Since May 15th, 2008 as much as 10 to 20 inches of rain has fallen over eastern Iowa based on data from the National Weather Service, Southern Region Headquarters. Today we will examine how the pattern that produced this heavy rainfall developed and what we can expected over the next several weeks.
Over the past 30 days, the overall pattern has featured two distinct features that has lead to heavy rainfall over the upper Mid-West, specifically in eastern Iowa. The first feature has been a strong and persistent ridge over the Southeast that climaxed with an extent up the entire East coast on the Week of June 9th. While the East coast baked under temperatures in the 80’s and 90’s with dew points in the 60’s and 70’s, a strong southerly flow out of the Gulf of Mexico supplied the upper Mid-West with a large amount of lower and mid level moisture and warm temperatures. A very unstable environment developed over the region, which would support the development of strong to severe thunderstorms capable of significant tornadoes, large hail, damaging wind gusts, and of course heavy rainfall.
The second feature was a strong upper low over the northern Rockies that at times deepened into a large trough over much of the Rockies and western central Plains. This trough had several different influences over the upper Mid-West. A series of jet streaks developed around the trough, at times to the west, east, or both. The placement of these jet streaks depended on the exact placement of the trough axis and upper level low. However, for the Upper Mid-West, the jet streaks constantly were situated in a position that strong supported a rising motion in the atmosphere. Another feature that developed out of the jet streaks was the intensification of low level jet streams at 850 MB. The low level jet stream increased moisture advection at the surface and aided in the development of nocturnal thunderstorm development. An additional feature was seen at 500 MB as strong short waves or disturbances rotated around the upper low and trough. These short waves produced several features that lead to the development of the thunderstorms.
The first feature was the development of strong divergence over Iowa that continued to support rapidly rising air. The second feature was strong vorticity. Vorticity is the rotation of air around a vertical axis. Think of vorticity as a cork screw. As the “cork screw” approached the upper Mid-West, air was forced to rise as a result. The rising air would begin to condense higher in the atmosphere, form clouds which lead to the development of rain drops and eventually the thunderstorms. The third feature was the introduction of much drier air at the mid levels of the atmosphere, which accelerates the rise of an air parcel on that “cork screw”. Finally, the fourth feature added to the environment was strong vertical wind shear due to the introduction of northwesterly upper level winds. The wind shear added in the development of meso lows, which allowed the development of tornadoes and very heavy rainfall. This pattern, which repeated itself over a four week period, focused thunderstorm development over the upper Mid-West. This continuous development over the same area produced rainfall amounts range from 10 to 20 inches, which naturally forced the river flooding. The combination of river flooding over Des Moines River, Boone River, and North Raccoon River had lead to significant flooding for Des Moines. However, the combination of river flooding from Shell Rock River and the Cedar River has lead to significant flooding and the breaking of levees in Cedar Rapids, Iowa.
The good news is that the pattern will begin to change for the upper Mid-West over the next few days. The upper low that has been so prevalent over the northern Rockies will progress to the east and settle over the Northeast United States. This will produce a dry northwesterly pattern over the upper Mid-West. A large ridge is then forecasted to build over the Plains and eventually into the Mid West and western Great Lakes. While the development of thunderstorms can not be ruled out through the next four weeks, the continuous production of heavy rain that has been seen will not be repeated. So relief will be on the way for the region to allow the flooding waters to drop below flood stages. As for the current flood situation, the water over Iowa will eventually move south towards the Mississippi River and will impact much of the mid and lower Mississippi River Valley over the next few weeks. The good news is that the public further to the south will have the ability to prepare for the additional river water and hopefully decrease the impacts of flooding over the Mississippi River Valley.