Most of Louisiana has received at least one foot of rain since Friday — with some places getting as much as 30 inches, according to the National Weather Service. Although the water has receded in some areas, it's still rising in others as the floodwaters move downstream toward the Gulf of Mexico.
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The scale of the devastation was only starting to come to light.
"The current flooding in Louisiana is the worst natural disaster to strike the United States since Superstorm Sandy [in Oct. 2012]" said Brad Kieserman, the Red Cross's vice president of disaster services, operations and logistics.
He said in a statement Tuesday that the relief operation would cost at least $30 million — warning that the price tag "may grow as we learn more about the scope and magnitude of the devastation."
Grammy award-winning musician Taylor Swift said she was donating $1 million to flood relief because of the warm welcome she was given when kicking off her world tour in the state last year.
"The fact that so many people in Louisiana have been forced out of their own homes this week is heartbreaking," the 26-year-old performer said in a statement. "I encourage those who can to help out and send your love and prayers their way during this devastating time."
The Red Cross said volunteers from all 50 states, plus the District of Columbia and Puerto Rico, were helping thousands of residents get to a safe place, as well providing them with food and water.
Twenty parishes were under a federal disaster zone and more than a dozen were subject to overnight curfews. At least 10 people have been arrested for looting in East Baton Rouge parish, according to Sheriff Sid Gautreaux.
Gautreaux told a news conference Tuesday that there were "significant" power outages and some homes and businesses had experienced "total devastation."
Among the worst affected was Livingston Parish, where more than three quarters of all homes have already been "lost to floods," Lori Steele, a spokeswoman for the parish, told NBC News.
"We're devastated in Livingston," Livingston Sheriff Jason Dore told the news conference.
Floodwaters were slopping over the top of the the Laurel Ridge levee, which protects the parish in the Baton Rouge metropolitan area from the Amite River, according to the Ascension Parish Homeland Security Office.
A third of Ascension's 45,000 homes have been flooded — and waters there are expected to rise.
"The next 24 to 48 hours is going to be a significant indication of just how much risk the parish remains in," said Rick Webre, director of the Homeland Security Office.
Forecasters said the worst of the rain is likely over, but the southern part of the state is still expected to see some 2 inches more of rain through Friday, the NWS said.
Louisiana residents are struggling with how to cope amid the uncertainty.
Ascension Parish resident Nick Babbin had just bought his home in February, and was forced out by the floodwaters Saturday. He returned Tuesday to find it completely destroyed.
"I try to hold back as many tears as I can," Babbin told NBC affiliate WDSU.
Flood victim Samuel Ancar was evacuated Saturday from his Baton Rouge home with his 3-year-old daughter and mother in tow, he told NBC affiliate WVLA. He said he survived Hurricane Katrina in 2005, when it seemed like "the world was coming to an end."
This time around, he has lost all of his belongings — again, he said. But he is thankful his family is OK, and remains hopeful that they can rebound.
"Just leave it in God's hands and it will all work out," Ancar said.