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El Nino ripening

The tropical Pacific is warming up, and that may have some impact on winter here.

Predictions of a mild hurricane season referenced a developing El Nino, in which vast expanses of the tropical Pacific become warmer than normal and agitate the west-to-east upper-air winds.

The season did set yet another record for gentleness – no major hurricane struck the United States, and no hurricane of any strength hit Florida for an unprecedented ninth-consecutive season.

But evidently that had little to do with El Nino, which never took hold.

Finally the sea-surface temperatures have warmed, and the U.S. Climate Prediction Center and the International Research Institute have upped the chances to 64 percent that El Nino will hold serve during the winter.

It is expected to be a weak one – sea-surface temperatures in the key region of the Pacific between 0.5 and 1 degree Celsius above normal, but has an outside chance of becoming moderate, 1 to 1.5 degrees above.

And so far, the atmosphere has been slow to respond to the warming.

The interaction of the El Nino and the overlying air can affect weather across the entire nation, and that includes Alaska and Hawaii, and it would have some impact on the Philadelphia winter.

So far, however, the atmosphere has been slow to respond to the warming, and sorting out how El Nino would interact with the rest of the atmospheric matrix is immensely problematical.

El Nino tends to energize the Gulf-East Coast storm track, the runway for our biggest snowstorms, but a friskier track wouldn't necessarily mean more snow.

Storms can veer off into the Atlantic, or come along when it's too warm for snow, or draw in warm air from the ocean to change any snow to rain.

Tony Gigi, a lead meteorologist at the National Weather Service Office in Mount Holly, has made a heroic effort to put together an "analog" list of seasons in which the prelude conditions are similar to those that have taken shape so far.

You'll find his analysis here.

To summarize, he found eight winters that coincided with weak or moderate El Ninos and came after temperatures in Philadelphia were above normal and November's, below.

The big snow winter on that list is 2002-03, the season of the monster President's Day storm. But also on that list is 1941-42, with a grand total of 10.3 inches for the season.