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Snow, ice, and the 'fall line'

Geology, and a few hundred feet, might make huge differences in tomorrow’s storm.

If the National Weather Service snow map is an indication, snow totals are going to vary steeply within the space of a few miles.

On the maps, the line between the 2- to 4-inch zone and the 4-to-6 is almost right along the "fall line," where the coastal plain gives way to the land that begins the long and gradual rise toward the Appalachians.

While elevations along the fall line, which runs all the way to Georgia, may be only a few hundred feet above sea level, those elevations can make dramatic differences in a storm such as the one expected tomorrow.

Almost from the outset, warm air will be trying to nudge aloft to change snow to liquid, but all signs suggest that it's going to be a battle royal north and west of the I-95 corridor.

The dynamic processes in the upper atmosphere are going to be bedevil accumulation forecasts to the very end. And if snow accumulates early on, that could stall the changeover by refrigerating the overlying air.

But elevation will have at least one obvious impact: Temperatures decreases with height – 300 feet can a mean a difference of 2 critical degrees.

That can mean the difference between a harmless changeover to rain, and lacquered, perilous coating of ice.

It also going to have an impact on precisely when any changeover occurs, and that will have a whole lot to do with who winds up with the heaviest snow.

Said Mitchell Gaines at the weather service office in Mount Holly, "It's going to be a really tight gradient."