Thunderstorm Wind in Butler County, Iowa

Large portion of a tree down in Allison.

A late-night Mesoscale Convective System (MCS) tracked southeastward through central Iowa from the night of July 27th into the morning of July 28th. This line of storms moved through a highly unstable environment, characterized by CAPE values of 4000 to 5000 J/kg. Effective wind shear was also favorable for organized severe weather, with 35 to 45 kts of shear ahead of the line. The structure of the system allowed areas along the eastern fringe, tracking east-southeastward, to remain balanced with the environment and produce strong winds across northern Iowa. Widespread crop, tree, and powerline damage was reported from straight-line winds throughout Winnebago, Hancock, Cerro Gordo, and Worth counties. Thunderstorm wind gusts of 60 to 70 mph were measured, with a 71 mph wind gust at the Mason City Municipal Airport.

The portion of the line tracking more southward was oriented from east to west. This orientation was less favorable for system organization, as the wind shear vectors were parallel to the cold pool. Consequently, this portion of the line quickly became outflow-dominant, which limited the threat of widespread wind damage. However, the environment remained conducive for strong downburst winds, which occurred sporadically as new updrafts developed along the line through north-central into central Iowa. Many of these downbursts were near or slightly below severe limits, generally producing small tree branch damage, including within the Des Moines metro area. However, pockets of severe winds causing tree damage still occurred within this less organized portion of the storm.

Behind the initial line, a few clusters of storms oriented from north to south tracked east-southeastward, taking advantage of a more favorable shear orientation. One of these storms produced a 66 mph wind gust in Emmet County. However, these trailing storms generally remained sub-severe as they developed in the more stable, rain-cooled air behind the initial MCS.

After reaching Interstate 80 around 2:00 AM on July 28th, the storms became disorganized, and only general, non-severe thunderstorms continued southward through the remainder of the morning.