Why has it been so #$%@ing cold this winter?

"The latest bout of brutally cold weather that has beset the eastern U.S. for weeks sent wind chills into the negative teens and 20s Fahrenheit (negative mid-20s to negative low 30s Celsius) in the U.S. Northeast over the weekend. Meanwhile, out West, winter has brought record-breaking warmth that is more suited for spring and even summer. I'm sitting here in a T-shirt in early February, a mile high in Colorado, says climate scientist Daniel Swain of the California Institute for Water Resources."

"To explain what's happening, let's review a favorite winter weather bugaboo: the polar vortex. The vortex is like a circular rushing river of wind that corrals the bitterest cold air up in the Arctic. When the vortex weakens, that tight circle becomes wavier, akin to how a slow-moving river tends to meander in bends across the landscape, Swain says. Where the vortex bends southward, cold air follows."

A persistent atmospheric pattern has driven a stark temperature contrast across the United States, producing brutally cold wind chills in the Northeast and record-warm conditions in the West. A weakened polar vortex makes the jet stream wavier, allowing Arctic air to plunge southward where the vortex dips and permitting warmer Pacific air to surge northward where it bulges. Geographic features like the Rocky Mountains and the Pacific–land boundary favor a ridge over the West and a trough over the East, amplifying the split. That waviness is forecast to break, shifting colder conditions westward and moderating temperatures in the East.

Read at www.scientificamerican.com