A storm system moving slowly through southern Illinois since March 17 has produced rainfall totals that have already exceeded 8 inches in some places south of Interstate 70, according to State Climatologist Jim Angel of the Illinois State Water Survey (http://www.sws.uiuc.edu), a division of the Illinois Department of Natural Resources.
The two largest observed totals come from Anna with 11.64 inches and Carbondale with 8.05 inches. Radar-estimated precipitation indicates a similar range in totals in this region.
Generally, rainfall totals of more than 8.8 inches in 48 hours have exceeded the 100-year design storm for southern Illinois. The measured amount of 11.64 inches in two days at Anna exceeded the threshold; radar estimates suggest that some other areas around Anna and Mt. Vernon may have exceeded it as well.
"What this means is that these events are exceptionally rare with less than a one percent chance of occurring at a specific location in any given year," says Angel.
Rainfall totals in other parts of the southern third of Illinois ranged from three to seven inches. As a result, the National Weather Service has issued a flood warning throughout the area.
"Our two largest measured precipitation totals, one at Anna and the other at Carbondale, came from our new volunteer-driven Community Collaborative Rain, Hail, and Snow network (CoCoRaHS). You cannot have enough rainfall observations during events like this. These volunteers, using standard equipment and standard observing practices, are a valuable addition to our existing observation networks. We are always looking for new observers," concludes Angel.
To find out more about this program and to look at rainfall maps and tables, visit http://www.cocorahs.org/.
