CHEHALIS, Wash. - The heavy rains fell everywhere.

AP

A closed Interstate 5 is shown running directly into the flooding Chehalis River near Centralia, Wash., earlier this week.

So why was Lewis County hit so hard by flooding?

Federal experts say a freak of nature may be to blame. They emphasize they're still investigating, but they say they're zeroing in on a small but high-intensity rain event that fueled the Chehalis River.They also can't rule out some sort of "x" factor -- an unexplained event that accelerated the flood waters.

The mild-mannered Chehalis River was transformed earlier this week.

"It's a small river and it turns into a monster," said Larry Schick, of the Army Corps of Engineers.

Schick says one gauge recorded a record river flow of 385,000 gallons, the equivalent of 120 dump trucks per second.

"This is an extraordinarily rare event," Schick said. "A huge event. Double the previous record."

The river rose nearly 30 feet, apparently destroying the gauge near Doty and depositing logs on Highway 6.

Experts say a mini super storm parked itself over the area where the Chehalis forms, with an intensity perhaps unprecedented in lower elevations.

The rain pounded the area of southwest Lewis County known as the Willapa Hills -- the headwaters of the Chehalis River. Nearly 9 inches fell there in one 24-hour period. By comparison, just a third of that amount fell on Centralia.

The rainfall gorged the Chehalis, with the water rising so fast many people were certain that levies broke.

The Corps of Engineers says it's still investigating, but has not found evidence that any of its four river levies were breached.