Santa will have to trade his snow gear for rain boots as a moisture-packed storm moves across the nation's midsection and South over the long Christmas weekend, bringing a threat of flooding downpours. The storm, fresh on the heels of soaking Southern California and the Southwest deserts, will spread rain across most of the Plains and lower Mississippi Valley through Christmas Eve, while also delivering some snow to the Intermountain West and Rockies. By Christmas Day, the rain will expand acros