Fast Facts

• After one round of early morning storms brought isolated wind damage to parts of far northern Illinois, scattered severe storms redeveloped late in the afternoon and into the evening on July 13th. 

• Large hail was the most common severe weather phenomenon in the area, with dozens of reports of golf ball size or larger received by the NWS.  The largest hail stone reported was in Marseilles, IL in LaSalle County, and was estimated to be 3.75" in diameter. where at least damage to some vehicles occurred.  This happened around 5:50 pm.  A Severe Thunderstorm Warning had been issued for central LaSalle County at 4:59 pm through 6:00 pm, with a new warning at 5:37 pm noting Marseilles.

• Many of the storms were supercells meaning they were well-organized and rotating.  This rotation was observed by trained storm spotters with several of the storms and for long durations, including wall clouds and funnel clouds.  Their reports were especially helpful to the warning process.  For more on why only one tornado formed, please see the Meteorology & Radar section.

• The possibility of severe weather on July 13th had been mentioned in NWS forecast products since the morning of July 10th.

Reports

• Text Listing of NWS Chicago Local Storm Reports (pdf)
• U.S. Storm Report Listing

Hail

Hail Safety Reminder

The National Weather Service received many photos of the large hail on July 13th and these visual reports assisted us with warning messages to better stress the dangerous nature of the storms, as well as verification efforts after. Thank you!

Meteorology & Radar

The primary storm mode during the mid-afternoon through late evening on July 13th was supercellular, that is the storm by definition had a rotating updraft.  Rotating updrafts tend to be stronger and result in a tilting of the updraft, which keeps the storm better organized and is more supportive of a longer lifecycle, and thus increased chances for severe weather.  

Many of the storms on July 13th had rotation in the lower levels that was visible from the ground by trained storm spotters.  However, only one of storms in the NWS Chicago County Warning Area produced a tornado despite relatively favorable, broad (synoptic) scale parameters for tornadoes such as moderate shear, very high instability, and at least somewhat of a defined focus at first (old outflow boundary).  The lack of tornadogenesis was thought to be due to a few variables on July 13th, some of which may have been more of a key factor than others.  These deterrents were hypothesized to be:

• Lack of much low-level flow and thus helicity (ability for storm inflow air to rotate).
• The high surface dew points were only in the immediate surface layer, with a drier wedge of air immediately above this.  This was reflected on the 7 pm NWS Lincoln, IL weather balloon launch from that evening.
• A lack of well-defined or consistent rear-flank downdrafts in the persistent supercells, which may have been a result of some of the aforementioned factors.    

NWS Chicago Doppler Radar 0.5° Base Reflectivity Loop:  5:15 pm - 10:30 pm

Other Links

• Severe Storms in Eastern Iowa and Northwest Illinois - NWS Quad Cities
• Heavy Rain, Flooding, and Two Tornadoes in Central Indiana - NWS Indianapolis, IN
• Event Archive Page for July 13 - NWS Storm Prediction Center
• NWS Chicago Science & Past Events Page