An upper-level low and associated frontal boundary are steering anomalously high moisture into the Midwest, Ohio Valley, and Great Lakes on September 20, supporting scattered to widespread showers and thunderstorms with locally excessive rainfall per the National Weather Service Weather Prediction Center outlook. Model consensus from the HRRR and RRFS indicates the highest probabilities of measurable precipitation (>0.01 inch) concentrated across eastern Iowa, northern Illinois, Indiana, and into the southern Great Lakes, with secondary chances expanding toward the Mid-Atlantic and Northeast as the system shifts east. South Florida also shows elevated thunderstorm risk tied to residual moisture. Official quantitative precipitation forecasts and updated model runs issued overnight will refine placement ahead of local climate reports used for market resolution, while soil saturation from prior days increases flash flood potential in the primary corridor.