Isaac weakens but drenches La., Miss. as possible dam failure threatens thousands

Crews work to try to prevent a dam from failing at Lake Tangipahoa in Mississippi's Percy Quinn State Park Aug. 30, 2012.

Updated at 3:03 p.m. ET

(CBS/AP) NEW ORLEANS - Tropical Storm Isaac continued to pound Louisiana and Mississippi Thursday as a dam in a sparsely populated area near the Louisiana-Mississippi border threatened to break, prompting officials to order a mandatory evacuation for as many as 60,000 people.

Louisiana Gov. Bobby Jindal said at an afternoon news conference that the dam was damaged at Lake Tangipahoa in Mississippi's Percy Quinn State Park. He said officials there would release water at the dam.

Mayor Whitney Rawlings of McComb, Miss., which is north of the park, told CBS News that there was a "50-50 chance" of the dam failing. He urged people south of the dam to evacuate.

"People need to be moving," Rawlings told CBS News.

In Louisiana, Tangipahoa Parish President Gordon Burgess issued an emergency alert warning of an "imminent failure" at the dam. Burgess said between 50,000-60,000 people had 90 minutes to evacuate around 11 a.m. ET, CBS New Orleans affiliate WWL-TV reports.

The National Weather Service warned that a break in the dam would raise the level of the already swollen Tangipahoa River from around 11 feet to 17 feet in Kentwood, La., which is near the state line. Jindal called for an evacuation of Kentwood after he and Burgess flew over Tangipahoa Parish in a helicopter, WWL-TV reports.

Even as Isaac weakened on its slow trek inland, it continued to spin off life-threatening weather including storm surges, inland flooding from torrential rain and potential tornadoes. By mid-morning Thursday, Louisiana's Public Service Commission said 903,000 homes and businesses around the state -- about 47 percent of all customers -- are without power.

Isaac pummels areas around New Orleans
Oil falls as Isaac heads inland
Escaping the floods in Plaquemines Parish

In the second reported death tied to Isaac, a tow truck driver was killed Thursday morning when a tree fell on his truck in Picayune, Miss., just across the state line from Louisiana. Authorities said Isaac was causing heavy rain and strong winds at the time. Pearl River County Coroner Derek Turnage identified the victim as 62-year-old Gregory Alan Parker of Picayune.

In Slidell, La., just northeast of New Orleans across Lake Ponchartrain, a flash flood emergency was declared Thursday morning, and officials called for evacuations in several neighborhoods, WWL-TV reports.

A canal near Slidell that helps Bayou Bonfouca drain into Lake Pontchartrain was being hit with a strong wind from the south, forcing water to pour into southern portions of the city, WWL-TV reports. Slidell Mayor Freddie Drennan told WWL-TV Thursday morning that water levels had reached 4 feet high in some neighborhoods.

"The real problem now is nobody can tell us how much water we're going to get," Drennan said.

Rescuers patrolled through Slidell's neighborhoods in heavy-duty trucks meant for driving through flooded areas, offering rides to people where water levels reached their waists in some areas and were above cars in others, WWL-TV reports.

Early Thursday, a Coast Guard helicopter hoisted a couple and their dogs from a home in LaPlace, near the lake, after storm surge poured into their neighborhood and local authorities called for help. The couple was taken to New Orleans and reported in good condition.

"The husband and wife and their two dogs were in an area where a lot of houses washed away," said Lt. Cmdr. Jorge Porto. "They used a flashlight inside the house as a signaling device, which made all the difference in locating them effectively."

The floodwaters "were shockingly fast-rising, from what I understand from talking to people," Lt. Gov. Jay Dardenne said. "It caught everybody by surprise."

Isaac's maximum sustained winds had decreased to 40 mph and the National Hurricane Center said it was expected to become a tropical depression by Thursday night, meaning its top sustained winds would drop below 39 mph. The storm's center was on track to cross Arkansas on Friday and southern Missouri on Friday night, spreading rain as it goes.

The hurricane center said in an advisory at 2 p.m. ET that Isaac was about 25 miles southwest of Monroe, La., and was heading north-northwest at 9 mph.

CBS News hurricane consultant David Bernard reports from CBS Miami station WFOR-TV that Isaac's final rainfall totals for southern Louisiana could be 18-30 inches. Bernard reports that the storm's feeder bands still posed a flooding threat for the Louisiana capital of Baton Rouge, north of New Orleans.

With water still trapped between two floodwalls in Plaquemines Parish, a sparsely populated area south of New Orleans that is outside the federal levee system, officials are considering cutting 10-15-feet-wide holes in a levee to let the standing water drain back into the Mississippi River and marshland, CBS News correspondent Byron Pitts reports from New Orleans. Jindal said that intentional breach would happen between 3 and 4 p.m. ET.

At left, watch video of a levee in Plaquemines Parish being overtopped as filmed by storm chaser Jeff Piotrowski

The rain fell almost constantly for more than a day, flooding neighborhoods in a rural part of the state and in neighboring Mississippi. Officials had to respond quickly because the waters were rising fast — even as Isaac meandered slowly northward Thursday on a path toward Arkansas.

Along the shores of Lake Ponchartrain, officials sent scores of buses and dozens of high-water vehicles to help evacuate about 3,000 people as rising waters lapped against houses and left cars stranded. Floodwaters rose waist-high in some neighborhoods, and the Louisiana National Guard was working with sheriff's deputies to rescue people stranded in their homes.