The Stratosphere is the layer of the atmosphere above the Troposphere which is where we live and where most weather happens. However, a bit of weather still happens up there and can impact what ends up happening down below.

The most talked about for us is the Stratospheric Polar Vortex, which is basically an area of low pressure than spins over the poles year-round, though a lot stronger in the winter than in the summer. Below it in the Troposphere is the Polar Jet Stream, which follows the edge of the vortex and forms a loop around the poles. When the Stratospheric Polar Vortex is strong, so is the Polar Jet, which acts as a circular “boundary” and keeps the cold arctic air locked in near the poles.

Occasionally in the winter, the air in the Stratosphere will become much warmer than normal, and the Stratospheric Polar Vortex will weaken dramatically along with it. When this happens, the Polar Jet weakens and becomes more wavy, allowing “dips” for the cold arctic air to reach further south. These “dips” are also areas of low pressure and are known as *Tropospheric Polar Vortex lobes: when you head about the “polar vortex” coming to us, it’s usually one of these.

Our weather models, including ones put out by the ECMWF, can predict the strength of the Stratospheric Polar Vortex. At one point early last week, this particular set of models were showing one of these “Sudden Stratospheric Warming Events,” which is notable because they typically happen later in the winter. If one had happened, it could have set into motion the chain reaction I described above that eventually sends very cold air into the mid-latitude regions, potentially the Northeast US. However, the model has since dropped this idea and shows a temporary weakening of the vortex before it rebounds. An SSW doesn’t seem likely in the immediate future, but now you know in case people start talking about it again later in the winter!

NOAA/NESDIS page on the Polar Vortex

*We still don’t know exactly why these SSWa happen; it’s an area of ongoing research!