Sunday, September 28, 2014

Above-Normal Stratospheric Temperatures May Signal Blocky Winter Ahead

Warmer than normal temperatures in the lower stratosphere are raising the possibility of a "blocky" winter ahead.

The above image shows a pretty daunting image of the stratosphere, so let's decipher it. The red line shows observed temperatures at the 70 millibar level of the stratosphere, between the 65N and 90N latitude lines. The dashed green line illustrates average temperatures for any given time, while the gray outlines give an indication of the record high and low stratospheric temperatures for any given time period over the past few decades of records.

Gazing over the image above, specifically around the most recent records of temperatures, we find ourselves on the above-normal side, with that red line bursting up into above-normal territory more than once in the last few weeks. This year's warmth at the 70mb level looks to be a bit more prevalent than that of last year, as you can see just to the left of the center of this image.

Why is this important to the coming winter? Above-normal temperatures in the stratosphere allow for a higher threat for persistent high pressure to form over the Arctic Circle, and general upper-latitude area. The polar vortex, a strong low pressure system of cold air located across the troposphere and stratosphere, can be strengthened during times when the stratosphere is colder than normal, and weakened when warmth prevails.

When this warmth prevails, strong bodies of high pressure can punch north from the lower latitudes into the Arctic, disrupting the polar vortex. When this happens, fragments of the vortex can break off and be sent to the lower latitudes (as was seen last winter), or the whole vortex can be shunted down south. If the high pressure sticks around in the Arctic for long periods of time, it can be referred to as "blocking" high pressure, for the way it "blocks" the pattern from flowing east to west around the globe, since the high pressure remains stagnant and backs everything up.

The risk for sustained high pressure is increased as we see above-normal temperatures affecting the surface of the Arctic. The graphic above shows us observed temperatures north of the 80th parallel from the first day of the year to present day. Over the summer, we saw sustained below-normal temperatures, as evidenced by the red line being below the green average-temperature line, and I had discussed this as showing a potential cold air build-up for the upcoming winter.

Things certainly have changed since then! In the past couple of months, temperatures north of the 80th parallel have stayed predominantly above-normal, only reaching the average line a few times. The most recent observation shows temperatures diving, but the anomaly is still above-normal for this time of year.

For winter weather fans and warm weather fans, this is a double-edged sword.

On one hand, the lack of colder-than-normal air means not as much cold air may be available for the coming winter, barring a regime shift that would cool down the upper latitudes. Such a scenario could then mean a general 'warm-ish' feel for the coming winter, where cold weather would still occur (since it's the winter season), but the intensity of cold air may not be that extreme.

On the other hand, the warmth both at the surface and into the lower stratosphere suggest a pretty elevated risk of that blocking high pressure this coming winter. All of this warm air in the Arctic has a pretty decent chance of depleting the polar vortex this winter, and this risk is increased even further per some items that will be discussed in our Official 2014-2015 Winter Forecast on October 11th at 12:00 PM Central Time.

To summarize, above-normal temperatures across both the stratosphere and troposphere appear to be raising the risk of blocking high pressure in the upper latitudes this winter, which could then raise the risk of a cold winter in North America.