Three years ago this September, a storm surge driven by winds from a weakening Hurricane Isabel produced the worst Chesapeake Bay flooding in 70 years.

Waters up to 8 feet above normal tides surged into lower Fells Point and across Pratt and Light streets into downtown Baltimore. Hundreds of basements and businesses flooded.

Hundreds of homes in Bowleys Quarters and elsewhere on the bay shore were badly damaged or destroyed. Property damage reached $410 million in Maryland alone.

But as destructive as Isabel was, recent computer simulations by government scientists - the most extensive ever for the Chesapeake Bay - show that hurricane storm surges here could get much, much worse.

Under some conditions, they discovered, a Category 4 hurricane making landfall in the Carolinas could produce storm surges as high as 18 or 20 feet in Baltimore at high tide. That's at least 10 feet - a full story - above Isabel's high-water mark and enough to carry floodwaters much farther inland.

The storms were simulated on the latest version of the government's computerized SLOSH model. The name is an acronym for "Sea, Lake and Overland Surges from Hurricanes," produced by the National Weather Service and the National Hurricane Center.

The projections show that 18-foot storm surges are also possible all along Baltimore County's shoreline. Parts of Harford and Anne Arundel counties, and the upper tidal reaches of the South, Severn, Patuxent, Potomac and Anacostia rivers are just as vulnerable.

"I guess I'm a little surprised the values are as high as they are," said Wilson A. "Will" Shaffer, chief of the National Weather Service evaluation branch in Silver Spring and a leader of the project.

The precise combinations of tide, storm intensity, track, size and forward speed needed to generate an 18-foot storm surge on the Chesapeake are surely rare, Shaffer cautioned - nothing like it has ever been recorded.

But New Orleans had never before experienced anything like Katrina's flooding, either.

Here, with the new SLOSH model data, the conditions needed to produce an 18-foot storm surge are "within the realm of possibility," Schaffer said. And emergency managers must consider them when they plan for storms.

Those managers agree.

"A 20-some-foot storm surge up the bay is not something we want to take lightly," said Robert A. Ward, all-hazards planner for the Maryland Emergency Management Agency. The new simulations are "a useful planning tool for us that we are going to take seriously."

The agency is working with local officials and the U.S. Army Corps of Engineers to turn the simulation data into updated maps for emergency planning. The new maps will demonstrate how much farther inland storm-surge floods might reach under the right conditions.

"We have to make sure, as emergency managers, that we understand where those areas are, and provide adequate warning to those residents," Ward said.

Managers also will use the data to rethink where and when to order evacuations. They'll reconsider where they can safely establish operations and evacuation centers, store emergency supplies and park emergency vehicles as storms approach.

Shaffer is scheduled to present his findings to the state's emergency managers at a training session Thursday at MEMA headquarters in Reisterstown. He'll also demonstrate the SLOSH model software.

"We are involving a lot more people in this SLOSH training ... so we can be better prepared for these situations," said Lt. Mark Demski of Baltimore County's Fire Department and its office of Homeland Security Emergency Management.

Although he has not seen the most extreme SLOSH simulation, Demski acknowledged that the old model failed to predict the extent of surging with Isabel, which made landfall as a Category 2 storm.