Blog: Why so warm?
Okay, what’s the deal with the warmth? If you’re a snow lover, this is getting ridiculous. On the heels of the third warmest November on record, we’re running nearly 7 degrees above normal through the first 7 days of December. There has to be an explanation. We know we have a weak to moderate La Nina underway. Typically, la ninas are not big snow producers for us. Last year was an exception, when we experienced the snowiest December ever in Syracuse and ended up with the 4th snowiest winter with 179”.The basic atmospheric flow with la nina looks like this:
With the polar jet stream rising over the central Pacific, cold air is tapped in Alaska and sent southward toward the lower 48. Depending on where the upper trough sets up, dictates where the cold goes. Last year, the cold air poured down over us. This year, it’s barrelling south into the western and central US.
While this is one piece of the puzzle, there is something else that we look at. It’s called the arctic oscillation or AO. The AO changes between a positive and negative phase and is defined by air pressure differences between the arctic and the mid latitudes of the Northern Hemisphere. Here is a diagram showing the difference in phases and differences on where the cold air goes.
If you want snow and cold here in central New York, you want the AO to be negative. The more negative, the deeper and stronger the cold.
Negative AO values also create big snowstorms along the East Coast. Remember the Snowpocalypse of 2009? Or how about the big snowstorm that buried New York City just after Christmas last year? Those occurred during times of negative phase of the arctic oscillation. Ready to compare those 2 December to December of this year? Here you go:
December of ‘09 and ‘10 had an negative AO that was nearly 4 standard deviations below normal! This year, the AO is nearly 4 standard deviations above normal. Thanks to Stu Ostro of the Weather Channel for cropping this information together so nice and neat.
In basic terms, the arctic cold which was allowed to spill south over us the past couple of winters, is for the time being locked up in the high latitudes of our hemisphere. When we start to see the AO trending downward to even negative, that’s when we should see more and more intrusions of cold air.
It’s important to say here, this doesn’t mean we can’t get cold. There will be shots of cold air in a positive AO. What the index tells us is that the underlying weather pattern doesn’t support prolonged cold and snowy weather. So if we do turn cold (like we’re doing the second half of this week) it won’t stick around long. Sure enough, we have temperatures in the 40s again by early next week.
I’ll leave you with the forecast of the AO for the next couple of weeks. If you click on the image, it’ll take you to the latest forecast. If you’re interested, you can bookmark it for future reference.
:
(source: NOAA Climate Prediction Center)
The takeaway is that overall, the AO is forecast to remain essentially positive into the final 2/3s of December. “The other shoe” likely won’t drop between now and Christmas. After that, we’ll have to wait and see.
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