Thunderstorm Wind in Grundy County, Iowa

Six inch tree limbs broken in Grundy Center. Time estimated via radar.

A deeply unstable air mass was in place across the area with high humidity as dewpoints rose through the upper 70s to near 80 and air temperatures warmed to near 90. This sent heat indices into the low 100s in the afternoon. While convergence was quite weak across the area, slightly backed winds across eastern Iowa provided enough focus for scattered thunderstorms development in the late afternoon and evening. With a lack of shear across the area, pulse thunderstorms were the primary mode. These storms formed and died quickly, and collapsing updrafts produced gusty winds, including a gust to 60 mph in Grundy county that resulted in tree damage.