Invisible snow

Any snow that lands on a surface this afternoon and this evening is going to stick. The question is how much of is going to be able to land.

Right now, the atmosphere around here is quite dry. The idea that the air can be too cold for snow is a myth -- temperatures were in the teens during the blizzard-like conditions of Jan. 7, 1996, and Feb. 11, 1983.

But the air can be too dry.

Snow could start falling overhead by early afternoon, but with so little moisture in the atmosphere, some of that snow is going to evaporate before it gets anywhere near the surface.

That's a phenomenon known as "virga." Often, the blue that indicates "snow" shows up on radar images, while ground stations report nothing more than cloud cover.

For example, the most recent images show the blue over western Pennsylvania. But at last look, not a flake out that way was reported from surface stations.

For the Philadelphia region, the National Weather Service is calling for a 40 percent chance of snow by 6 p.m., and 80 percent tonight.

We are going out on a limb and calling for a 100 percent chance of virga this afternoon, and it is possible that no accumulating snow will land until nightfall.

The official forecast is calling for 1-2 inches, with perhaps a little more in southwestern Chester County.

Right now, the more likely outcome looks to be closer to 1 in the immediate Philadelphia area, but expect heavy virga.