Here is the forecast for the Iowa State Fair in Des Moines for Tuesday August 12th.

NWS: 40% Chance of Thunderstorms otherwise partly cloudy High 80°
WX Underground: 40% Chance of Thunderstorms High 79°
KCCI-TV: Chance of Thunderstorms High 80°
WHO-TV: Chance of Thunderstorms High 80°
Weather Channel: AM Thunderstorms High 80°
AccuWeather: Chance of Thunderstorms High 82°

Iowa WX Blogs Forecast Chance of thunderstorms, otherwise partly to mostly cloudy. High 80° Wind South at 5-10mph. Most of the rain should be out of the Des Moines area by the evening hours with just an isolated chance at a shower or storm thereafter for the rest of the night.