(CNN) -- Authorities were warning people in a handful of towns outside of New Orleans to get out early Thursday ahead of a storm surge brought on by hurricane-turned-tropical storm Isaac as it dumped more than 20 inches of rain in some areas.
The warnings for residents in Washington Parish followed news that the Bogue Chitto River was expected to rise by 14 feet overnight, while National Guard troops looked for people stranded in St. John Parish after thousands were forced to flee when the surge forced water over the banks of Lake Pontchartrain.
"What we're doing is we have got law enforcement and fire personnel who are going door to door to notify people," Tommy Thiebaud, the Washington Parish director of emergency services, told CNN early Thursday.
The eye of the storm was 55 miles southeast of Alexandria, Louisiana, early Thursday, moving slowly at about 8 mph with maximum sustained winds of 45 mph, the National Weather Service said.
Issac is believed to have spawned three tornadoes overnight in Mississippi and Alabama, the weather service said.
Tornadoes are suspected to be behind the damage in Gulfport and Jackson, while a twister in Geneva, Alabama, was believed to have knocked down power lines and damaged a home.
President Barack Obama signed major disaster declarations for Louisiana and Mississippi after slow-moving Isaac pummeled the Gulf Coast, dropping more than 20 inches in some locations and creating a dangerous storm surge.
Some 840,000 people in Louisiana, Mississippi, Arkansas, Texas and Alabama were without power, and water boiling advisories were being issued in a number of towns and cities along the Gulf Coast.
Forecasters warned "life threatening hazards from storm surge and inland flooding are still occurring," the weather service said.
Isaac made landfall as a Category 1 hurricane, seven years to the day Hurricane Katrina swept ashore in Louisiana and Mississippi. Katrina killed 1,800 people, most in New Orleans after the levee system failed and the city flooded.
Isaac's greatest punch bypassed New Orleans, taking aim at surrounding areas.
National Guard troops and authorities searched St. John Parish for people possibly trapped by up to six feet of water.
By late Wednesday, about 1,500 people were evacuated from the parish, and another 1,500 were expected to leave their homes, Gov. Bobby Jindal's office said.
Dozens of buses were used to move residents out of flooded portions of the parish, while authorities worked to rescue others trapped by up to six feet of water.
"We're continuing to rescue people from different areas throughout the parish," Paige Falgoust, communications director for St. John Parish, said early Thursday.
"Our main focus right now is getting people out of their homes."
Dusk-to-dawn curfews were in effect from New Orleans to Baton Rouge, where authorities were urging residents to stay inside and ride out the slow moving storm.
The situation was particularly dire in Plaquemines Parish, southeast of New Orleans, where 3,000 people remained in one area close to an 8-foot tall levee that waters are threatening, Jindal's office said.
An initial estimate by local parish officials showed as many as 800 homes may have received significant water damage, Jindal said earlier.
The U.S. Army Corps of Engineers reported significant storm surge in the parish, scene of many rescues. One involved National Guard troops who moved 112 residents from the Riverbend nursing home to another facility.
Dozens of Louisiana families that had ignored mandatory evacuation orders in a low-lying area retreated to their attics and roofs and sought rescue amid the howling wind and pounding rain.
Officials were considering intentionally breaching the levee downstream to allow some of the floodwater to flow back out of the inundated area, Jindal said.
That could happen as early as Saturday, said Billy Nungesser, the parish president. A report, though, in The Times-Picayune said digging could begin as early as Thursday.
"We are still looking for stranded residents," Nungesser said Wednesday night.
"We will resume a double check tomorrow on the homes on the east bank (of the Mississippi River). We're checking the west bank for anyone who may have been trapped."
Airports from Baton Rogue to Biloxi were closed until at least Thursday as the stubbornly, slow moving storm continued to saturate the region.
In Mississippi's Hancock County, near the Louisiana border, National Guard troops rescued dozens of people stranded by the storm surge on Wednesday.
Joey Amann thought he and his family would be safe after Isaac made landfall until he saw the fast rising water.
At that point, he knew he had to get his family out of their home, Amann told CNN affiliate WLOX.
That's when he called for help, and the National Guard responded.
While New Orleans reported relatively minor damage from the storm, officials said there were at least 12 incidents of looting in the city. New Orleans Police said arrests were made in each case, but didn't specify how many people were involved or where the arrests occurred.
St. John Parish was virtually cut off Wednesday after Isaac's torrential rain and storm surge made nearly all roads in and out impassable, she said.
Parish officials were cross-matching records to make sure no residents were overlooked in the haste to get people out, Falgoust said.
The parish, with a population of about 32,000, was spared Katrina's devastation.
This time it wasn't so lucky.
The storm surge from Lake Ponchartrain came quickly and "in a different way from what we were expecting," Falgoust said.
The surge was unusually bad in LaPlace, about 25 miles northwest of New Orleans, where many people had been rescued or still needed to escape rapidly rising water.
"We have established pickup points in certain subdivisions that are easy access for our residents to get to by foot," she said.
People were being taken to a processing center at a church then bused to state shelters outside the parish.
"In some areas the water levels rose in 10 minutes to where they could not get out of their homes," she said.
CNN's Leslie Tripp, Brian Todd, Soledad O'Brien, Ed Lavandera, Martin Savidge, John Zarrella, Chandler Friedman, Anika Chin, Mike Ahlers, Aaron Cooper and Ed Payne contributed to this report.