So…I took a day off from forecasting weather on Wednesday and enjoyed another one of my passions (high school athletics) but that never truly means that I’m totally away from the science of Meteorology.
I was asked on a couple occasions…why are the computer models changing so much in recent days and why are snow chances going downward in such a predicted cold pattern? Those questions have piqued my interest and I’m starting to think that I have come upon something that might answer those questions to an extent…
First off let’s state the obvious players on the table. It’s definitely cold, there’s no debating that at all. North America with the exception of about a seven-day span has been pretty cold the entire month of December. It was something that models began to hint on back in late October and all of November and sure enough it has verified well to this point in the month. Just a look at these frigid temperatures as of late Wednesday evening, a significant chunk of the country was dealing with temperatures in the teens or colder and for the northern-third of the US, its below zero temperatures. An arctic invasion has taken over here after Christmas for sure.
Now for us here in the Southeast US its temperatures that usually are the issue in getting winter weather but we’re dealing with the opposite issue right now, a lack of moisture. Of course some want to say we’re too cold for winter weather. First off technically you can never get too cold for snow but in theory related to weather patterns there may actually be a tiny bit of truth to the old saying.
With all that said, I think just one look at satellite images may tell a better story. For this forecaster one look at the water vapor and we’re seeing things that are a lot more obvious…
We are in the midst of a split-flow regime that is more dominant by the northern stream. This is actually a lot like the main synoptic pattern that our weather team recognized in the Winter Weather Outlook that was released back in October.
The only caveat in all this is that the early December cold outbreak (+ PNA/ -NAO) resulted in cross-polar flow which helped direct a lot of the built-up cold and forced it more into North America rather than in Siberia or Northern Europe. Plus a good snowpack across Canada and the Northern US has allowed that cold to remain in tact longer than normal. IF this was just about any other winter, we will be in a very mild pattern right now. The big arctic surface high’s continue to show up daily and are dropping into the US repeatedly.
Now, what needs to happen in order to change this pattern to where our part of the country could get more in the way of winter weather?
Again, just look back at the satellite images and its very obvious that we have no Pacific-North America ridging in place. The PNA was a huge player in the cold and snow from early December and we will need the PNA to jump back in the positive phase. When we have a +PNA, short waves can dig and amplify a lot more pulling up moisture and bringing it into the cold air. Right now the flow is just way too fast for that to happen. The zonal flow is why models lately have been all over the place for winter weather potential, the margin of error is so small that a wild variation of solutions are coming out.
The encouraging sign for us is that the PNA ridge might just re-establish itself after we ring in the New Year. The latest tele-connections which show a firmly – PNA shows an increase firmly into the positive phase by around January 3rd and hanging in there through the 9th.
This by no mean is an end-all be-all determination of winter weather but the chances of seeing snow or ice again definitely increases in that scenario. So that is the message to really take here. The cold air while impressive right now isn’t going anywhere.
The 500 mb look isn’t real pretty through the end of this weekend but we see positive signs next week at the jet stream level. Once this transitions over, then we’ll look for those individual pieces to the puzzle that might line up just right.
For snow lovers, patience is the virtue right now.
Foothills Weather Network