The image below shows an estimate of the accumulated rain that fell over parts of Wisconsin, northeast Iowa, and northern Illinois between 12:48am CDT Wednesday, July 27, and 5:43pm CDT Thursday, July 28. The estimates are produced by the radar system located at our office in Sullivan (located in the dark circle in the center of the image). The radar can determine where precipitation is occurring at a given time, and also can estimate the rate at which the precipitation is falling. The type of accumulation product seen here is a result of multiplying the estimated rates by the amount of time that precipitation lasted over an area. Depending on the type of storm producing the rainfall, the season of the year, and other factors, radar estimates of precipitation can be reasonably accurate.
The different colors in the image represent different amounts of rainfall. Total accumulation of one inch or more was widespread across the region. The pink shades over and to the southeast of Dubuque, Iowa (roughly left-center in the image) represent totals of 12 inches or more! Click on the image to see the full-size version.
It is important to note that radar estimates are not accurate enough to replace "ground truth" precipitation measurements like those measured by NWS cooperative observers and storm spotters. A technology upgrade to the NWS radar network is underway that may one day make radar precipitation estimates more reliably accurate.
The Dubuque Airport measured 10.31" of rainfall for the period from 7:00am July 27 to 7:00am July 28. For roughly the same timeframe, a cooperative observer in Galena, IL (just southeast of Dubuque) measured 13.45". Localized areas in the vicinity may have actually received more or less precipitation than these observations suggest, but we don't know for sure without having observations from more locations. This shows one particular benefit of radar precipitation accumulation estimates: coverage from areas where observations are not available. While not perfect, the radar estimates are the best we can do to determine how much rain fell at every location within an area.