Capping one of the most radical winter flip-flops on record, the region has just endured its coldest February since Jimmy Carter was president and Frank Rizzo was Philadelphia's mayor.

Last month's official average temperature - a nippy 28 - was 10.2 degrees lower than its predecessor's, the second-biggest January-to-February temperature swing ever measured in Philadelphia, according to the National Weather Service.

Weather extremes are a local tradition, observed Paul L. Meyer, longtime director of the Morris Arboretum in Chestnut Hill. In the last several weeks, he has seen premature blooms followed by premature deaths. "This year," he said, "has been far more crazy than usual."

The warm spell started Dec. 10 and stopped abruptly Jan. 16. By the time February ended, it was the coldest since the bitter February of 1979, when the mercury hit zero three times and the average was 23.3 degrees.

The only thing "normal" about last month was its snowfall, 6.5 inches. The season's total for Philadelphia stands at 8.7. In a weird twist in a weird winter, that's twice as much as Boston has received.

The next threat isn't snow. It's flooding. Rather than a lion or lamb, March will come in like a wet dog.

Heavy rain is expected tonight, and a flood watch is in effect for the region through tomorrow evening, with one to two inches of rain forecast.

Runoff is a concern. As the ever-homelier snowpack dwindles, not much is left to soak up what falls atop frozen ground.

March is a notoriously volatile month, but it is unlikely to match the meteorological winter - Dec. 1 through Feb. 28 - for peculiar behavior.

The consequences could appear in the spring, perhaps in more muted blossoms and more aggressive weeds, horticultural experts say.

Daffodils that popped out in early January were assassinated by the cold, Meyer said. The early budding quince? "Those are brown mush."

The winter of 2006-07 also took a toll on the reputation of the seasonal-forecasting trade.

Almost nothing went according to script. It was a winter in which El Niño - a warm pool of water in one of the warmest places on earth, the tropical Pacific - was supposed to be a dominant factor.

Instead, El Niño was upstaged by a stubborn air pattern in the frozen north.

"We have to go back to the drawing board and figure out what happened," said Vernon Kousky, a researcher with the government's Climate Prediction Center who has spent 30 years studying El Niño and its cooler opposite, La Niña.

He is confident, he said, that the winter was driven by the Arctic Oscillation, an index that measures seesawing air-pressure changes between the polar region and the middle latitudes, closer to Philadelphia.

When the pressures are high at the pole and lower farther south, cold air can spill southward, and the index is said to be negative. When the index is positive, the pressures are high farther south, and the cold air gets locked up north.

The index approached record positives in December and early January. Then it went negative.

The result? January was about 6 degrees above average in Philadelphia, February about 6 degrees below.

So what happened to El Niño? Kousky and his colleagues are trying to figure it out. It appeared out of the blue in August, and helped snuff out the hurricane season.

It also fueled speculation that this would be a stormy winter on the East Coast.

When the Pacific heats up, it warms the overlying air. Water evaporates and then condenses into storms that release heat into the high atmosphere. The storminess typically incites the west-to-east jet stream that carries weather to North America.

Except this time, it didn't happen. The storms never formed in the tropical Pacific, the East Coast was generally quiet, and the Mid-Atlantic was spared big snowstorms and even big-snow threats.

El Niño has vanished, and La Niña is about to take hold. The cooling Pacific waters could enliven the 2007 hurricane season, beginning June 1. Active hurricane seasons are well correlated with La Niña events.

In the short term, after the rain, the local forecast calls for a return of winter temperatures, with nights in the 20s and days in the low to mid-40s

It is unclear, however, whether this will be a finale.

After 30 years, Kousky has developed a healthy respect for nature's inscrutability. "It's funny," he said, "how Mother Nature will always throw you a curveball."

Anthony R. Wood writes about everything under the sun at http://go.philly.com/

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Contact staff writer Anthony R. Wood at 610-313-8210 or twood@phillynews.com.