Friday, November 28, 2014

Polar Vortex Split Occurring; Arctic Outbreak Possible in Mid-December

The stratospheric polar vortex has experienced a split as a result of its second interruption by warm air this season, and it appears some cold weather may be on the way as a result.

The chart above shows heat flux values over the 2014 calendar year, from January 1st to present day. In this graph, shifting our attention to November and December, we find that flux values have skyrocketed to near-record levels, only now beginning to drop down a bit. According to the Climate Prediction Center (CPC), strong episodes of eddy heat flux events to the poles may result in a Sudden Stratospheric Warming (SSW) event. You may recall that SSW events occur when wind direction in the stratosphere reverses from low pressure-favoring winds to high pressure-favoring winds. This decimates the polar vortex, and usually unleashes near-record intensity cold weather on whoever is in the line of fire about 2-4 weeks after the SSW event occurs. Considering we are seeing an eddy heat flux episode of this magnitude, it's no surprise the polar vortex is undergoing some serious damage.

The ECMWF forecast of height contours and temperatures (shaded) about 96 hours from now, at the 50 millibar level, shows a Wave 2 stratospheric intrusion event. For some clarification, let's remember that the stratosphere exists primarily from the 1 millibar to the 100 millibar level. Therefore, I'm using the 50mb level as a happy medium, per se, so we don't receive any tropospheric influences that may occur at the 100mb level, but also keep away from vortex events in the far upper stratosphere that will ultimately have little to no effect on the troposphere.
As for the Wave 2 event, stratospheric polar vortex split events are classified into a few 'Wave' numbers. Take a look at this image to see the difference between a Wave 1 and Wave 2 event.

This image here looks complicated to those who may not be as enthusiastic about the weather as others, so we will focus on the middle column. In that middle column, we essentially are seeing areas of low pressure in orange, and areas of higher pressure in blue. On the top row, a normal polar vortex form is displayed.

Wave 1 events indicate a very strong warming event or body of high pressure is forcing the polar vortex off the North Pole, while still keeping it in one piece.
Wave 2 events are seen on the bottom row, where a weaker, but still intense warming/high pressure event occurs, but now splits the vortex into two vortices.

Using that knowledge back on the ECMWF image, we can confirm that a Wave-2 stratospheric split has occurred.

Now, bearing in mind that it usually takes 2-4 weeks for stratospheric intrusions to result in cold air outbreaks on the surface, we flash forward to a potential chilly period in mid-December.

In the long range forecast from the Climate Prediction Center, the Arctic Oscillation (AO) looks to turn negative in about two weeks, right when the timeframe for any stratospheric consequences opens up. Many signals support the warm start to December ending around the middle of the month, and this stratospheric event only strengthens this potential.

To summarize:

- A stratospheric intrusion may result in a cold air outbreak in the middle of December.