Snow: Depth may depend on height

We don't know if we have ever seen so many different colors on the National Weather Service's regional forecast map. We counted nine.

To summarize: A winter storm warning is in effect for Chester County for up to 6 inches of snow; winter-storm watches and advisories are up elsewhere for 3 to 5 inches, and the Shore gets all the candy -- a winter-storm watch plus a coastal-flood warning, plus wind advisories.

The weather service's afternoon discussion suggests that we should allow a large margin of error.

The recent computer-model runs have been taken the storm farther south. That said, Walter Drag noted in his narrative that this is "still a very large storm, and uncertainty reigns."

As mentioned, the big enemy of accumulating snow is the March sun. For any significant accumulation in the immediate Philadelphia area, snow would have to continue well after nightfall as the storm pulls off the coast.

The precipitation is due to arrive before dawn and then continue throughout the day.

We may well see a familiar pattern this winter in which a few hundred feet of elevation could mean the difference between white rain and 4 inches.

Temperatures are going to be borderline and may not get below freezing until the snow is winding down.

Since temperatures decrease with height, however, it may be just cold enough in elevated areas such as Roxborough and Chestnut Hill for snow to accumulate during the day.

In addition, with above-freezing temperatures, snowflakes that land on elevated surfaces are less likely to melt than if they had to continue in flight an additional 200 or 400 feet.

This may be a case where 4 inches lands atop the mailboxes of Chesnut Hill, and half of that or less lands at Philadelphia International Airport.

So far, the No. 1 snow spot of the season belongs to Jan. 24, the night when 1.5 inches was squeezed out of a paltry amount of precipitation.

To date, Jan. 24 has withstood all challenges, and given that the snow is measured near sea level, it may well survive this one.