The Polar Vortex is a naturally occurring area of low pressure that spins around the north pole every winter. Our weather here at home changes as the strength of the polar vortex changes. When the vortex weakens, the typical cold it bottles up near the poles is displaced, allowing rounds of Arctic air to spill into the northern United States. When the polar vortex is unusually strong like during the winter of ’19-’20, cold air is kept at bay and we stay mild.
Typically, a weakening of the vortex is initiated by a process known as stratospheric warming. Remember the polar vortex meteorologists usually refer to is located in the stratosphere which is a layer of the earth’s atmosphere approximately 10-30 miles above the surface. Waves in the lower atmosphere can be so intense that they propagate upward into the stratosphere which can raise stratospheric temperatures by several degrees Celsius. If the warming is intense and fast enough, it can cause a complete disruption of the polar vortex as typical westerly winds aloft are weakened and reversed. The vortex can either be displaced from the pole or split into two, which can cause strong ‘blocking’ of high pressure over the pole and typically leads to more impactful winter weather over the mid-latitudes.
Below is a map showing the sudden stratospheric warming event that took place during the last week of December and into early 2021. Note the red colors which depict the warming event originating over Siberia.
Now here is a forecast of the state of the polar vortex this weekend. Note the two separate lobes, one of which is present over the north Atlantic, and the other over Russia. This is a split which was caused by the warming event approximately two weeks ago. So far this month, the location of the vortex has favored more cold outbreaks over Europe and Asia rather than North America, but this may change soon.
Not every stratospheric warming event leads to immediate, sudden changes in our weather. In fact, it can typically take weeks for effects in the stratosphere to spill into the troposphere, the lowest layer of the atmosphere. But these effects can linger for several weeks to even a couple months, depending on the exact strength, timing, and placement of the warming. This is why our weather team expected the second half of January to be more busy than the first, and it appears that remains the case.
Following strong warming events like this one, the probability of cold and snow outbreaks in the United States increases. Over the next few weeks, the presence of strong ‘blocking’ or high pressure over the North Pole will lead to outbreaks of cold air, although the La Nina based pattern suggests the most persistent, Arctic chill should stay over the northwest US and southwest Canada. Even so, the block over Greenland will keep cold air centered near New England which raises the probability of snow and mixed events as storms develop and run into the block (see forecast image below of the mid tropospheric pattern). Bottom line is we expect the second half of January through mid-February to be the busiest period of winter for us before a gradual weakening of stratospheric warming induced effects as we approach the end of winter.
-Meteorologist Adis Juklo