Precipitation Distribution for July 2011

This July was the wettest on record for Chicago (O’Hare International AP); along with the region that encompasses Dubuque, IA to Freeport, IL. At O’Hare, 75% of July’s rainfall occurred within a two day span (22 July-23 July). Before those dates, the last significant rainfall that had been recorded at O’Hare was 11 July, 2011; when 0.39” of rainfall was recorded. By comparison, the station at Galena, IL received 13.45” of rain in a 24 hour span on 28 July, 2011. That same mesoscale convective complex did indeed affect areas further east, but was not as efficient upon reaching Chicago O’Hare. The Galena area had an initial complex come through, and the outflow boundary left from the first complex was enough to initiate new storms. These new storms trained over areas which had already recorded excessive rainfall.


 Surface Frontal Analysis 22 July 2011

Figure 1: Note the orientation of the quasi-stationary front draped across north central IL.


The start of the two day rain event for Chicago O’Hare had a quasi-stationary front draped across north central IL. This created moisture gradient in which an MCS trained W-E across northern IL. In general, the heaviest rain fell from O’Hare and northward, north to Waukegan.


Surface Analysis 23 July 2011

 Figure 2: Note the advancing cold front in central Wisconsin, and its W-E orientation.


On 23 July, 2011, there was an advancing cold front in central Wisconsin; of importance with this feature was the orientation of it: Its W-E orientation, which was parallel to the jet stream aloft, generally advocates a slowing in the progression of the front, and promotes the training of thunderstorms due to its slowed progression and orientation. Moisture pools along the front (especially in this case, due to strengthening low level jet), and the increase of moisture leads to higher perceptible water values (which indicates the amount of available moisture in the atmosphere). In this case, we could expect WNW-ESE storm tracks with these storms. Along with those storm tracks, we had very efficient precipitation producing thunderstorms. There was more than adequate MUCAPE (Most Unstable Convective Available Potential Energy; measurement of how favorable/unfavorable the atmosphere is for thunderstorms) to allow for continued thunderstorm development. Most of the thunderstorms were driven by the approach 500mb shortwave, which induced rising motion (synoptic lift), which is a needed ingredient for thunderstorms. This thunderstorm threat was highlighted by SPC in their Mesoscale Discussion #1700:


SPC Mesoanalysis Graphic

In a case like 23 July, 2011 one cannot predict the exact areas which will be affected by these thunderstorms, and how much precipitation the thunderstorms may produce. In general, most of the July precipitation was confined to areas north of I-80, leaving areas south of I-80 rather dry for the month. For instance: DeKalb, IL received 5.28” of rain for July, whereas O’Hare received 11.15”. DeKalb was outside of the heaviest precipitation on the morning of 23 July, 2011 due to the track(s) of the thunderstorms. This can be noted in the following graph:


July Precipitation Graphic


The issue with these rain events is the amount of precipitation in a short amount of time. There is no doubt that the National Weather Service Chicago’s area lacked precipitation for July; but when large amounts of rain occur in a short amount of time, not as much water is absorbed deep in soils. Instead, it causes flash flooding because of the run off. So, while the crops of northern IL could have used the rain, it’s likely that most of the rain was not as useful for some areas: Whilst other areas, such as DeKalb, had beneficial rainfall, because it was distributed more evenly throughout the month compared to Chicago O’Hare.

Jeff Kilburg
Senior Meteorology Student
Northern Illinois University

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