For symmetrical and practical reasons, Dec. 1 is the first day of winter on the meteorological calendar. And according to forecasters and computer models, winter weather is almost certain to arrive well before the solstice.

Both the European and U.S. computer models are in "excellent agreement" that the upper atmosphere is on the verge of executing a dramatic flip, the government's Climate Prediction Center said Thursday afternoon.

"This major pattern change is likely to usher in outbreaks of below- to much-below-normal temperatures to the eastern two-thirds of the continental United States through at least mid-December," the Climate Prediction Center said in its updated monthly forecast.

It has quite a different look from the December outlook it posted in mid-November.

December outlook issued on Nov. 16.
Climate Prediction Center
December outlook issued on Nov. 16.
December outlook posted Thursday.
Climate Prediction Center
December outlook posted Thursday.

Storm threats frequently attend such radical temperature swings, and we would be surprised if at least one wild snow rumor didn't surface, but the best advice would be to believe nothing yet.

The climate center also updated its two-week outlook in which the aforementioned pattern change is evidence. It calls for a significant trough of lower pressure, associated with cold and storminess, to dominate the atmosphere over the East, while a mirror-image ridge of higher pressure settles over the West.

The 8- to 14-day outlook.
Climate Prediction Center
The 8- to 14-day outlook.

The three-month outlook still favors warmth for the Dec. 1-Feb. 28 period that constitutes the meteorological winter.

(Meteorologists break down the seasons in tidy, three-month segments, the logic being that both winter and summer weather aren't strictly tied to the solstices and equinoxes.)

All the winter outlooks we've seen thus far have cited the potential influence of the periodic La Niña cooling in the tropical Pacific, where vast expanses of sea-surface temperatures are below normal, that is forecast to persist through the winter.

Weather does move west to east and the Pacific tropics are critical to winter, but around here the 20 La Niña winters since 1950 have coincided with a veritable wintry mix of conditions.

Ultimately the career of shorter-duration phenomena – pressure patterns in the North Atlantic and Arctic, and the position of the high-speed jet-stream winds that are the bearers of warm and frigid air and storms  – will determine the character of the winter.

Those things are marvelously complex, interactive, and chaotic and thus far more difficult to predict than slow changes in the Pacific.

Nonetheless, the long-range forecasters keep trying. Let's take a look at how they fared with their outlooks for last winter:

AccuWeather saw temperatures averaging 3 to 5 degrees lower than those of the 2015-16 winter. At the time, that looked like one of the safest bets in the history of seasonal prognostication.

The winter of 2015-16 was one of the warmest on record in Philadelphia, with an average temperature of 41.4 degrees, a good 7 degrees above the long-term average.

But the winter of 2016-17 ended up being a worthy rival, finishing at 40.4. Together those two seasons constituted the warmest back-to-back winters since 1932-33 in Philadelphia.

The commercial service WeatherBell Analytics — which is calling for about 30 inches of snow this season, by the way — last year had warned of the potential for "extreme cold" in December. In Philly, the December temperature was 1.3 degrees above normal.

The Weather Co., a New England concern that serves energy interests, gets points for seeing above-normal temperatures overall, but it said February's would be "slightly" above. Here and all over the Northeast, it was one of the most wildly mild Februaries on record.

As for that December forecast issued by the government Thursday, we point out that on Oct. 31 the climate center updated its November forecast, which had quite a warmish look for the East.

Nov. outlook issued Oct. 31,
Climate Prediction Center
Nov. outlook issued Oct. 31,

The November temperatures came in quite close to normal from Washington to Boston.

Evidently, chaos still can outwit computers.