Louisiana governor warns that the state's historic flooding is 'not over' - here's why

Over the weekend, parts of Louisiana and Mississippi saw more than a foot - two feet, in some neighborhoods - of rain from a stalled storm. NBC News reported that more than 20,000 people have been rescued.

The worst of the rain is over now, but that doesn't mean the flooding is too.

"I'm still asking people to be patient. Don't get out and sightsee," Louisiana governor John Bel Edwards told the Associated Press. "Even when the weather is better, it's not safe."

There are a handful of factors coming together to keep the risks high over the next few days.

First, there are scattered pockets of rain in the forecast for the area this week. While an inch or two of rain usually wouldn't be a problem, with so much water already in and on the ground, there isn't anywhere for more to go.

A second factor is that floodwaters upstream are still making their way down toward the Gulf of Mexico. That's a huge amount of water, and there's a chance it won't all stay within a river's banks as it makes its way south.

According to the Weather Channel, at least six rivers in the area have hit record high levels - one river broke its previous record by as much as six feet. And parts of that same river, the Amite, weren't expected to hit their highest level until sometime today.

Moreover, the National Weather Service believes these will be "long crest" events, lasting potentially days, rather than lighter crests that fade quickly.

"It's not over," Edwards warned, according to NBC News. "The water's going to rise in many areas. It's no time to let the guard down."

Even after floodwaters begin to recede, there's a chance they'll rise again because of backwater flooding, which occurs upstream when something blocks the flow of water downstream. The National Weather Service warned backwater flooding was a risk along the streams that flow into the Amite River.

Taken together, these factors mean that the National Weather Service is concerned about flooding continuing "during at least the first half of the work week."

This flood is so unusual that experts aren't even sure precisely what will happen and where water will go. "It's unprecedented and it's not anything, anyone will know for sure," hydrologist Jeff Grashel told a local newspaper.

All of this is happening against the backdrop of climate change. According to one meteorologist's count, there have been at least eight 500-year rainstorms in the US within the past year - that means rainfalls so heavy they are only predicted to occur once every 500 years. Part of the reason this storm was strong is it fed off unusually warm water in the Gulf of Mexico.