This is a repost I did from yesterday from liveweatherblogs.com, but it’s very relevant this morning as some of the models are beginning to pick up on what I discuss below. It’s a bit technical, and more for the hard core weather enthusiast (nerd), but I’ll try to simplify it some so that most will understand.
Since people continue to follow the ups and downs of the models, including teleconnection forecasts like a yo-yo on a string, I wanted to get away from that this morning and briefly explain why I’m not as pessimistic as some on the winter as a whole, including December.
Two days ago, I stated that “the QBO continues to fall and stratospheric temps continue to rise, which mean eventually the Polar Vortex will slide south. Eventually the NAO and AO will go neutral to negative due to this, based on physics.”
But what exactly does that mean? Here’s an excerpt from Steve DeMartino’s discussion from yesterday morning, which sums it up well:
“when the QBO is negative at a level, that means the warmer stratospheric air at the tropics can be transported at a much faster rate into the northern latitudes. The warming leads to expansion of that level towards the surface, which leads to a cooling effect on the troposphere, where we live. Under areas of strong stratospheric warming, deep upper lows develop. A negative QBO phase leads to the Polar Vortex to drop south and strongly supports a negative AO/NAO pattern.”
Here’s the latest cine loop showing stratospheric temperature changes across the globe:
You can easily see the warming taking place at the upper latitudes (at the top of the image above), because the QBO index continues to fall. I’ve simplied this a little, but the bottom line is the NAO and AO should improve as we go through December. Just looking at the GFS through 384 hours and it’s teleconnection forecasts doesn’t cut it for me. If you look beyond these, I think it helps. The GFS has in its defense trended colder, the NAO is on the downturn on the latest GFES forecast:
One could argue that the teleconnection forecasts have been all over the place the last few weeks, which they have, so why believe them, but with the above evidence I’m more inclined to believe them this time around than not. If you look at the cine loop above for mid November, the stratosphere was pretty cold – favoring a positive NAO. Now that the startosphere is warming, this should, and the NAO should head toward neutral and then weakly negative. We’ll have transient cold, and nothing extremely warm, for the next 10-14 days, and even a chance of some snow for those farther north. After that, we’ll know a lot more, and we should be in a pretty good spot if things unfold as I think they will. Will the blocking be like the last two years? It very well may not. But there will be some. Which is why I still believe those farther away from the coast will benefit the most this time around.
The bottom line is this – it will become much colder mid week next week, with a chance at some snow with minor accumulations in the Wednesday-Thursday time frame in at least east central and northeast Pennsylvania. The cold will be somewhat transient at first, but if I’m right about the above, should be more prolonged for the second half of the month, with increased chances at bigger snows. A white Christmas is a definite possibility for a lot of people in eastern PA.