Low pressure will organize over Colorado today and move onto the Plains tonight and Tuesday. It will advance east through Missouri Wednesday and into the lower Great Lake Thursday night. Thunderstorms will develop tonight and continue intermittently through Tuesday night.Isolated severe storms are possible from Interstate 80 south. As the low passes to the east of Iowa, colder air will be drawn south into the region. The rain will gradually change to snow from the northwest. Accumulating snow will be restricted to far northwest Iowa and areas northwest with heavy snow possible. Moderate to heavy rainfall amounts over the next few days could result in ponding on roads and ditches Tuesday and Wednesday.