"High wind, a lot of rain and it's going slow," New Orleans Mayor Mitch Landrieu said in describing the system. "That's not a good prescription for New Orleans."
The system became a tropical depression Thursday evening, and forecasters issued tropical storm warnings from Mississippi to Texas.
"We've got a huge area of moisture, we've got a developing wind field ... we're probably going to see some tremendous rain amounts and the corresponding flooding that goes with that," NHC Director Bill Read told reporters in Miami.
Tidal flooding is possible along the coast from the Florida Panhandle to Texas, Read said.
Some computer models showed the developing system, which would be called Lee if it became a tropical storm, could pass over the coasts of Louisiana, Mississippi and Texas. Lee will be the 12th named storm of the busy 2011 Atlantic hurricane season.
Flash flood watches were issued for parts of southern Louisiana and Mississippi. Up to 20 inches of rain could fall in some area and tides could be one to two feet above normal, forecasters said.
Cities facing the greatest threat from the system include New Orleans and Mobile, Ala., The Weather Channel forecast online.
New Orleans' levees could be tested by the slow-moving system, forecast service Accuweather.com said Thursday.
"It's not just the rainfall, but perhaps days of pressure on levees, as storm surge water could be driven into Lake Pontchartrain," Accuweather.com Meteorologist Mark Mancuso said in a statement.
Officials in the New Orleans ordered that levees be pumped down to lower levels in anticipation of heavy rain, the Times-Picayune reported.
The Army Corps of Engineers said it did not expect the system to trigger much of a storm surge but was preparing for that just in case.
Landrieu echoed that, but added that the system is "very unpredictable and uncertain."
Texas, which is in the grip of a severe drought, isn't likely to get much rain from the system, Accuweather.com said.
The system, now over the central part of the gulf, already has prompted some major international oil companies to evacuate workers from offshore oil platforms.
The Miami-based hurricane center said the low pressure area was producing a large area of clouds, thunderstorms and gusty winds as it headed slowly to the northwest.
"This system has a high chance ... 80 percent ... of becoming a tropical cyclone during the next 48 hours ... Interests along the entire northern Gulf of Mexico coast should monitor the progress of this disturbance," the NHC said Thursday afternoon.
"Considering potential for damage, impact to the petroleum industry and commerce in the Gulf Coast region," Accuweather.com stated, the system "could be the next billion-dollar disaster in a mountainous year of costly storms for the U.S."
Meanwhile, still far east out over the Atlantic, Hurricane Katia formed late Wednesday and was churning west but posing no immediate threat to land.
The Atlantic hurricane season typically brings 11 or 12 named storms. Katia is already the 11th and with half of the season still ahead, it is shaping up to be the unusually busy year that was predicted.
reuters