Tornadoes and Flash Flooding in Iowa, May 16 to 17, 1999

As was mentioned above, the weather pattern was very active over the central U.S. A cold front had become stationary to the west of Iowa during the afternoon of the 16th. This set the stage for a serious weather situation. During the afternoon, 70 degree F. surface dew point temperatures were widespread for the first time of the season. The airmass became very unstable as the upper levels were still relatively cool. CAPE values during the few hours just prior to the development of the thunderstorms were in excess of 5000 J/kg. The situation became explosive as thunderstorms formed along and just ahead of the cold front over western Iowa. A little later in the afternoon, thunderstorms formed over northeast Iowa along the nose of the low level jet and just north of the surface warm front. These two features combined to produce a variety of severe weather across much of Iowa. The first of the severe weather broke out over west central Iowa. This was on the form of tornadoes. The strongest tornado in the Des Moines CWA touched down in Crawford County near Ricketts. The tornado was F2 strength and caused damage to 10 farmsteads along the way. One farmstead reported the house as being destroyed. The tornado had a duel structure with the two tornadoes about 2.5 miles apart north and west of Ricketts. This tornado was on the ground from near the western county line to a point north of Deloit. This was by far the strongest tornado in the CWA. There were several brief touchdowns and one that was on the ground for a couple miles, west of Denison. During the late afternoon and early evening hours, there were several reports of brief touchdowns from west central into central Iowa. One tornado touched down briefly in Audubon County, another southeast of Bedford in Taylor County destroyed a trailer there and tore a roof off of a shed. Another tornado touched down briefly in rural areas west of Perry in Dallas County. The tornado did little damage however. The same storm knocked down power lines and trees south of Perry as winds were recorded at around 70 MPH a short time later. The final touchdown was brief near Ames, in Story County. There were a few reports of high winds as the line in western Iowa transformed into more of a bow echo structure. A 67 MPH wind gust was recorded at the Creston Airport in Union County. As the storms moved across Dallas County, winds of around 65 MPH blew trees down on a commercial chicken house, killing 5000 laying hens. Winds gusted to 61 MPH at both the NWS office in Johnston and at nearby Ankeny, both in Polk County. As the bow echo continued to move north and east, winds around 65 MPH swept through the Marshalltown area. Damage was reported at a trailer court there as the skirting on some of the trailers was torn off and one of the trailer houses was nearly blown off its supports. The line of storms and high winds made its way northeast into the Wellsburg area of Grundy County, causing extensive damage on a farmstead just southeast of the town. Hail was widespread with all of the thunderstorms as they moved across the state. Many areas reported hail nearly dime size. Reports of hail of up to an inch were quite common as well. Some of the larger hail reports included golf ball size hail in Cass County at Atlantic and baseball size hail reported at both Boone in Boone County and south of Plainfield in Bremer County. Once the severe weather started to wind down, flash flooding was a fairly widespread problem, especially near the warm front over that was over northeast Iowa. The storms associated with the cold front produced flash flooding in Cass County at both Atlantic and Griswold. For the most part, the storms with the cold front were moving too quickly to cause much in the way of flooding, though urban flooding was reported in some areas. The big flooding was over the northeast part of the state. Those areas were hit twice with heavy rain and severe weather, once in the morning and once with this event. Widespread flash flooding took place in Black Hawk, Bremer, Butler, and Hardin Counties. Rains of 4 to 6 inches for the day were common in these areas, as well as areas to the northeast of that. Governor Vilsack declared 12 Iowa counties disaster areas. Butler, Bremer and Black Hawk Counties in the Des Moines CWA were declared disaster areas. All of these same counties were later declared Presidential Disaster Areas. Some of the more serious damage in the Central Iowa CWA was in Black Hawk and Bremer Counties. In Black Hawk County, U.S. Highway 218 was closed for a time by flooded water. The Cedar River caused considerable flooding in the area. The town of Dunkerton was approximately 50% evacuated as waters rose. The Sewage plant there was inundated by the high water. There was damage caused to many public roadways around the county. Some of the bridges over smaller creeks were declared unsafe. The County Engineer stated damages in Black Hawk County were at least $1,200,000, including $560,000 in damage to ditches, roads, culverts, and bridges. Damage in the town of Dunkerton were placed at $500,000 to public infrastructure. The county also reported $183,000 in damages to parks. In Bremer County, numerous homes in the town of Tripoli reported sewer backups into the basements. In the town of Denver, 25 homes reported at least 6 feet of water in the basement. Of those 25 homes, five of them had basement wall damage. Fifteen business in the town had anywhere from 2 inches to 8 feet of water in the basement. Bremer County reported damages to 19 culvert sites, 55 road sites, 2 bridge abutments, and 8 driveway washouts. Damage from these items alone were at least $200,000. In Butler County the county Engineer reported 20 to 30 sites affected with estimated damage of $150,000. One Bridge sustained at least $60,000 damage. Needless to say, numerous county and state roads were under water and closed as well throughout the area. The flooding in these areas was described as worse than the flooding in 1993. In many areas it was worse than the record floods of 1968. Farther to the southwest flash flooding also occurred in Cass County. Damage was not as serious there as the amount of rainfall was not as great. One basement was completely flooded in the town of Griswold. Flooding was serious enough in Atlantic to not only flood several homes but also buckle the pavement on one of the streets in town.