More than 18 million people across the West and Midwest are under winter weather alerts as a series of storms brings heavy snow, dangerous wind chills and the threat of flooding, according to forecasts from CBS outlets and the National Weather Service.A strong weather system is already generating heavy snowfall across the Northern Rockies, with higher elevations expected to receive a foot or more through Saturday, CBS Meteorologist Andrew Kozak said. As the system shifts eastward, snow is forecast to spread into the Northern Plains and Upper Midwest, from South Dakota to northwest Illinois, where 4–6 inches and isolated higher totals are likely.Chicago is under a winter weather advisory, with 2–5 inches expected to create slick roads and sidewalks by early morning, CBS Chicago reported. The cold outbreak following the snow may send temperatures plunging below zero across parts of the Dakotas and Minnesota by Sunday night into Monday. Another round of snow is expected to hit the metro region and southern Minnesota on Saturday afternoon, CBS Minnesota reported, accompanied by bitter temperatures.The National Weather Service has also projected moderate to heavy snow over parts of the upper and middle Mississippi Valley on Saturday night.Farther west, winter storm warnings and advisories are in effect across Colorado, with mountain regions expected to bear the brunt of the snowfall, CBS Colorado reported.Meanwhile, the Pacific Northwest is bracing for a series of incoming Pacific systems beginning late Sunday into Monday. Kozak said the region is likely to see widespread heavy rain across western Washington and Oregon, with flash flooding possible due to saturated soil. Flood watches are already in place, and forecasters warn that the heavy rain threat may persist for 5–10 days.