When a hurricane comes ashore, there are so many dangers: tree-snapping winds, torrential downpours and even tornadoes spawned by the tempest itself.

But it’s the wall of water that tropical systems push onto land, known as storm surge, that tops the list.

“Storm surge has historically taken the most lives,” Rick Knabb, director of the National Hurricane Center, said by telephone yesterday.

For the estimated 22 million Americans living in hurricane- prone coastal areas the question is: Am I at risk?

A new online tool released by the hurricane center this week should provide some of the answers.

Overlays showing water heights from hurricanes of varying power are projected on a U.S. map that users can scroll down into to show street-by-street detail.

Hurricanes come in different sizes and follow unique paths, so the tool isn’t a prediction of exactly what will happen if a Category 3 storm hits Southampton, Long Island.

It does show that people living there may want to consider their options. The same is true for people in any of the other places that face the Atlantic and Gulf of Mexico.

The maps also show neighborhoods that are behind levees, which can be vulnerable, as New Orleans was in Katrina, if they should break because of a storm.

“It is a valuable tool for showing who is at risk and who needs to be putting together a personal or family or business plan for evacuation,” Knabb said. “I hope the big result is that people realize they are vulnerable to storm surge and that they put together a plan now.”

Insurance Coverage

As recent years bear out, there is no region that is safe from a strike. The last hurricane to hit Florida was Wilma in 2005, Knabb said. Since then, the Northeast has been hit by Irene and Sandy as well as a number of near misses.

People in the Northeast may be more aware of storms in recent years than those living in Florida and along the Gulf coast, two areas that have been hit the most since records began in 1851.

Eight storms have formed during this year’s Atlantic season, which runs from June 1 to Nov. 30. The 30-year average calls for 12 storms with winds of at least 39 miles (63 kilometers) per hour.

Evacuation Calls

Regardless of what the map indicates, Knabb stressed that people need to heed evacuation calls. Emergency managers take a lot more than just storm surge into consideration when they advise people to leave their homes and businesses.

Tropical systems regularly cause deluges of fresh water flooding far behind where they come ashore, as did Tropical Storm Irene when it inundated Vermont in 2011.

Sandy dropped three feet of snow, closing roads and knocking out power, throughout the Appalachians in 2012. Hurricanes and tropical storms touch off tornadoes, collapse buildings and can hurl debris through the air like missiles.

Danger comes in a lot of different forms in a hurricane.

The biggest of them is now measurable on your desktop.