Louisiana declared a state of emergency on Friday. Why? The state is experiencing severe flooding that's left at least three people dead, caused more than 7,000 people to need rescuing from their homes, and sent waist-deep water across the state in a disaster described as "truly historic" and "unprecedented."

But the sobering news is that the weather now pummeling Louisiana is a sign that climate change has upended all expectations of what's unusual.

One Bossier City, Louisiana, resident described the flooding as looking like a "horror movie." Other state residents described it as the worst they've seen in their lives. Even Gov. John Bel Edwards and his wife, Donna, were forced to evacuate.

The National Weather Service warned of "significant to catastrophic flash flooding," adding: "Obviously, we are in record territory."

The National Weather Service description of this weekend's storm couldn't be more accurate.

Louisiana's downpour is being called a "classic signal of climate change" because of its extreme nature. And there's a good reason: In just 12 hours, the state saw as much rainfall as it should during a typical three-month hurricane season.

Looked at another way, some parts of Louisiana experienced a downpour of an intensity that should occur only once every 500 years.

This isn't good. And worse, it's not unique.

You can expect to see more of these kinds of superstorms as the planet warms, according to the National Climate Change Assessment, a study produced by more then 300 experts working with a 60-member Federal Advisory Committee.

We've already started to experience them. "By my count, it's the eighth 500-year flood to strike the USA since May of 2015," meteorologist Paul Douglas said with tongue in cheek. "Probably nothing to worry about."

He's right. The same type of extreme 500-year flooding has already been recorded at least eight times so far this year, in Maryland, South Carolina, Texas, Oklahoma, and Virginia.

Other parts of the country are experiencing severe weather of their own.

Unless you live under a rock, you probably know that California is undergoing a severe drought. The last five consecutive years have been the driest the state has ever seen, according to official climate records.

And it's not expected to get better anytime soon: Weather forecasters are predicting that the La Niña weather pattern will occur again this winter, which is supposed to bring cooler waters to the Pacific Ocean and drier weather to the state. So much for a wet winter.

The takeaway: The old standards of climate prediction are officially dead.

Meteorologist Eric Holthaus said that weather prediction is based on the ability of meteorologists to look at climate data and infer patterns over time. But because of our changing climate, that's no longer a sure bet, as extreme weather events that should be rare become more and more commonplace.

"Statistical calculations like these make a major assumption: That the climate of the past is the same as the climate of today," Holthaus wrote. "That's no longer a very good assumption."