More than 83 million people Thursday were under some kind of winter alert from the Pacific Northwest to New England.
The first storm meteorologists were tracking was producing heavy snow through Tennessee, where snow falling at 1 to 2 inches per hour was likely to cause a difficult morning commute along Interstate 40, including Nashville.
Through Thursday, the snow is forecast to move across Tennessee, Kentucky, West Virginia and southern Ohio and into the Appalachian Mountains.
By Thursday night, the snow is forecast to be in full swing across the mid-Atlantic and the interior Northeast, affecting cities from Fredericksburg, Virginia, northward to Washington to Pittsburgh and eventually spreading over the I-95 corridor from Philadelphia to New York to Boston.
Friday morning's rush hour is when the storm is expected to be heaviest.
By sunrise Friday, the snow is forecast to be ending for Washington and Philadelphia but still heavy across New York City; Hartford, Connecticut; Boston and all points in between.
It is a fast-moving storm system, so all snow will be off New England by early afternoon.
Meteorologists were keeping a close eye Thursday morning on the potential for a narrow but very heavy snow band that could set up near or along the I-95 corridor during peak morning rush Friday.
Snow totals as high as 8 inches will be possible where the heavy, narrow snow band sets up, with snow falling at 1 to 2 inches per hour. While the exact location is impossible to predict, forecasters warned that it could be anywhere from New York City to southern New Jersey or farther east across Long Island up through Cape Cod, Massachusetts, and Boston.
A separate winter storm Thursday was causing lake effect snow to blanket parts of the Great Lakes.
By Thursday morning, heavy snow was falling downwind of the lakes, especially across the southern suburbs of Buffalo, New York. Snow falling at 2 to 3 inches per hour Thursday will affect areas east of lakes Erie and Ontario, with thundersnow also possible.
The snow will last through Thursday night into Friday morning before it winds down.
About 1 to 2 feet of snow will be possible, and the heavy snow, combined with wind gusts up to 40 mph, will lead to whiteout conditions, making travel difficult or nearly impossible.
Finally, meteorologists were also cautioning about heavy rain and mountain snow in the Pacific Northwest.
About 7 million people were under flood watches Thursday across parts of Washington and Oregon, including Seattle and Portland.
Heavy rain and snow will persist across the region through Friday, with the flood watches in effect because of high running rivers, creeks and streams.
Rain up to 5 inches and snow up to 18 inches will be possible through Friday.
And behind the snow will come dangerously cold temperatures.
About 19 million people woke up Thursday under wind chill alerts from North Dakota to the Texas Panhandle. Wind chills were forecast to be as cold as 50 degrees below zero, which can cause frostbite in just minutes.
Temperatures on Thursday morning were below zero across the Northern Plains and the Upper Midwest, with wind chills 20 to 30 degrees below zero.
Across the Great Lakes, temperatures were in the single digits and wind chills were below zero.
The cold air will head east Friday and stay locked in through early next week.