Tropical Depression Bill has brought torrential rainfall totals to southern Oklahoma since Wednesday evening, prompting dangerous flash flooding and potentially a record-breaking river flood around the Arbuckle Mountains.

The storm, which is still hanging on to its tropical characteristics despite not having been over a warm ocean since Tuesday, slowed down on Wednesday evening as the center crossed from Texas into Oklahoma. In the nine hours between 7 p.m. Wednesday and 4 a.m. Thursday, Tropical Depression Bill tracked just 86 miles — crawling at an average speed of less than 10 mph.

Bill’s excruciatingly slow forward speed set up a dangerous rainfall event in southern Oklahoma along parts of Interstate 35, which were still flooded and closed on Thursday morning, according to the Oklahoma Department of Transportation.

Around 9 p.m. Wednesday, the National Weather Service issued an extreme rainfall forecast for southern Oklahoma, warning of widespread totals of four to six inches, with the possibility of as much as 12 inches in isolated areas south of Oklahoma City.

At the time of the forecast, the Oklahoma Mesonet was already recording relentless rainfall rates in the Arbuckle Mountains in Davis, Okla., where winds from the east were riding up over the mountain range and wringing out all of the moisture over locations around Ardmore, Healdton and Newport. At this point, rainfall rates were as much as five inches in just two hours.

By Thursday morning, the rainfall totals south of Oklahoma City had exceeded a foot, with widespread totals over five inches that spread northeast toward Tulsa. As of 5 a.m. Central time, with more rain on the way, Newport had seen an astonishing 13.07 inches, Healdton was at 12.53 inches, and Burneyville had reported 10.09 inches.

The result of Bill’s inundation will be more than dangerous flash flooding. With the ground saturated to the max, and most reservoirs at or well-beyond average capacity, this rainfall has nowhere to go but into rivers and streams that will bulge far beyond their banks. This infrastructure-crippling river flooding often doesn’t peak until after the rain has stopped, especially downstream.

That will be the case for the Washita River, which runs through the Arbuckle Mountains, in locations downstream of where the heaviest rains have fallen over the past 12 to 24 hours. As of about 8:30 a.m., the National Weather Service was forecasting the Washita River near Dickson, Okla., to crest nearly two feet above its previous record, and a full 20 feet above flood stage on Wednesday evening.

The National Weather Service says that “catastrophic” flooding at this location begins around a river height of 42 feet. Here’s what they say about 47 feet:

Expect catastrophic flood depths of 22 feet or more to wreak havoc in the Washita River valley … from Murray County … to near Dickson in Carter County … to the shores of Lake Texoma. Many homesteads are flooded and may be cut off for days. Over 10 thousand acres of agricultural lands are covered. Major highways and railroads are threatened. Flood levels are nearly 2 feet higher than the flood crest of May 30 … 1987.

Ardmore, Okla., saw particularly devastating flooding on Wednesday evening as the rain came down in buckets, flooding roads and parking lots. Storm chaser Cody Robben captured some stunning visuals from Ardmore after cars were abandoned in high water, and tanker trailers were left stranded.

The tropical depression will inch slowly to the northeast on Wednesday, pushing the threat of excessive rainfall into eastern Oklahoma, northwest Arkansas and southwest Missouri. Widespread totals of four to eight inches are expected across these areas, though isolated amounts of 10 to 12 inches are possible where the heaviest storms set up.

Through Saturday, Bill will be moving into the Ohio River Valley and potentially interacting with a frontal boundary that’s expected to sag south across the Eastern U.S., which could provide a focal point for more heavy rainfall and thunderstorms.