Polar vortex breakdown early in the season raises forecast uncertainty
A disruption of the polar vortex, which usually keeps Arctic air locked in place, has begun unusually early, potentially affecting weather in the coming weeks.
The polar vortex, a band of upper-level winds that typically contains the coldest air near the Arctic Circle, has weakened and reversed course. This breakdown began on Tuesday and is expected to last through Thursday, according to computer model guidance. Such a disruption this early in the season is unprecedented in the historical record; these events normally occur in the latter half of winter.
When the polar vortex weakens or reverses, it can allow bitterly cold air to spill southward. The specific weather outcome depends on how the vortex breaks down. Brief cold spells can result from southward-dipping lobes of the circulation, while a splintering of the vortex can cause cold air to linger over one region for days or weeks. A similar event in early 2015 sent prolonged cold across the eastern US, with Chicago recording its coldest February in nearly 140 years and temperatures in Lexington, Kentucky dropping to -18 degrees Fahrenheit.
Because weather patterns typically take a few weeks to respond to a polar vortex disruption, and reliable forecasts only extend about 10 days, it is not yet clear where in the northern hemisphere the cold air may travel. Additionally, this winter is already being shaped by the strongest El Niño on record, which has warmed sea surface temperatures in the eastern Pacific and is expected to bring warmer-than-normal conditions to much of North America. The overlap of these two unprecedented phenomena makes forecasts less certain.
Source: Wired