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Don Paul: Warming stratosphere will likely chill us out

We live in the troposphere, the lowest layer of the atmosphere. It’s where nearly all of our weather occurs, although a few towering thunderstorms can cross over a boundary called the tropopause and extend up into the stratosphere.

In the troposphere, the temperature generally decreases with height  – except when there are temperature inversions, where a warm layer aloft interrupts that dropoff in temperature. In the stratosphere above the troposphere the temperature does the opposite, increasing with height.  At our latitude, the stratosphere begins at about 33,000 to 43,000 feet (59,000 feet at the equator) and extends upward to about 160,000 feet.

The stratosphere is extremely dry, and contains our protective ozone layer, which shields us from much of the sun’s ultraviolet radiation. It is generally a very stable layer of the atmosphere, with very little vertical motion, unlike the usual busy, bubbly troposphere. The winds at this altitude typically are westerly.

On fairly rare occasions, a wave from the troposphere moves up/propagates into the stratosphere and disrupts this westerly flow, with a reversal to easterly winds at this high altitude. This reversal can distort, disrupt and weaken the polar vortex. When the polar vortex is stretched and weakened, that can allow the polar jetstream to buckle and drop to the south, delivering polar air to the south. The most potent of these warmings are called Sudden Stratospheric Warmings/SSW.

As for the polar air itself, it is nearly always found under an area of stratospheric warming. When the stratosphere warms, it expands and these easterlies high up allow very cold temperatures to develop beneath the warming down to the surface. Right now, the cold temperatures in much of Europe are there due to other processes, since the SSW isn’t forecast to develop until next week. But confidence on the actual forecast of an SSW is fairly high.

From a private British forecasting company called Metcheck, here is current and modeled positioning of the ongoing SSW based on the American GFS model. The warmed stratosphere can be found where the yellow contours are. If you hit the right-facing arrow in this link and keep clicking on it, you’ll see where the warmth is modeled to head between now and the beginning of February. The date can be found on the lower right, below the map. This model is mirrored by the European model as well as others.

This Sudden Stratospheric Warming and its forecasted movement and effect on the polar vortex are the reasons there is growing confidence among many meteorologists that winter in eastern North America will be making a comeback toward month’s end but mainly into February.

In fact, past SSWs of this magnitude tend to happen only once every two years. The disruption of the polar vortex and the surface cold which results have sometimes lasted up to two months.

So when it develops around the end of the month into February this return to below average temperatures may have more persistence than what we’ve experienced so far this cold weather season. The coming cold doesn’t necessarily equate to above average snowfall for all of Western New York. We’ve already had numerous lake effect events in which the northwest low level wind direction did not favor heavy accumulations on the Niagara Frontier from lake effect.

The impact of this SSW on snowfall remains unclear. But the impact in producing colder temperatures is higher confidence. Just how quickly it turns colder, and how cold it gets is something we’ll know better by next week.

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