People who think they might be in a hurricane’s path tend to focus on the storm’s wind speed, which defines its category. But what matters in deciding whether to evacuate is the trickier calculation of storm surge.

This year, the National Hurricane Center will be using a better model on faster computers to help emergency managers update hurricane evacuation plans.

“The categories put a focus on wind; people are absorbed by wind,” said Jamie Rhome of the National Hurricane Center, but “water is what kills the vast majority of people. Evacuations are done for surge.”

Surge is the water a storm pushes ashore. Rhome said “surge is not just a coastal event; the tidal Potomac River up to Washington, D.C., is vulnerable.”

The height and extent of a surge depends on many factors, including the storm’s wind speeds, the direction the storm is coming from, how fast it’s moving, its size, the depth and shape of the ocean bottom, and what the surge encounters on land, including dunes, buildings, embankments and rising land.

Only a powerful computer can make sense of all of this.

The hurricane center’s storm-surge model is called SLOSH, for Sea, Lake and Overland Surges from Hurricanes. (There’s also a version of SLOSH for extratropical cyclones such as our winter Nor’easters.) It uses the model to make advance calculations of potential surges for 37 “basins,” which include all of the East and Gulf coasts, the Bahamas, Puerto Rico, the Virgin Islands and parts of Hawaii and Guam.

To prepare the “hazard analysis” for each basin, the hurricane center runs models for hundreds of simulated hurricanes. They are done for all five storm categories, changing various factors such as forward speeds, directions of approach and landfall locations. Each model run for a category creates a Maximum Envelope of Water (MEOW). They are combined into a Maximum of Maximums (MOM) for each category.

Faster computer speeds are enabling the hurricane center to run more scenarios than in the past, which gives a better picture of potential surges. “In the past, we did 10,000 to 15,000 scenarios for the hazard analysis,” Rhome said. “We’re working on one now for Louisiana and are more than doubling the number of scenarios to 50,000.”

Emergency managers use the hazard analyses to create evacuation zones long before storms hit.

The hurricane center normally updates surge data for five or six basins a year after a storm changes the shape of a coastline, the high-resolution land elevation for the area has been updated, or local emergency managers need updated surge data for planning, Rhome said.

Surge data for the Lower Chesapeake Bay was updated last year, and the Upper Chesapeake Bay is now being updated.

When a hurricane threatens a particular basin, the center runs a few hundred scenarios, with slight changes in size, strength, where and when it could hit, and other factors that could affect surge. The results help emergency managers decide which zones to evacuate.

Another big change this year is that potential surge is being described in terms of depth of water above ground level, instead of “feet above normal tide levels.” If you’re in a potential surge area, you won’t have to know how far above “normal tide levels” your house is to know how deep the water around it could be.

As with all forecasting, surge predictions are done in terms of probabilities. Rhome said there is a fine line between crying wolf and giving adequate warnings. “Under-evacuation is a terrible thing to think about. Imagine if I told an emergency manager five feet and it turned out to be 15 feet.”

Still, “its an exciting time,” he said. “This year’s changes are the biggest ever.”

Leading hurricane researchers say to expect an above average Atlantic hurricane season, but slightly less active than once thought.

Phil Klotzbach and William Gray of Colorado State University, who have issued seasonal hurricane outlooks for 30 years, are forecasting 18 named storms (including the four that have already formed), eight hurricanes and three major hurricanes (category 3 or higher).

They plan to publish updated outlooks every two weeks into October. Hurricane season spans from June 1 to Nov. 30. There have been five named storms so far, but no hurricanes.

Jason Samenow of the Capital Weather Gang contributed to this report.