A swirling storm with the potential for more than a foot of snow clobbered the mid-Atlantic and the urban Northeast on Tuesday, grounding thousands of flights, closing government offices in the nation's capital and giving students another day off from school.

The storm stretched 1,000 miles between Kentucky and Massachusetts but was expected to hit hardest along the heavily populated Interstate 95 corridor between Philadelphia and Boston, creating a perilous commute home for millions of motorists.