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The Appalachian Mountains often trap the dense, icy air layer, known as a cold air dam, while Atlantic winds usher in a milder air mass that glides up and over the top.

The highest impact ice event of that winter unfolded Feb. 10-11, with a massive cold air dam that stretched east of the Appalachians from Maine to Georgia.

There, a stalled frontal boundary enabled milder air to slide up and over the cold air dam.

The cold air gets trapped against the eastern slopes of the Appalachians, forming a shallow dome of subfreezing air called a cold air dam or wedge.

The subfreezing air wedge/cold air dam is shown in blue; the milder air wedge is shaded orange.