Tornadoes and Flash Flooding in Iowa, August 28, 2007

A seasonally strong cold front moved southeast into the state during the afternoon and evening hours of the 28th. There was a strong contrast across the front with highs in the low 90s south of the front with dew points in the low to mid 70s, and temperatures in the 60s and 70s north of the front. The airmass was quite unstable with CAPE between 3000 and 4000 J/kg and lifted indices in the -6 to -8 C. range. There was sufficient shear in place and was in the 35 to 50 kt range. CAPE in the -10 to -30 C. layer of the atmosphere was 500 to 700 J/kg, however with freezing levels 15,000 to 15,500 feet hail was not realized in severe size at the ground level. The LCL was between 1000 and 1500 meters along the lead edge of the thunderstorms. As the front progressed southeast, a line of thunderstorms developed just ahead of the front. They developed quite rapidly as the cap was broken. The air was fairly dry in the lower levels leading to downdraft CAPE of 1000 to 1500 J/kg. In spite of the dry layer at low levels, precipitable water values were quite high, in the 1.5 to 1.7 inch range. Many of the storms remained below severe levels. An upper level vorticity max lifted northeast across eastern Nebraska into western Iowa during the late afternoon and evening hours. This intensified storms over eastern Nebraska, which moved into western Iowa. The storms became severe and produced high winds and several tornadoes from west central into central Iowa. The line took on a very evident bow echo as it moved through Crawford County, and that remained in tact as far east as Story County. Numerous reports of tree damage and power line damage were received, with winds of 60 to 70 MPH common. Winds were strong enough to partially blow over a utility pole near Jamaica in Guthrie County. A semitrailer truck was blown off off of County Road E-57 west of Kelly in Story County. Tornadoes touched down in Carroll and Greene Counties. They were short lived and were only on the ground for a short time in rural areas. One of them touched down in Carroll County north of Dedham, while a short time later a second touched down north of Coon Rapids downing utility poles. An aerial survey showed a very intermittent track, beginning about 3 miles north of Coon Rapids in Carroll County, continuing east for approximately 15 miles. It dissipated in Greene County just west of Cooper. Maximum path width was 50 yards. Crop damage and power poles were bent but no structures were significantly impacted. There was a two mile continuous swath of corn damage occurring about four miles west of Cooper with numerous meanders and turns. EF0 damage ended at approximately 1825 PM CST. Another tornado was on the ground for about 5 minutes in an intermittent track from southwest to south of Rippey. Minor roof damage was done and some windows were blown out by the tornado southwest of Rippey. Another tornado touched down in Guthrie County west of Bayard. An aerial survey showed a 3/4 mile long track that was embedded in the west to east outflow of the storm. A 50 yard wide track of damaged corn and a dozen trees downed at a farmstead were observed. A few shingles were blown off the house, but otherwise no other significant building damage was noted. The tornado was and EF0. Flash flooding was also a concern with flash flooding taking place in Calhoun and Wright Counties. Water was flowing over roads and into some basements. Heavy rainfall of 1 to 2 inches was common over already saturated ground. Nearly three and one half inches /3.42/ fell in a short time in the Rockwell City area.