Millions of Americans should prepare for what could be the most significant winter storm of the season as a large cold front sweeps across the U.S. this week.Ahead of the system, winter storm watches are in effect from New Mexic o east into Tennessee and Kentucky. Major cities under winter storm watches include Dallas, Oklahoma City, Little Rock, Memphis and Nashville.Additional winter storm watches are expected to be issued Wednesday.RELATED STORY | Cold winter, natural gas prices, data centers blamed for rising carbon pollutionMeanwhile, extreme cold watches and warnings are posted for the Upper Midwest. Chicago, under an extreme cold watch, could see wind chills near minus 30 Friday morning. Wind chills in Minneapolis could plunge to minus 40 Thursday.The National Weather Service said parts of the Great Lakes could receive 6 to 12 inches of snow by the weekend.For the upcoming weekend storm moving across the South, the National Weather Service says it is too soon to know snowfall totals. Amounts will depend on how much of the precipitation falls as freezing rain or sleet.Although no winter storm watches are in place yet, forecasters say the Mid-Atlantic including Washington, Baltimore and Philadelphia could be hit hard. Those areas have several days to prepare for potential impacts from a major winter storm.RELATED STORY | Growing up in extreme heat linked to slower childhood development, study finds ...read more read less