Tornadoes and Hail in Iowa, May 18, 2000

A very dynamic weather situation took shape over Iowa. The strength of the system was more typical for for mid April rather than mid May. An intense low pressure center developed over the central Rockies. A warm front extended east from the low across Iowa. To the south of the warm front, humid air was in place with dew point temperatures in the mid 60s to around 70. Strong winds above the surface lifted the moist and unstable air over the top of the front. Initially a round of thunderstorms fired north of the front. The line became severe quite quickly and produced hail of three quarters of an inch to one and one quarter inch in diameter. This east to west line of storms produced a strong outflow boundary that propagated south across the central and southern counties of the state. By late morning, a round of thunderstorms developed over southern Iowa as the upper level low pressure system advanced into southwest Iowa. The storms became severe as they moved north, then intensified even more as the line of storms from southern Iowa interacted with the outflow boundary from the storms to the north. The storms produced several tornadoes, however most of them were very small and could be best described as spinup tornadoes. The strongest one formed at the intersection of the of the line of storms and the outflow boundary in the Des Moines metro area. A tornado touched down in Warren County east of Hartford. This tornado was on the ground for a couple of miles, though did little damage as it was in open country moving from Warren into Marion County. Another small tornado touched down in Poweshiek County south of Deep Rock. It formed along the boundary with a strong updraft into the parent cumulonimbus cloud. The dirt it picked up made it visually apparent even though there was no condensation funnel visible. The greatest damage occurred from the hail. Many of the storms produced one inch or larger diameter hail. The storm that developed southeast of Des Moines and spawned the largest tornado also produced very large hail. Hail up to baseball size was reported in Jasper County in a few places, with one report of three inch diameter hail southeast of Prairie City. Some areas reported a hail depth of 3 inches, with drifts a few feet deep, after the storm had passed. The hail was still on the ground six hours after the storm moved through. Needless to say, the hail caused considerable damage to cars and roofs in the area. With the exception of the tornadoes, there was not a great deal of wind associated with the storms. Most of the wind gusts were under 40 MPH. One of the storms did produce a wind gust of around 60 MPH in the Waterloo area.