Tornadoes and Damaging Winds in Iowa, August 9, 1999

An unstable airmass was in place over Iowa during the afternoon and evening hours of the 9th. Satellite and sounding data suggested the atmosphere was capped at about 775 mb by a warm layer of air with temperatures as high as 18 C. or more. Thunderstorms had a hard time firing off. A cold front moved southeast into the state as rich low level air with dew point temperatures in the mid 70s preceded the front. While the front moved southeast, a speed max near 100 kts in strength moved into the north central U.S. This combined with an upper level short wave helped a few of the storms break the cap. The wind profile was favourable with a strong shearing environment. Thunderstorms moved into north central Iowa and became tornadic quickly. There was basically one cell that produced at least two tornadoes as it slipped southeast into Iowa. The first tornado touched down in Winnebago County near Scarville. The tornado moved southeast across the county and advanced into Worth County by a few miles. The tornado caused crop damage and some building damage along its path. The corner of a bank building was torn off in the town of Joice in Worth County for example. The same cell produced another brief tornado touchdown near Burchinal in Cerro Gordo County near the intersection of I-35 and County Highway B-43. No significant damage was reported with this tornado. As the large cell that produced the tornado moved on, the rear flank downdraft produced high winds once again in the Scarville area. Power lines were downed and a few buildings were damaged by the high winds in the town of Scarville. High winds also struck the airport in Mason City with a wind gust of 58 MPH.