Heavy Midwest Rainfall and Flooding
Extreme rainfall amounts have been observed over parts of southern Iowa, Missouri, and northern Illinois. An axis of extreme rainfall, with pockets of 6 inches, was indicated from Moberly Missouri, across Quincy Illinois, northeast towards Chicago. Pockets of darker yellow/orange and red indicate the areas of heaviest rainfall.
When I see these extreme rainfall amounts, I often suspect a tap of upper-level moisture. Take a look at the following image showing mid- and upper-level atmospheric moisture. The yellow arrow indicates a flow that originates from the Pacific Ocean. The Madden-Jullian Oscillation (MJO) has been recently active in this area. Also, note the green arrow, Looking closely, you can see streaks of low- or mid-level moisture streaming in from the Gulf of Mexico. When you have a low-level tap of moisture from the Gulf of Mexico, supported by a tap of upper moisture from the Pacific Ocean – heavy rain often results.
Precipitable Water (PW) is a measurement of the total amount of atmospheric moisture – both at low and at higher levels of the atmosphere. The next image shows that PW levels as high as 1.7 inches has pooled ahead of the area of convection/thunderstorms. PW readings of 1.7 represent unusually high levels, above 90 percent, for this time of the year for the Midwest.
Major Flood Threat
The recent heavy rainfall, in addition to current high flows and past heavy rainfall, has resulted in the forecast for major flooding across portions of northern Illinois, southeast Iowa, northern Missouri, and portions of the Mississippi River. Areas in purple represent a forecast of major flooding while red indicates moderate flooding. Flooding of secondary roads and inundation of property is possible with these levels.
In addition to river flooding, widespread flash flooding, urban flooding, and pooling of water in low or poorly-drained spots can be expected.
(Be sure to refer to official government sources for forecast or warning information)
Southeast U.S. Rainfall
An area of heavier rain also occurred over southern sections of Mississippi and Alabama.