Snow: Band is back

Although the snow took its time getting started, snowfall rates in parts of the region have been astonishing, up to 2 inches an hour, according to the National Weather Service.

So far 6 inches has been reported in Radnor Township, Delaware County, and it didn't start snowing out that way until just after daybreak.

Right now a band of heavy snow has set up over the region, a shade to the northwest of what was expected, said Jim Bunker at the National Weather Service office in Mount Holly.

In his forecast discussion, Bunker's colleague, Mitchell Gaines, said that some areas were reporting rates of 1 to 2 inches an hour, with computer models suggesting that banding would continue this afternoon.

Banding was a frequent visitor in the winter of 2013-14, when 68 inches was measured by the official observer in National Park, Gloucester County.

"Banding" occurs in geographically narrow areas that get caught under an ideal "snow growth" region.

They typically are confined to about 50 miles or so, and areas outside the band can be snow-deprived, at least temporarily.

For a band to set up, temperatures in the clouds have to be just right - around 5 degrees Fahrenheit - with strong winds blowing upward.

Upward motion is a sine qua non, because snow falls when vapor-filled warmer air is forced to rise above cooler air and then condenses.

It is all but impossible to predict precisely where bands will develop, and that's one reason why accumulation forecasts typically carry plenty of wiggle room.

Assuming the banding continues, Philadelphia is likely having its biggest snowfall of the season. Before today, the biggest one so far was 4.8 inches, on Feb. 28