Saturday, January 12, 2013

What Are The Chances The Polar Vortex Drops Into The US?

The chances are on the rise for the potential of some serious Arctic cold hitting the nation going into the latter part of the month. But could the polar vortex actually enter the United States? Let's take a look.

This is the GFS model's 500 millibar forecast for Hour 264, about 11 days away. We see a deep depression of heights in the Great Lakes, shown as blues and purples and even whites. These colors symbolize extremely low heights, showing that the polar vortex is in that area. There are a few crucial things we need to get straight before we dive into this forecast.

First and foremost, it is now probable that the polar vortex will drop south from the Arctic. The thing the models are having trouble with is just how far south. Initially, when this vortex drops south, you will see a depression of heights in Canada, not unlike the one seen in the image above. In response, high pressure should form in western Canada and may propagate south into the West US as a positive Pacific-North American (PNA) index. The positive PNA opens the doors for cold air to flow south into much of the central Plains, Midwest and into the East US. I find it only logical that the PNA, which has been strongly negative recently, will have to push to neutral or slightly positive levels as this polar vortex pushes down. It's plain physics- if there is a force pushing down, there must be another force pushing up; in this case, the polar vortex will be pushing down and the PNA must then go positive with high pressure building up in the West US and west Canada.

This positive PNA will then open the door for chilly air to make its way as far south as the North Plains. Depending on where the polar vortex as a whole shifts through the following 2-4 days, the track of the vortex could be dramatically changed. Right now, it would seem likely that the vortex would want to keep pressing south due to the positive PNA starting to enforce its will upon the storm track, which then dips south in response. Something else supporting the Plains and Midwest starting off with the coldest temperatures is a very weak east-based negative NAO. If you look on the above image, you will see Greenland in the top-right corner with a sash of blue across it. That is low pressure and is the anti-cold positive NAO. However, there is a small green bubble of slightly higher heights that would signify the presence of a weak east-based negative NAO. The negative NAO has a west-based and east-based phase. The former option supports the cold hitting the East Coast, and the latter has the Plains/Midwest receiving the brunt of any cold and stormy activity. The GFS supports the idea of an east-based weak negative NAO, adding influence to the positive PNA that then supports a general cold regime taking place in the Plains/Midwest (possibly in the form of the polar vortex). It would eventually  move east (the cold would), but with more of a moderated stance, not as strong as the Plains/Midwest.

This is Hour 240 of the new ECMWF model run, showing 500 millibar height anomalies. High pressure appears in warm colors, low pressure in cool colors. Here, we see the polar vortex has pushed east into the Canadian Maritimes, but another piece is reloading in south central Canada and would likely push south and east with time if this model went out further in time. This would also ensure a cold shock to the system, especially for those in the Plains, Great Lakes and Midwest. We still have that strong positive PNA paving the way for enhanced cold in the aforementioned regions, and what could be a weak east-based negative NAO trying to push north into southeast Greenland. Both models have a really nice cross-polar flow going on with high pressure pushing the jet stream north across the Arctic (hence cross-polar flow) and down into south Canada or even north US.

I made a check-list of things that need to happen in order for the polar vortex to begin pushing into the US. Confidence in these events occurring is added as well:

1: Polar vortex must shift south into Canada (Confidence: 85%)
2: PNA must go positive (Confidence: 55%)
3. At least a weak NAO must be in place (Confidence: 30%)

As you can see, there is still a lot to do before there is a good shot at the polar vortex sliding south into the States. However, we're already on our way to our best chance for winter yet- even if the vortex does not come crashing into the US, cold shots will make much of the north central and far northeast US colder.

Please don't ask what your location will be like, there's no answer to give at this point.


1 comment:

Rob ice said...

By the time all these teleconnections and pv's and mjo's go where there suppose to its gonna be spring...I guess we could get a couple of cold shots come our way in the northeast...but nextyear hopefully all this could occur sooner so we could have a decent shot at snow and cold..