MADISON, Wis. — Southern Wisconsin is facing a heightened risk of heavy rain and localized flooding Monday afternoon through Tuesday morning as warmer, more humid air moves into the region. Monday is designated a First Alert Day because the greatest concern is expected Monday night into early Tuesday, when widespread rain and thunderstorms could develop across the southern half of the area. A stalled weather boundary may keep storms moving over the same locations, increasing the potential for water to collect quickly on roads and in low-lying areas. Forecasters said it remains uncertain where the heaviest rain bands will form. Some of the strongest thunderstorms could also produce gusty winds. The day is expected to begin quietly, with rain arriving by Monday afternoon. Storms may be mixed with periods of heavier rain as humidity rises. After rain tapers to lighter, more scattered showers Tuesday afternoon, conditions could briefly dry out Tuesday night into Wednesday morning. Another round of rain is possible Wednesday afternoon, with on-and-off chances continuing through the end of the week. Forecast details later in the week remain less certain. Residents should monitor updated forecasts Monday, allow extra time for wet travel and avoid driving through flooded roadways. Even relatively shallow moving water can hide washed-out pavement or sweep vehicles from the road.