With saturated soil and streams already full from summer rains, Maryland and neighboring states are ripe for serious flooding when heavy rains from Hurricane Isabel finally arrive Thursday, weather forecasters said.
While the precise path of the storm's center and its highest winds remained uncertain, Isabel's tropical storm conditions sprawled 200 miles outward from its eye yesterday, so the likelihood of heavy rains across the Middle Atlantic States was growing more certain.
"Assuming the storm doesn't make any dramatic changes in its path, there is a high potential for widespread flooding across Maryland and parts of Virginia and Delaware," said Jeff Warner, a meteorologist with Penn State Weather Communications in State College, Pa.
The rain, if it comes, will simply have nowhere to go. So officials cautioned homeowners to start taking precautions.
"If you're a homeowner, one of the things you should do between now and the time the storm gets here is take a look at your property and see what you have to do to keep the water away from your house," said Quentin Banks, a spokesman for the Maryland Emergency Management Agency. "We consider this [storm] very serious."
That means checking sump pumps and clearing gutters, downspouts and nearby storm drains. It may also be wise to add dirt around the foundation to be sure the water flows away from the house, he said.
Those warnings were driven home early in northern Cecil County and its neighbors in Pennsylvania and Delaware yesterday, where heavy rain unrelated to the hurricane triggered flash floods along creeks and rivers, closed roads, and disrupted traffic and school bus runs.
In Harford County, three cars were caught in waist-high water along Route 7 in Riverside. One driver was rescued after she was swept 30 feet downstream.
In Delaware, firefighters and police rescued more than a dozen motorists from stalled cars. A Coast Guard helicopter from Atlantic City, N.J., plucked at least three people from the roof of a flooded chemical facility.
Flooding aggravated by high tides was also reported yesterday in Chestertown on the Eastern Shore and Deale in Anne Arundel County.
Isabel is likely to bring much more of the same. Yesterday, the National Weather Service compared the approaching storm with the 1933 hurricane that cut the inlet at Ocean City and to Hurricane Hazel in 1954.
Both storms followed a path close to the one tentatively predicted for Isabel, with landfall in North Carolina and a track northward across Central Maryland near Hagerstown.
Winds on the east side of Hazel's rotating center pushed water north into the Chesapeake, producing 2- to 6-foot tides and flooding on streets and in basements adjacent to Baltimore's harbor. Six Marylanders died. Homes, docks, crops, farms, boats, bridges and roads were damaged, and a half-million trees fell. Statewide, damages totaled $28 million - the equivalent of $190 million today after adjustment for inflation.
Hurricanes that pass east of the Chesapeake, such as Floyd in 1999, generally blow water southward, out of the bay, and cause less bay shore flooding.
The amount and nature of flooding from any hurricane or tropical storm depend on many factors, including the storm's path and forward speed.
"At the coast you get the strongest winds and most damaging winds, the storm surge and high tides," Warner said. But as the storm crashes inland, the winds diminish and the most pronounced impact is frequently from heavy rain.
"It is common to see rainfall amounts of 5 to 10 inches, and in some places 20 to 30 inches," he said. "It all depends on how fast the storm is moving. And how much water is associated with it."
"As a general rule," he added, "the faster the storm is moving, the less the highest amounts of rain are going to be. It's not in any one place long enough to produce prodigious amounts."
Isabel was expected to speed up as it approaches the coast Thursday afternoon. "Hopefully it will make a quick exit," Warner said.
The environmental effect of such heavy rains and flooding can be considerable.
Rain from Tropical Storm Agnes, which dumped as much as 14 inches on some areas of Maryland in 1972, "turned the bay fresh for at least one-third, maybe even two-thirds, of its length," said Bob Wood, a fisheries biologist with the National Oceanic and Atmospheric Administration's Chesapeake Bay Office in Annapolis.
"When it happens slowly, or in a small region very quickly, that's one thing," Wood said. "But in an event like a hurricane, it's very hard for those animals that don't move, such as shellfish."
Heavy rains could also flush nutrients and sediment into the bay - which won't be fully apparent until the spring, when they feed algae blooms that deplete the bay's oxygen.
Scientists believe that Agnes may also have killed up to two-thirds of the bay's underwater grasses, which provide a critical habitat for fish and shellfish. Wood said he was worried about a repeat from Isabel.