Southern California woke up Thursday to emergency alerts as a second day of intense rain soaked the West Coast, triggering flash flood warnings.

Mandatory evacuation orders are in effect as local officials scrambled to clear 50,000 truckloads mud and sediment on slopes to stave off potentially deadly mudslides ahead of the storm’s next wave of rain, the LA Times reports. Mudslides in the region have already forced freeways to close.

Upward of 9 inches of rainfall was reported in parts of the region, and the downpour will likely continue through Thursday night.

“The storm is living up to the promise,” David Sweet, meteorologist with the National Weather Service in Oxnard, California, told the Los Angeles Times.

All this rainfall is due to a channel of tropical moisture starting in the South Pacific and barreling toward California called the Pineapple Express (because these systems originate over Hawaii). It’s an example of what’s known as an atmospheric river.

But don’t let the bromantic comedy with the same name fool you: The impacts can be devastating.

The water vapor image shows the "atmospheric river" of tropical moisture heading to CA.#cawx #LArain #CAstorm pic.twitter.com/ECbOZ95zYA — NWS Los Angeles (@NWSLosAngeles) March 20, 2018

The Pineapple Express forms when plumes of moisture in the Pacific glom onto larger storm systems. Sometimes years go by without one, while other times several can form in a year, channeling a huge volume of water toward the United States.

“In an instantaneous sense, they’re moving more water than the Mississippi River, in the sky, above your head,” said Daniel Swain, a climate scientist at the University of California Los Angeles. “It’s a tremendous volume of water already under normal conditions.”

The storm is creating some nifty satellite images.

This really is a spectacular, photogenic storm. #AtmosphericRiver becoming entrained in warm sector of deepening cut-off low to the west, exhibiting "textbook" #PineappleExpress structure. Can't take my eyes off the satellite imagery. #CAwx #CAwater pic.twitter.com/cgCpu82Neg — Daniel Swain (@Weather_West) March 20, 2018

The National Weather Service is showing this Pineapple Express is hitting San Luis Obispo County the hardest. It’s put out flash flood warnings for many other communities in the region too:

Radar estimated rainfall for this rainfall event through 7AM this morning. The Rocky Butte rain gauge in the NW corner of SLO County has measured over 9" so far. #CAwx #CAstorm #AtmosphericRiver #LArain #LAweather #SLOweather #SoCal pic.twitter.com/FAq8GG7vdb — NWS Los Angeles (@NWSLosAngeles) March 22, 2018

The rainfall may be a welcome relief to parts of California after years of drought, but the land may not be able to handle all the water. Compare the above rainfall forecast to this map of wildfires in Southern California late last year:

All that rain falling on the 282,000 acres scorched by the Thomas Fire, the largest wildfire in California history, in California’s worst fire season ever, has already caused mudslides and could lead to more. These earthen cascades have already killed 21 people in California this year.

#CAstorm- Montecito Creek flows Wednesday morning March 21 under East Valley Rd. in Montecito. Nearby are homes that were destroyed from the deadly Jan 9 debris flow. pic.twitter.com/yNJNYNLy8R — SBCFireInfo (@EliasonMike) March 21, 2018

As a precaution, thousands have already been ordered to leave their homes Santa Barbara, Ventura, and Los Angeles counties.

Researchers expect that atmospheric rivers will grow more intense as average temperatures go up, allowing more moisture to evaporate and reach the sky.

“There is now emerging evidence that these atmospheric rivers, at least along the West Coast of the US, have become more intense over the last few decades, and there’s certainly an expectation that they will become more intense as the climate continues to warm,” Swain said.