A rapidly forming thunderstorm inundated Natrona County with as much as 1.5 inches of rain in 40 minutes early Friday evening, according to the National Weather Service in Riverton.

The heavy rains sometimes reduced visibility to a few yards, and flooded streets and underpasses.

About 1.5 inches - 1 inch in 13 minutes - reportedly fell in Mills, about 1 inch fell at the Casper-Natrona County International Airport, and 0.9 inches fell at a residence in the 1700 block of South Wilson Street, said Todd Baker, a meteorologist with the National Weather Service.

Such storms are typical of the monsoonal moisture passing through Wyoming usually later in July, Baker said.

But Friday's tempest was a bit different, he said. "That developed on top of the airport in a matter of minutes."

A similar storm may form this afternoon and Sunday, Baker said.

Because the ground is saturated, the consequences of more thunderstorms could be worse than Friday, he said.

"Any more rain that falls isn't going to get absorbed, so it will flood," he said. "It depends on how efficiently the storm sewer system can disperse the water."

A storm sewer on N. Lincoln St. was dispersing water - straight up.

Scott Laird was watching the storm from his house when the pressure blew a sewer lid, he said. "It shot about 15 feet in the air."

That storm sewer water, joined by streams along streets in east central Casper, inundated East Yellowstone Highway with up to a foot. Four teenagers pushed their non-working vehicle through a large puddle.

C Street west of N. Lincoln St. was in even worse shape with water rising three or four feet, enough to float cars. Some people cruised the area in boats.

The Wyoming Department of Transportation Web camera overlooking the Bryan Stock Trail overpass showed Interstate 25 flooded beneath the nearby railroad bridge.

WYDOT also announced I-25 was closed between Casper and Glenrock because of the flooding.

At the North McKinley Street underpass, a Casper police officer was warning passers-by to not walk into the water because they could step into an open storm sewer and be swept away.

At the Natrona County Fairgrounds, the carnival rides slated to open Friday evening were shut down because of the storm, said marketing coordinator Angela Berry.

"We have three feet of water in the Midway, and it's draining into the fairgrounds," Berry said.

Law enforcement officials reported no injuries; however, Casper police received calls from people trapped in stranded vehicles.

Reach Tom Morton at (307) 266-0592, or at tom.morton@trib.com. Read his blog at tribtown.trib.com/TomMorton/blog.