WIUM Tristates Public Radio

A Combination of Factors Contributed to Deluge

May 5, 2015

The National Weather Service reported Macomb received 2.98 inches of rain in about two hours Monday evening. Streets flooded as the city's storm water system struggled to keep pace.

Portions of McDonough and Hancock Counties were under a severe thunderstorm warning for a while but other parts of the region did not get nearly as much rain.

Meteorologist Tom Williams of Western Illinois University said McDonough and Hancock were hit by a cluster of storms for a weather event sometimes referred to as “training” or “train echoes.”  He said it’s similar to a train pulling a series of coal cars, with each car being stopped in the same place to dump its load.

“Pretty soon you would have a pretty big pile of coal.  Well that’s it,” Dr. Williams said. 

“A thunderstorm dumps its rain. Another thunderstorm follows behind, following essentially the same path and dumps its rainfall. And that’s how we get in this situation.”

He said these storms were slow moving and had a lot of water with which to work. He said precipitable water was an inch-and-a-half.

“What that’s doing is measuring how much water is in a column of the atmosphere. And an inch-and-a-half is quite high,” Williams said, adding the storms were “efficient rainfall producers.”

Williams said it’s not unusual for thunderstorms to bring heavy rainfall.  And more rain appears to be on the way, especially toward the end of the week.  But it’s too soon to know how much precipitation to expect.

In addition to the storms, temperatures were in the 80s for the early part of the week.

“It’s an indication that we’ve changed over to warmer and more humid air,” Wiliams said.  “We have transitioned into a more summer-like weather pattern that has chances for showers and thunderstorms about every day.”