White Christmas outlook

Yes, December barely has begun and we are getting only our first hint of wintry temperatures, but Accu-Weather's Joe Bastardi already is thinking about Christmas.

He believes that chances are better than usual that Philadelphians and other folks in the northern half of the country will wake up to a snow cover on Dec. 25.

Not to throw cold water on a 23-day outlook, but loyal readers will note that in any given year, the odds are overwhelmingly against a White Christmas in Philadelphia, loosely defined as an inch or more of snow on the ground on Dec. 25.

And Bastardi is not looking at anything terribly specific -- just a favorable storm pattern and a persisting of December cold.

Speaking of cold water, as of Monday the La Nina cooling in the equatorial Pacific once again registered in the strong category.

A moderate to strong La Nina is expected to continue through the winter. La Nina, as Tony Gigi at the National Weather Service in Mount Holly notes, is no friend of snow.

In 13 winters coinciding with moderate or strong La Ninas, Philadelphia has never had an official snowfall of 10 inches or more.

Further, October and November both finished on the warm side, and in La Nina years with warm Octobers and Novembers, the snowfall has not been impressive.

Snow totals ranged from a high of 20.8 in 1973-74 to a sad 4.5 in 1950-51 in those years.

Last year's record snows were juiced by an El Nino warming, characterized by copious moisture streaming from the Pacific on a strong subtropical jet stream. That doesn't happen with La Nina.

Snow-lovers, however, may have a friend near Greenland. Last year the North Atlantic Oscillation index was consistently negative, indicated a pressure pattern over the North Atlantic that favored cold air in the East.

Gigi pointed out that while negative NAO winters tend not to occur in pairs, the index has been stubbornly negative for 14 consecutive months.

While the consensus forecasts have called for cold December and milder the rest of the winter, Judah Cohen, scientist at the Atmospheric and Environmental Research Inc., thinks the winter will end up being on the cold side around here.

He's looking for a warm-up the second half of December, with a negative NAO reasserting itself in time for a cold January. 

In the meantime, we still await our first decent snow rumor.