With temperatures currently hitting the upper 40s to 50s, we are all wondering where old man winter is. Especially because the next 10 days will likely be just as warm heading into Mid-December. However it appears winter will begin to set it around the 17th-20th. What is leading us to predict this?
Let’s get into some of the biggest factors of winter. So immediately the El Niño probably popped into the minds of many of you. Well actually, a strong El Niño does not have much to do with this. The main factors are the AO, NAO, MJO, and QBO.
To begin with, here is a little about the AO. It stands for Arctic Oscillation and it’s guided by sea surface temperatures in the Arctic Region. There are two phases of it, positive and negative with neutral obviously between. With a negative AO, below average temperatures are common across the Eastern US. A neutral AO brings near normal or slightly below average temperatures. Lastly, a positive AO favors far above average temperatures for our area. For the past month we’ve had a neutral but primarily positive AO, which has clearly shown with our near record breaking warm November.
Furthermore, we anticipate the AO to remain positive through about December 15th. But after that, almost all models indicate it will drop to neutral and potentially even negative. For now we will go with neutral just because there is still a lot of time for things to play out. Below is an info graphic illustrating how different our weather can be with a neutral AO.
The majority of ensembles predict a negative AO will unfold. Now let’s move on to the NAO. The same goes for the NAO, with a negative NAO, below average temperatures are common across the Eastern US. A neutral NAO brings near normal or slightly below average temperatures. Lastly, a positive AO favors far above average temperatures for our area. The NAO is very similar to the AO in that we have been looking at a neutral but mainly positive NAO for quite a while. Models predict we’re on a path to a neutral NAO. How much will this effect our weather?
So now that we have stated the biggest factors that guide our weather in the wintertime, let’s dive into the models. More specifically, the GFS Ensemble. Currently we have far above average temperatures in the Eastern two-thirds of the Northern US, up into Southern Canada. This is a result of the Arctic Region experiencing well below normal temperatures caused by a cold pool centered over the Arctic. However by between the 16th-20th we will likely be looking at a much different setup.
Above average temperatures will move over the Arctic Region and the jet stream will dip south into the US. How far south? Likely south of Pennsylvania, into Central Mid-Atlantic area. We predict this will bring the long anticipated winter-like temperatures opening the door for potential winter storms/events. As you would expect, we have yet another graphic illustration.
There is even more model support for our expectations which we will not get into. As stated on our Long Range page, slightly above average precipitation is forecasted for the second half of December. But will our temperatures even at average be cold enough to support snow? Yes, highs on average in Late December range from the 20s to 30s depending on what part of Pennsylvania. The first winter storm we see will catch many of us off guard without a doubt. And coincidentally the time-frame we believe has potential is December 17th-20th lasting to at least the 28th-31st.
We hope you found this forecast informational. The wait will soon be over ladies and gentlemen! Be sure to like Pennsylvania Weather Action on facebook for updates/forecasts.