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La Niña winter likely … but how much for Philly?

Data argue against abnormal cold; snow is a whole other matter.

City snow pile in South Philly after 2016 storm; feds say odds favor above-normal temperatures this winter.
City snow pile in South Philly after 2016 storm; feds say odds favor above-normal temperatures this winter.Read more( MICHAEL BRYANT / Staff Photographer )

Thanksgiving is three weeks away. The atmosphere evidently thinks it's closer to Memorial Day.

Friday's forecast high of 74 for the region would be about perfectly normal — if this were mid-May.

Instead, with summer-like high pressure in control, Philadelphia has an outside shot at matching the official maximum for a Nov. 2,  set in 1990. The overnight low Friday morning was expected to reach 60, which happens to be the normal high for the date.

Temperatures are due to reach the 70s again Monday,  a continuation of a profoundly warm October that saw monthly records set in a third of all the major observation stations throughout the Northeast, according to the Northeast Regional Climate Center.

It is impossible to know what all this might mean for the months ahead, but the government's Climate Prediction Center has the odds tilted toward a mild winter for Philadelphia, the entire Northeast, and roughly two-thirds of the rest of the country in the period from Dec. 1 through Feb. 28 that constitutes the meteorological winter. (In the weather community, the seasons are divided into four three-month segments.)

In its outlook, the climate center cited a recent trend of milder winters and the burgeoning La Niña, in which temperatures over a vast expanse of the tropical Pacific are running about a degree Fahrenheit cooler than normal —  the mirror opposite of the situation this time last year. The cooling is forecast to persist through the winter, and a La Niña reference has shown up in every winter outlook we've seen. Since weather moves west to east, what happens over the Pacific always has an impact on North America.

Through case histories, the effects of a La Niña are evident in the Northwest, where it tends to be cold, and the Southeast, warm and dry.

But in Philadelphia, La Niña's influence on winter is about as clear as a frost-covered window pane.

The data would argue against this being unusually cold. Not one of the 21 La Niña winters finished among the top 20 coldest in records dating to 1874.

The median December-through-February temperature for those La Niña winters was 35.4, about a degree above the long-term average.

The snow totals are more chaotic. The median for La Niña-influenced seasonal totals comes in at 18.3, about 80 percent of the long-term average. But the range for total snow is dramatic: from 2 inches in 1949-50 to 65.5 in 1995-96.

Speaking of chaos, the atmosphere's behavior can be more erratic than that of most politicians, which is why seasonal outlooks tend to come with more hedges than an English garden.

The tropical Pacific is important, but what happens in the shorter terms in the North Atlantic and Arctic  — events that are far more volatile and unpredictable than La Niña events — has everything to do with cold outbreaks and winter storms.

That explains the ambiguity of the climate center outlooks, which don't stray beyond listing probabilities for above- or below-normal temperatures and precipitation.

Mike Halpert, the Climate Prediction Center's deputy director, said he was aware that other outlooks were more ambitious. From the climate center's perspective, the science simply doesn't support more specificity. Based on the performance of some past winter outlooks, they might be onto something.