EF1 tornado in Franklin County, Iowa

The tornado damaged a barn roof and about a dozen trees on two farmsteads.

Low pressure moved northeast out of north central Kansas into central Iowa by mid day on the 12th, then into northern Illinois by evening. A strong push of very warm and humid air pushed into the state south of the warm front extending east from the low. Temperatures warmed into the upper 80s to low 90s, with dewpoint temperatures in the low 70s. The airmass became unstable with CAPE rising to 2500 to 4000 J/kg by mid afternoon. Precipitable water values rose to around 1.8 inches by late afternoon. The atmosphere was strongly sheared with 35 to 55 kts of effective shear available. The lifted index fell to -7 to -9 C. A very strong cap was in place over much of the state, so in spite of the downdraft CAPE of 900 to 1400 J/kg across the state, it was not realized until very late in the event. CAPE in the -10 to -30 C layer of the atmosphere was in the 600 to 800 J/kg range. The freezing level was quite high, at 15,200 feet, limiting the hail potential through much of the event. During the evening, as the surface low pressure moved to the east of the area. Temperatures aloft fell rapidly and the character of the storms changed from supercellular to short line segments. They began producing hail with the largest in Bremer County east of Readlyn with golf ball to baseball size hail. The LCL was low, around 750 meters, making for a favorable environment for tornadic development. Seven tornadoes occurred during the afternoon hours in Wright and Franklin counties with the strongest tornado rating of EF3. Flooding occurred in north central Iowa in parts of Cerro Gordo and Worth County. Rainfall of 2 to 3 inches in under an hour occurred, resulting in flash flooding initially, then general flooding overnight. Numerous roads were flooded and closed. Rescue operations took place in Worth County as motorists were stranded by the rising water. The motorist hydroplaned off of a highway into a ditch full of water. After the rain ended, flooding continued through the night as standing water took several hours to recede.