Tropical storm Isaac reached hurricane status early Tuesday afternoon as it bore down on a shuttered southeastern Louisiana.
And while the storm is expected to make landfall Thursday night with maximum sustained winds of about 80 miles an hour, the storm's biggest threat is posed by water – from the storm surge initially, along the coast, and flooding inland.
For the first time since it was built following hurricane Katrina in 2005, a $1.1-billion surge barrier has been closed to help protect New Orleans from surge-related flooding. The barrier, some 26 feet high and 1.8 miles long, is designed to prevent a storm surge from rolling up a shipping canal the US Army Corps of Engineers built at the behest of Congress to cut the amount of time it takes for ships to travel between New Orleans and the Gulf.
In addition, following Katrina, the Corps of Engineers was given the job of repairing and upgrading the system of pumps that draw storm water out of New Orleans, just under half of which is below sea level. New Orleans Mayor Mitch Landrieu pronounced all of them ready for operation during Isaac's downpours.
Several parishes have ordered mandatory evacuations for communities on the coast. Isaac's category 1 rating, however, is too low to trigger mandatory evacuations for New Orleans. Even so, at a press briefing Tuesday, Mayor Landrieu strongly encouraged people who live in Orleans Parish outside the city's levee system to evacuate.
Meanwhile, in Washington, President Obama declared states of emergency for Louisiana and Mississippi, setting the stage for the Federal Emergency Management Agency to coordinate relief efforts and provide other forms of assistance to stricken areas.
As residents prepared for Isaac's arrival, summer rainstorms served as its warm-up act, setting up conditions familiar to anyone in the eastern US who endured hurricane Irene last year. There, the storm – at $15.6 billion the fifth most costly hurricane on record in the US – dumped its heavy rain onto already-saturated soil.
The portion of the Gulf coast expected to feel Isaac's effects most acutely has seen rainfall in July and August significantly above normal.
To an already saturated ground Isaac is expected to dump 7 to 14 inches of rain along a swath that stretches along most of Louisiana's coast and continues east to the western end of the Florida Panhandle.
Landrieu acknowledged the improvements to the city's pumping system. But, he added, expect flooded streets anyway.
"There are no pumps in the world that can keep the land dry when that much water is falling," he said.
Isaac, a large, lethargic system, is moving at about 10 miles an hour – and is expected to slow somewhat as it moves over land.
"We look at our forecast path out through Thursday morning, and the center might not be out of Louisiana yet," says Rick Knabb, director of the National Hurricane Center in Miami. Even when it leaves the state, the storm's large size, slow motion, and mighty load of moisture pose a significant flood hazard even after it moves farther along its projected track into Arkansas, Missouri, then up into Illinois and Indiana.
"And after the rains fall, river flooding can commence," Dr. Knabb warns, and the runoff from mountains and hills works its way into the region's rivers.
At the same time, however, Isaac's relatively short stint as a hurricane before landfall has gradually reduced storm-surge estimates to levels that give some of the lowest coastal areas "a fighting chance," in the words of one official in a coastal parish outside New Orleans.
The National Hurricane Center is forecasting a surge of from 6 to 12 feet along the coast from southeastern Louisiana to Mississippi and 4 to 8 feet along Alabama's coast, where Isaac's counterclockwise winds will drive the ocean up against the coast. Other portions of the Gulf coast will see a storm surge as well, although in lesser amounts.
One aspect of the surge parish officials say they will watch is the extent to which Isaac stirs up oil and oily sediment and moves it into coastal wetlands.