As T.J. Pignataro wrote on Tuesday, meteorologists are watching carefully something called a Sudden Stratospheric Warming/SSW, expected to develop soon.
Don’t worry, I’m not going to wax with great verbosity on the complexities of SSWs. I’ve made the judgment call it’s a topic better left for a future article, until I can figure out how to keep reader eyelids from drooping. When a portion of the stratosphere, which is the next atmospheric layer up from the troposphere – the layer in which we live and where most weather occurs – warms very suddenly by a large amount, it can transfer wave energy down into the troposphere and bring sometimes sharp to extreme cooling at the surface. Here is a modeled animation of an SSW from last February.
The most important impact we surface-dwellers need to be concerned with is how much an SSW disrupts and weakens the now-famous polar vortex. Many in the press gave the public the impression the polar vortex was something new, some term meteorologists recently conjured up.
But the polar vortex has always been there, and the term was first introduced in scientific literature in 1853. It is a mainly permanent area of low pressure which girdles the globe in polar regions, with a belt of counterclockwise winds aloft (clockwise in the southern polar region). The colder polar air lies to the north of this belt of westerly winds.
The phrase “strong polar vortex” sounds as though it’s more likely to bring colder weather to places like WNY. The truth is quite the opposite: A strong polar vortex tends to be more compact and closer to the North Pole. Those winds act as a barrier to the transport of polar air southward and keep the frigid air covering a smaller part of the northern hemisphere.
A weaker polar vortex tends to buckle in multiple places, sagging southward in some global regions and bulging northward in others. When the polar vortex slows, weakens, and destabilizes, that can allow polar air to take the plunge southward. In this diagram, the strong polar vortex and its effects are seen on the left. The weaker vortex is on the right.
When a SSW migrates from Siberia to far northwest North America, we have learned that often results in the weaker polar vortex dropping down toward our region. That is certainly what occurred in February 2015, Buffalo’s coldest month on record. Multiple lobes of the polar vortex dropped closer to us, and we ended up with an astoundingly bitter average temperature of 10.9 degrees for the month.
One of the problems we’re having right now is model disagreement on how strong the SSW will be, and where it will center itself. One model actually shows three separate SSWs in three locations, none exceptionally strong. Others show an SSW in the ideal location to send a weaker polar vortex closer to us during January. Overall, there is good agreement January will evolve into a colder month than the average January, but as to how much colder and when in January, agreement is not good.
Personally, I’m not ready to rush the possible sharply colder pattern until I see better agreement on the SSW. For now, model ensembles are not showing anything extreme between now and the first week in January. The American/GFS ensemble shows a western North American ridge beginning to form by Jan. 4, but the eastern trough, which allows colder air to drop south, has not yet become deep.
And a different, longer-range American model also shows nothing out of the ordinary in that time frame.
My best estimate is an eventually weakened polar vortex will bring some true, below-average cold to our region and much of the east, but the biggest transition will hold off until mid-January.
On a more mundane matter — a White Christmas — the best chance will be on higher terrain, where a few inches may accumulate on Saturday and again toward Christmas Eve and maybe late again on Christmas Day. A thinner, patchier coating will be possible on the Niagara Frontier. Either way, no major snowfall is anticipated, which would snarl WNY travel. But there may be just enough to pretty things up around here.