Outdoor plans in states from Texas and Oklahoma to Florida could be in jeopardy through the rest of Memorial Day weekend as a slow-moving storm system threatens millions with rounds of torrential rain and severe weather.

Some parts of the U.S. could see triple-digit temperatures over the weekend, while severe storms threaten other areas, according to forecasters.

A slight chance of showers is forecast for Memorial Day in Denver, with the odds of rain and thunder increasing in the afternoon.

Thunderstorms could threaten motor sports activities at Charlotte Motor Speedway during Memorial Day, forecasters say. Rain, though, should hold off at Indianapolis.