Hail in Iowa, May 16, 1999

A very active weather pattern was in place over the central U.S. A cold front had become stationary west of Iowa and had held there for two days prior. A strong upper level jet stream was in place with Iowa falling into the rear entrance region of the speed maximum. While the cold front was off to the west of Iowa, a warm front was located to the south of the state. This set the stage for an over running precipitation event. A low level jet stream developed during the early morning hours of the 16th with very moist air flowing over the top of the warm front to the south. Surface dew points south of the front were in the low to mid 70s. The air flowing over the front at the 850 mb level had dew point values around +15 degrees C. During the predawn hours, a mesoscale convective system developed to the west of Iowa and moved east into the western part of the state. The system weakened after dumping hail over northwest Iowa, then began to intensify again during the later morning hours. Strong warm air advection continued throughout the day. This continued to feed the thunderstorms well into the afternoon. As the thunderstorms moved across the north half of Iowa they dropped very heavy rainfall. Though the ground was able to absorb the rain initially, it did set the stage for flooding later in the day. The thunderstorms produced gusty winds in many areas as well, however the main severe weather associated with the storms was hail. There were widespread reports of hail three quarters of an inch or larger in diameter. There were several reports of hail as large as golf ball size. The largest hail measured nearly the size of baseballs. Webster County was hard hit as the ground was covered by golf ball size hail that fell for a period of 10 minutes. There was damage reported from the city both to roofs as well as vehicles. Another area that was hard hit with golf ball size hail was north of the Waterloo area. The ground was also covered with hail in Bremer County.