Forecasters said the snow that fell in the Mid-Atlantic Tuesday was dry and fluffy to the north of Washington, D.C., but heavier and wetter to the south. Why is this? Air temperature is the primary factor in determining the kind of snow that falls.

The warnings are issued by the NWS "when a significant combination of hazardous winter weather is occurring or imminent."

More than 90 million people were under winter weather alerts overnight, with the heaviest snow due for northern Kentucky into West Virginia.