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Despite a cool La Niña shaping global weather, here in North America the weather has been amazingly warm. Winter weather sulked in the West and refused to move east. Indeed, the weather got so strange that on Christmas there was more snow in El Paso, Texas, on the Mexican border, than in Toronto, Canada!
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.
This did not happen this December. Indeed, throughout the month, the weather was more similar to the conditions normally produced by a warm El Niño. Almost all of Canada and the Northern Plains were substantially warmer than average (>5˚C or 9˚F) and the Southern and Central Rockies were freezing, averaging 8˚F (4.4˚C) cooler than normal. The Gulf was as warm as expected, but so was 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.
December 20 – 31 – Temperatures (top map) and Precipitation (bottom) Top – Red means temperatures are more than 9˚F (5˚C) warmer than normal and dark blue is 9˚F (5˚C) cooler Bottom – Dark blue means 200% more precipitation than normal while brown is less than 50% of normal
So what happened? Scientists have centuries of historical and tree ring data that indicate how La Niña events effects climate. Why is this event behaving so differently?
Scientists really are not sure, but here are some theories that I have heard so far:
Man-made global warming.
La Niñas fluctuate in strength. This one weakened through November to mid-December but is beginning to grow stronger. When the La Niña is weak, it lets the warm sub-tropical jet stream warm the East.
The Arctic Oscillation, the “wild card” of winter, remained positive and trapped the cold polar air mass north.
The North Atlantic Oscillation remained positive, which also traps the polar air mass north.
The Gulf Stream in the Atlantic is very fast this year, so the Atlantic has been extremely warm. That heat seeped inland throughout the East.
The combination of all these factors is rare, but it has happened in the past. I hope that you are enjoying the warmth (if that's what you like).
If snow and cold are more to your liking, there's lots of winter left in the calendar! In fact, we predict that winter will make a comback across the eastern U.S. and Midwest by mid-January!
How is your winter weather to date? Please report the weather in your area—and let us know what you think of it!