A new look at climate data shows that, by the end of the century, the heaviest days of rain and snowfall across much of North America will likely release 20 to 30 percent more moisture than they do now. Much of the increased precipitation will occur in winter, potentially exacerbating flooding in regions such as the upper Midwest and the west coast. Researchers also found that heavy precipitation days historically experienced once in a century will become more frequent — as often as once every 30 or 40 years in the Pacific Northwest and southeastern United States.