Snow still possible, cold to endure

As expected, the computer models continue to change their minds about the weekend snow prospects, but for now a coastal storm remains in the realms of possibility for the weekend.

In its morning updated, the government's Hydrometeorological Center said it is favoring the U.S. model, which is more bullish than the European.

Again, we will caution that it's only Tuesday, so don't take any of this too seriously just yet. We've mentioned before that the computer is the world's most prodigious producer of snow, and the most-efficient eraser.

The cold, however, is more than virtual. December chill has descended upon the Eastern United States, and it has been mightily cold in the United Kingdom.

A big factor is the pressure pattern over the North Atlantic, known as the North Atlantic Oscillation, which is measured by the NAO index.

When pressures are higher over the Greenland area than in the area around the Iberian Peninsula, the index is said to be negative. In winter, a negative index generally means cold here and in Western Europe. (Summer is another matter.)

The index has been negative now for 13 consecutive months, the first time that's happened since at least 1950, according to the Climate Prediction Center.

It is forecast to stay negative for at least the next week, and that would position December to end a run of nine consecutive months of above-normal temperatures in Philadelphia.

Today looks to be the first time since Feb. 6 that the temperature fails to get out of the 20s here. By the end of that day, 28.5 inches of snow was on the ground, you may recall.

We'll keep you posted on the weekend snow threat.