When is a La Niña not like a La Niña? The answer is - NOW.
An even better question is why is this La Niña not like a La Niña? How long will it continue to be producing such atypical weather? Those questions are much more difficult to answer.
Notice how a La Niña is supposed to shape the winter weather. It is supposed to produce cold wet weather in Western Canada and the Pacific Northwest. The South and Midwest are supposed to be hot. Texas and the Southwest are supposed to be dry while the Ohio River Valley is normally wet.
So far, it hasn't happened. Indeed, during December the weather was more similar to the conditions normally produced by a warm El Niño. Almost all of Canada and the Northern Plains have been substantially warmer than average (>5?C or 9?F) and the Southern and Central Rockies have been freezing, averaging 8?F (4.4?C) cooler temperatures than normal. The Gulf has been as warm as expected, but so has the entire East. The Desert Southwest and Southern Plains have been buried in snow which is great news for drought stricken Texas but very unusual for a La Niña.
So what happened? Scientists have centuries of historical and tree ring data that indicates how a La Niña event affects climate. Why is this event behaving so differently?
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