The Southern Plains and Southeast United States are bracing for a bout of thunderstorms and potential flash flooding on Sunday, according to weather forecasts. An upper-level wave and surface frontal system are set to bring unsettled weather conditions across these regions. While cooler conditions may limit the overall instability, the risk of locally heavy downpours remains a significant concern. These intense rainfalls could lead to isolated flash flooding, particularly in low-lying areas and urban environments with poor drainage.