On Mon, 07 Mar 2016 23:19:00 -0000, "Mr Macaw" wrote:

Ah, this explains how they happen - warmer air higher up. But what I want to know is, surely if the warm air is moving to the right, then everything will melt soon anyway? And why do they happen in America and not the UK?

You may just not have the right combination of the jet stream, warmer
water bodies and cold fronts.. I suspect that wedge of warm air that
causes this ice thing in Maryland comes from the Chesapeake bay. The
moisture comes in as snow west of DC, east of DC it is sleet and
freezing rain.
When Southern Md gets it's big snows, the weather is coming up from
the south because the jet stream has driven way down, enough to make
it cold in Florida and that train comes up the coast like a hurricane
overwhelming warmer water in the bay. They might even get thunder with
it. I did it for 38 years and I am glad to be out