Here's what we know about the winter storm hitting the eastern United States:

— What is it? Possibly one of history's ten worst winter storms to hit the East Coast. Heavy snow and high winds are moving across the northern mid-Atlantic region. Forecasts show Washington and Baltimore are in the bull's-eye, but Philadelphia and New York City also will get socked.

— Why now? All the ingredients have come together to create a blizzard with brutally high winds, dangerous inland flooding, white-out conditions and even the possibility of thunder snow, with Washington at the epicenter, forecasters said. The storm initially picked up warm water from the Gulf of Mexico, then gained much more moisture from the warmer-than-usual Gulf Stream off the East Coast.

— How long? The snow arrived Friday afternoon in Washington, and it's expected to continue into Sunday as the slow-moving storm moves up the coast.

— How bad? Life-threatening conditions are possible as the storm moves from Kentucky and Tennessee through the Virginias and up the East Coast. Already, several people have died in accidents on icy roads. Extremely heavy snow and high wind gusts could reduce visibility to zero, toppling trees and causing power outages. Forecasters expect flooding, though not as bad as Superstorm Sandy's.

— Is snow the only worry? Snow is only a small part of this blizzard. It is a blizzard because of high sustained winds. Those winds reached near hurricane levels with 70 mph in Wallops Island, Virginia, said forecaster Patrick Burke at the National Weather Service's Weather Prediction Center in College Park, Maryland. From Virginia to New York sustained winds topped 30 mph and gusted around 50 mph, he said.

On the Eastern Shore, Dewey Beach, Delaware, and Langley Air Force Base, Virginia, recorded hurricane-force 75 mph winds, the weather service's storm tracking page reported.

— How much? It could fall as heavily as 1 to 3 inches an hour and last for more than 24 hours in places, said meteorologist Paul Kocin with the National Weather Service. That puts estimates at more than 2 feet for Washington and Baltimore, a foot to 18 inches for Philadelphia. Forecasters had to increase their snow predictions for New York City and points north. New York should now expect 15 to 20 inches.

— What to do? STAY INSIDE! Authorities pleaded with people to hunker down by 3 p.m. Friday and stay there until the storm is over.