainage systems underneath were never designed for that volume.

(MORE: Tips to prepare for flood)

Coastal cities are at a tipping point

Coastal cities are caught between two forces: the heat-driven rain falling from above and the tidal water backing up from below.

Coastal cities don't get a pass from the urban heat island. New York, Miami, New Orleans, San Francisco, and Los Angeles all carry the same heat-driven rainfall problem as any inland city —but with an extra layer of vulnerability built in.

Storm surge and hurricanes are the coastal flood story everyone already knows about.

What's missing from that conversation is what's happening on an ordinary day.

High tides now routinely push water back through storm drain outlets — the same aging pipes that can't handle the rain coming from above. Water backs up from below before a storm ever arrives.

And when the storm does arrive, it lands on a system with nothing left to absorb.

Use the NOAA inundation dashboard to see how coastal communities are impacted by high tide flooding.

For inland areas, check your local flood maps through FEMA. Knowing what's driving the flood is the first step to understanding your own risk.

Faiza Mohammed is a meteorologist intern at weather.com. She studied geography and worked with the Weather Dawgs team at the University of Georgia. She also received her master's in Earth and Atmospheric Sciences from the Georgia Institute of Technology.