WASHINGTON — The storm that is threatening 60 million Americans in the eastern third of the nation in just a couple of days with persistent high winds, drenching rains, extreme tides, flooding and probably snow is much more than just an ordinary weather system. It’s a freakish and unprecedented monster.

What forces created it? Start with Sandy, an ordinary late summer hurricane from the tropics, moving north up the East Coast. Bring in a high pressure ridge of air centered around Greenland that blocks the hurricane’s normal out-to-sea path and forces it west toward land.

Add a wintry cold front moving in from the west and colliding with that storm. Mix in a blast of Arctic air from the north. Add a full moon and its usual effect, pulling in high tides. Factor in immense waves commonly thrashed up by a huge hurricane plus massive gale-force winds.

Do all that and you get a stitched-together weather monster expected to unleash its power over 800 square miles, with predictions in some areas of 12 inches of rain, 2 feet of snow and sustained 40- to 50 mph winds.

“The total is greater than the sum of the individual parts” said Louis Uccellini, the environmental prediction chief of the National Oceanic and Atmospheric Administration meteorologists. “That is exactly what’s going on here.”