Tornadoes and Flash Flooding in Iowa, July 2 to 3, 1999

A very active weather pattern was in place over the central U.S. A strong west-northwest wind flow was in place over the state. Meanwhile a warm front was lifting north from the southern Plains. The warm front separated the cooler and dry air to the north from very humid air to the south. Dew point temperatures in the air to the south of the front were in the mid 70s to low 80s. Precipitable water values were near 2 inches as well. A meso scale convective system formed over north central Nebraska and south central South Dakota during the previous night. The remains of this system drifted into west central Iowa during the predawn hours. One cell became severe during the morning. This was to be the pattern for much of the rest of the day as the cells that formed became super cellular, with each supercell lasting about 2 hours. By the early afternoon hours, surface based CAPE values were approaching 6000 J/kg just to the south of the thunderstorm genesis region. Once the storms formed they became supercells quickly. Very strong sheer was in place with strong southeast surface winds veering to westerly winds in the mid levels. Even though freezing levels were in excess of 15,000 feet, the storms were prolific hail producers. WSR-88D VIL values with many of the supercells exceeded 90 kg/m/m. There was also some high winds with the storms at times as many of the cells were rotating as was seen with WSR-88D imagery. The first high winds occurred early in the severe event as 64 MPH winds blasted into the Dow City area of Crawford County. Winds of 66 MPH were recorded at the Denison Municipal Airport as the storm moved through. Damage was reported in many areas of the county. During the day there were also other spotty reports of wind damage, however most of the were were in the 40 to 50 MPH range. A few other cells did produce severe criteria winds as they moved into the central and northeast counties of the state with wind damage reported in Hardin, Hamilton, and Boone Counties. These storms developed during the period of time the weather system was changing from a supercell type system into more of a multi-cell system. The main severe weather event of the day was the hail. Hail reports of one inch up to golf ball size were too numerous to mention. Some of the hail was larger than tennis balls. Baseball size hail fell in Guthrie County, into Adair and Dallas Counties as one of the cells advanced east-southeast. The largest hail actually fell east of Casey in Guthrie County. Softball size hail fell there, smashing windshields on numerous cars. Needless to say, crop damage was widespread from west central into central Iowa along the paths of the two to three supercells that moved through the area. Scattered crop damage occurred with the multi-cellular storms over central into northeast Iowa as well. An example of the damage caused by the hail came by way of a report from Crawford County. Reports indicated hail damaged 80,000 to 100,000 acres of crops with widespread damage. Roads and ditches sustained between $150,000 and $200,000 in damage. Tree removal in the county was #30,000, with $100,000 damage to homes and businesses in the county. By the early evening, the weather system had made a transition from severe weather to heavy rain. Intense rainfall occurred over northeast Iowa. Flash flooding was widespread over northern Grundy, southern Butler, southern Bremer, and most of Black Hawk Counties. A large swath of 6 to 8 inch rainfall occurred over the Beaver Creek basin, causing flash flooding in the area and an eventual flood of near record proportions. County officials reported the flooding as very serious with countless roads and highways under water. Cars were stranded and thousands of basements were flooded by the onslaught of heavy rain. One report indicated several head of cattle lost as they were drown by the flood waters in Grundy County. No doubt, this was not the only case of livestock loss due to the flooding. Another report from the Dewar area, east of Waterloo, indicated about a block of railroad was washed out. Damage there was reported to 33 houses, a car repair shop, and a tavern. Rainfall was in excess of three inches per hour in many areas. The heaviest rainfall officially was around 9 inches. Some unofficial reports in the Cedar Falls area included up to 9 inches of rainfall in a 90 minute period. The observer at Parkersburg picked up 6.55 inches of rain in under 3 hours. A bucket survey in the area showed rainfall of 11 to 14 inches at a few locations in Butler County. The heavy rain occurred as a series of storms formed and trained over the same areas. The complex of thunderstorms that developed was a back developing complex. Late in the life of the system, one of the storms became severe. This storm displayed a meso circulation on the WSR-88D in Johnston, IA, though it was not all that well formed. The storm was the last in the series of storms and layed down a path of damage. The swath was about 13 miles long, extending across southern Butler and northern Grundy Counties. The damage path looked to be more downburst in nature, though there were reports of a tornado imbedded in the flow pattern. The situation was not all that different from the meso cyclone/tornado system that moved through Boone, Dallas, and Polk Counties about a year earlier on 29 June 1998. The tornado caused damage to several farmsteads along the road. The worst hit was on farmstead where all of the farm buildings were destroyed and only the superstructure of the farmhouse remained. Debris was wrapped up in trees for many miles along the path. One other tornado did occur earlier in the day. The tornado came from one of the supercells over western Iowa. A relatively small tornado touched down in Audubon County west of Hamlin. There was little damage reported with this tornado.