Snow arrives ... So why 6' to 24'?

Snow begins to cover Germantown Avenue in Germantown on Friday night. (Ron Tarver / Staff Photographer)

Snow has spread across the great Philadelphia city-state, and while so far it's having trouble sticking to the roads, that's going to change very soon as the intensity picks up and the temperatures drop below freezing.

The National Weather Service is calling for up to 18 inches in the immediate Philadelphia area, and you probably have heard estimates ranging from 6 to 24.

Why the spread? In general, the totals are expected to increase from roughly south to north, the reverse of the usual, but this has been the upside-down winter, so why stop now?

Embedded in that general trend, however, meteorologists point to a few wild cards that could cause wide disparities in accumulations:

Snow-to-liquid ratios: On average an inch of water yields about a foot of snow. That's an average, however. The colder the profile of the atmosphere, the higher the snow ratio. If it's cold enough, an inch of water can yield 20 inches of snow. Less precipitation may fall to the north, but if it's colder, it would take less liquid to produce more snow.

Banding: In every major snowstorm, snow bands form over certain areas, migrating and re-forming. Sometimes, bands pick on one given area, and snow totals end up higher over a narrow corridor than those in the next county. 

Thundersnow: Winter thunderstorms are unusual but do occur in snowstorms, as they did during the Dec. 19 mega-snow. During thundersnow, the skies can shake out 3 inches or more within an hour over a given area.  

The consensus is that the Baltimore-Washington area will be in the bulls-eye of this one, with up to 30 inches possible. The Shore is expecting up to 2 feet with blizzard conditions.

Everyone else will have plenty to plow and shovel, thundersnow or no.