Over the last few weeks, storms slamming California have delivered torrential rain, extensive flooding, and a near-disaster at one of state’s largest reservoirs. But the atmospheric river superstorm sweeping across the state today could be the most dangerous this season, if not in decades. Here’s why.

The storm hitting Northern California is a particularly wet, slow-moving system in a string of atmospheric river events, which funnel moisture from the tropical Pacific towards California. These weather systems have saturated the ground and pushed stream flows to historic limits.

While it’s just drizzling in Southern California, many of the rivers that could flood in the northern half of the state today are the ones that supply Los Angeles with water, thanks to our extensive aqueduct system. The San Joaquin and Sacramento Rivers, for example, are the main tributaries of the California State Water Project, the hub which provides drinking water to 23 million of California’s 39 million people.

Many mainstem rivers across #NorCal expected to be in flood or danger stage in the upcoming days. #CAStorm #CAFlood #cawx pic.twitter.com/pebywRwYje — NWS Sacramento (@NWSSacramento) February 19, 2017

So it may not rain all that much at your house in LA today, but the storms that are inundating the northern half of California right now and into tomorrow could have severe impacts that will push the state’s water system to its limits. And—the rainy season is not over yet. The risk could go up again if another massive atmospheric river event strikes in the coming weeks.

What’s of greatest concern today is the potential breach of the California Delta levee system, which would contaminate the state’s freshwater supply with flooding sea water. This is what’s called saltwater intrusion, and it’s the worst-case scenario for the state’s water supply, according to meteorologist Eric Holthaus:

The implications of this flood would be huge: If the levee system is breached, Sacramento could have 30 feet of flooding, and much of the state's water delivery system could be paralyzed by an influx of saltwater, including much of southern California. Two-thirds of people in the state could lose fresh water. That's not to mention the potential loss of life. Of course, this is not a given based on the latest weather forecast—but the fact that it can't be ruled out should cause everyone in the region to pay close attention.

To prepare the state for this kind of flooding event, California scientists created a report called the ARkStorm Scenario, a fictional atmospheric river-powered flood based on the actual events of a winter storm event that happened 155 years ago:

From December 1861 to January 1862, it rained for 45 straight days in California. The Great Flood of 1862 wrought so much destruction that some parts of the state remained underwater six months later. The center of California turned into a 300-mile sea. Finance-wise, the storms “bankrupted the state, destroyed the ranching industry, drowned 200,000 head of cattle [and] changed California from a ranching economy to a farming economy” in essentially one stroke, seismologist Lucy Jones told NPR in 2013.

Even if the state got 45 days of rain in 2017, what happened in 1862 is unlikely to happen again today, thanks to a century and a half of building infrastructure to protect California’s cities. But now we have climate change.

What the information from the ARkStorm Scenario can do is take the weather data from the 1862 flood and reimagine that event in light of current climate trends. A hotter planet means warmer, more moisture-saturated air, which climatologists say means atmospheric river events such as this will be the norm in the future.

California’s infrastructure was built under the assumption that events like this were more rare, and they might not be able to withstand the new normal. That would lead to more disasters like what almost happened at Lake Oroville, for example, where the water from the over-capacity reservoir damaged two spillways, nearly causing the dam’s failure last week.

That’s the reason why the climate research being performed right here in the state is more valuable than ever: USGS, NOAA, and NASA are working together to bring real-time updates on the storm and how it will impact dams, reservoirs, and aqueducts. This helps California officials make quick decisions about how to move water around the state.

But for a storm like this one, it’s more than just making sure that California’s residents will have uninterrupted access to a clean, safe water supply. Today, that science will more than likely save lives.