Flooding from storm surge and heavy rain are two of the most dangerous threats posed by hurricanes and tropical storms. When they combine, the results can be devastating.

This double dose of danger occurs when onshore winds push water from the ocean into areas near the coast where heavy rain is falling.

The rain is unable to drain as it normally would since storm drains, particularly in urban areas, and runoff locations are being overwhelmed by the push of rising water from the ocean.

Hurricane Dorian's storm surge will cause higher than normal tides that, when coupled with the storm's heavy rain, will exacerbate the flood threat in the coastal Carolinas and southeast Virginia through Friday.

Inlets and rivers, near and just inland from the coast, are particularly problematic since they act as a funnel, allowing water to rise quickly above bankfull as the storm surge pushes inland.

Heavy rain runoff into the river basin before or during the arrival of the storm surge can worsen the flooding potential since the rainwater has nowhere to drain downstream.

In 2017, storm surge from Hurricane Irma together with runoff from heavy rain led to some of the worst flooding in Jacksonville, Florida's history.

A slow-moving storm only amplifies the effects of the storm surge, rainfall combo because of multiple high tide cycles and the accumulation of heavy rain over a longer period of time.

All of this is important because water is the biggest contributor to deaths from tropical cyclones, which includes tropical depressions, tropical storms and hurricanes.

Storm surge and rainfall flooding accounted for , according to a study by Dr. Ed Rappaport, deputy director of the National Hurricane Center.

Percent of deaths by cause from U.S. tropical cyclones from 1963-2012.

(Source: Rappaport, BAMS, March 2014)

So, if you live in a storm surge-prone location, and an evacuation is ordered, make sure you move inland to a safe shelter.