In a way, it was the perfect flood.
Shifts in Isabel's winds, a timely high tide and something called the "slosh" effect all conspired early yesterday to produce some of the highest water levels ever recorded in communities around the Chesapeake Bay.
The bay flooding was almost entirely the result of tidal phenomena, and not the unexpectedly modest rain that accompanied the storm in Maryland.
Meteorologists said the high water broke or tied 70-year-old records in Baltimore, Annapolis and Washington, as it inundated streets along the Inner Harbor and Fells Point, communities in Eastern Baltimore County and towns on the Eastern Shore.
"It was just the right conditions, timing-wise, of the tide, the surge and winds changing around right at the right time, to push all that water in there," said meteorologist Steve Zubrick, of the National Weather Service's Sterling, Va., forecast office.
Many hurricanes and storms that affect the Chesapeake pass to the east of the bay, and their counter-clockwise circulation usually means that winds drive water out of the bay. But Isabel tracked to the west of the bay, and its winds were fated to push the water northward, filling the tidal creeks and rivers like overflowing bathtubs.
Forecasters had warned emergency management officials on Tuesday that the approaching storm would produce tidal surges along the western shore of the Chesapeake.
"We were briefing people that even a Category 1 or 2 hurricane taking the right track can give you pretty high water levels," Zubrick said. "It doesn't have to be a real spinner."
But the size of the mound of water Isabel's winds actually piled up - 2 feet higher than predicted - surprised everyone.
"It's not an exact science," said Barbara Watson, who is the warning coordinator at Sterling.
"We're certainly further along than we were in 1954 when [Hurricane Hazel] came along," she said. But the Chesapeake Bay is such a complex natural system, and there have been so few storms like Isabel to study, that "it's difficult to come up with the forecasts." Given that difficulty, she said, a two-foot error is "a pretty good forecast."
The high tide in Baltimore was supposed to occur at 2:07 a.m. But that hour came and went, Zubrick said, and the water "was just going up and up."
It didn't stop rising until 8:06 a.m., according to Lt. Kevin Slover, of the National Hurricane Center's storm surge unit. By that time it stood at 8.15 feet above mean low water, or 7 feet above normal.
That topped the previous figure of 7.99 feet above mean low water, recorded during the unnamed hurricane that struck the bay on Aug. 23, 1933, Slover said.
Meanwhile, the tidal gauge at Wisconsin Avenue in Washington showed the Potomac River reached 11.3 feet above mean low water, Zubrick said. That beat the 11.1-foot mark set by the 1933 hurricane.
Annapolis reached 7.2 feet above mean low water, tying the 1933 record.
Newspaper accounts of the 1933 storm - the same one that cut the inlet at Ocean City - will seem eerily similar to accounts of Isabel's flood in today's newspaper.
"[Baltimore's] main waterfront thoroughfare, Light street, was in many places waist-deep in waters that came surging up over wharves and piers," The Sun reported the morning after the 1933 flood.
"Shore cottages at Dundalk were reported afloat. ... Fifty families living at Boley's Quarters grove on Eastern Avenue near Middle River bridge were forced to go into the second story of their homes by rising water. In some of the dwellings the water was six feet deep."
Isabel's surge was not unexpected. Hurricane statements issued Tuesday by the weather service as Isabel approached the North Carolina coast warned of an impending storm surge and tidal flooding.
"Along the western shore of the Chesapeake Bay, tidal surges of 3 to 5 feet are possible," the statement said. "This surge would be on top of the normal tide, and then waves of 4 to 5 feet may occur on top of that." Residents on the tidal Potomac were told to expect surges as high as 6 feet above normal tides.
The predictions were based on calculations by the hurricane center's "Slosh Model." Slover said the complex computer program crunches data on wind speed, the location of a storm's highest winds, the topography of the landscape and the depth of the waters around it. Then it spits out forecasts on storm surges for the affected shoreline.
Here's what actually happened when Isabel came ashore Thursday, according to the National Weather Service and the National Hurricane Center:
As the storm slammed into the Outer Banks around noontime, the northern edge of its counter-clockwise circulation produced winds across the Chesapeake out of the northeast, and then the east. That drove the bay water westward.
By 9 p.m., far to the south and west of Baltimore, a weakening Isabel was heading north-northwest into Virginia. As it moved, tropical-storm-force winds over the Chesapeake shifted to the southeast.
