After making landfall on Friday night, Hurricane Harvey has since been downgraded to a tropical storm but it is still wreaking damage throughout southeast Texas. Massive flooding is being reported in the Houston area, thanks to two bands of rain that merged and strengthened on top of the city overnight. In the last 24 hours, Houston and nearby Galveston have received 24 inches of rain, and it looks like it’s not letting up any time soon.

“We’re really dealing with a disaster that’s just now beginning in terms of rainfall and flooding,” Patrick Burke, a lead forecaster with the National Weather Service, tells The Verge. Harvey was an incredibly powerful Category 4 storm when it first hit Texas, and though it weakened after making landfall, it’s still taking a long time for the storm to dissipate, according to Burke. Plus, it’s August, which means that there is not a lot of steering flow to move the system away from the city. All of this means Harvey is not leaving the area for a while and could continue to dump rain for at least the next three days.

In fact, the National Weather Service is forecasting an additional 15 to 20 inches of rain for the region over the next few days, Burke says, leading to the most amount of flooding that Houston and surrounding areas have ever seen. “We’ve actually never forecast rain amounts this high.” Burke says. “It has the potential to break the record for any hurricane in Texas. We don’t know how high the water is going to get.”

“Catastrophic flooding in the Houston metropolitan area is expected to worsen and could become historic in association with Harvey, with potentially significant flooding also expected in other saturated areas of southeast TX,” according to a statement by the National Weather service.

Complicating things is Houston’s system of bayous and waterways. These waters are useful for draining rain waters during normal storms, but they aren’t regulated by dams or other human-built structures, so there’s no way to control the waters when they rise and overflow. It makes the flat area of southeast Texas prone to flooding during intense storms. “The terrain is so flat that the water is going to rise readily,” says Burke. “It can sometimes take two to three weeks for it all to subside.”

There was no evacuation order for the city of Houston and residents are being told to shelter in place. The 911 call centers in Houston are operational but are at capacity, according to the City of Houston’s Office of Emergency Management. People are being asked not to call 911 unless they are in imminent danger.

People should not drive in flood waters, and FEMA suggests people try to get to higher ground if possible. “If you are in a high-rise building and need to shelter in place, go to the first or second floor hallways or interior rooms. You want to stay on floors above floodwater or storm surge, but do not go to the highest floors due to wind impacts,” FEMA says on its website. However, residents should try to avoid going to the attic unless they have a way to get onto the roof.