Tornadoes and Flash Flooding in Iowa, July 26, 2000

After a brief break of about five hours, thunderstorms began to erupt in Iowa once again. As had been the case over the course of the previous several days to the north and west of Iowa, thunderstorms developed during the mid afternoon and became supercells within a short period of time. Wind profiles were favorable with a south to southeast flow in the low levels and a north to northwest flow in the mid and high levels. A cluster of thunderstorms developed over central and west central Iowa. Initially, the storms exhibited strong characteristics of being tornadic. There were a couple of tornado touchdowns during the first hour of the storms development. The first was northwest of Boone in Boone County. This tornado touched down in open country and was only on the ground for a short period of time. As the storms continued to develop, another tornado touched down in Guthrie County. This tornado was on the ground for a few miles before roping out and did cause some damage. Fortunately the tornado missed the city of Bagley. As the system progressed, the tornadic activity ended with the complex becoming a significant hail producer. Although there were some high winds reported with the storms, most of the winds were under 50 MPH. The hail from this complex did considerable crop damage as the entire complex tracked to the south-southwest. Most of the hail was in the one inch to one and three quarter inch range. Storms passing over Cass County produced baseball size hail near the town of Atlantic. The hail caused serious damage and even broke windows out of several houses. The other problem with the storms during this event was the extremely heavy rainfall. The airmass was very moist with deep moisture in place, even though the moisture ribbon was quiet narrow. Rainfall of one and one half inches was reported in Tama County in a 15 minute time period. This produced the first flash flooding of the afternoon. As the action shifted to the west, heavy rainfall occurred over Adair and Cass Counties. Rainfall of three to five inches was reported in both of these counties within a two hour period. Enough rain fell to cover a section of Interstate 80 to a depth of two to three feet with water in Adair County, forcing the closure of the Interstate for a period of time. Water was also over the interstate in Cass County north of Atlantic, resulting in a brief closure there as well. Meanwhile, the area of thunderstorms over central Iowa increased in strength. Heavy rainfall ensued over Tama County with an additional four inches of rainfall in a couple hour time period. Serious flooding was reported in Tama and Montour with water around three feet deep in the streets of both towns. The storms weakened over central and eastern Iowa during the early evening as they moved southeast. The storms over west central and southwest Iowa remained severe as they moved south-southwest out of the Des Moines County Warning Area.