Punxsutawney Phil’s predictions are in, and this year the science agrees
A tale of two (and counting) storms
Massachusetts is far from the only state to be hit with heavy snowfall this winter. The same storm that closed MIT campus on Jan. 26 led to the cancellation of over 19,000 flights as well as a series of power outages impacting over a million people. Extreme cold, ice, and winter storm warnings were issued in counties across 41 states, including Texas and other parts of the Deep South, where infrastructure required to withstand the extreme cold does not exist. A week later, a nor’easter hit the southeastern U.S., bringing snow as far south as Fort Myers, Florida, and up to 20 inches in North Carolina. Fortunately for Cambridge, the path of the storm curved eastward offshore, missing the city and triggering coastal flooding warnings in Nantucket and the Cape Islands. Though nor’easters have long been associated with winter in New England, snow in Florida is certainly unusual. So where did these storms come from?
Demystifying the polar vortex
These two storms, as well as the freezing weather they brought with them, were caused by a polar vortex event. The polar vortex is a rotating low-pressure cell of cold air located in the stratosphere above the North Pole. Directly below it in the troposphere is the polar jet stream, a circular wind system that keeps cold Arctic air close to the pole. When the stratosphere warms and the polar vortex weakens, the jet stream also weakens. A weakened jet stream tends to deviate from its usual path and wander southward, bringing cold air with it. While there is natural variation in jet stream strength, severe southern wandering like what the U.S. is currently experiencing is rare. These events are becoming more common, bringing Arctic temperatures to areas of the world that lack the appropriate infrastructure for such conditions, but scientists are unsure of exactly why.
A wandering jet stream will definitely bring frigid polar air. Nonetheless, this phenomenon alone is not the cause for winter storms; there needs to be a source of moisture. The Gulf is a suitable candidate if the polar vertex extends south enough. In addition to these moisture-catalyzed storms, polar vortex events can also lead to a process known as bombogenesis. According to the late meteorologist and MIT professor emeritus Fred Sanders, bombogenesis occurs when a low-pressure system’s pressure drops at least 24 millibars in 24 hours. These pressure drops are boosted by strong temperature differences between air masses, such as when smaller waves in the jet stream break off and form independent systems with a core of polar air. The storm from last weekend is an example of a weather system that underwent bombogenesis.
Ongoing research
As the Arctic warms, polar vortex events are becoming more frequent. Although it’s still unknown how exactly these two events are connected, scientists like MIT Visiting Scientist Dr. Judah Cohen are trying to find ways to predict these extreme cold spells. Dr. Cohen and his team are focusing on how the polar vortex is shaped by year-to-year weather conditions, such as Siberian snowfall and sea ice cover in nearby areas of the Arctic. According to their model, temperature differences between areas of Siberia may be linked to the instability and stretching of the polar vortex. Years where Siberia received high levels of early-season snow while the nearby Barents and Kara Seas had less sea ice tended to correspond to anomalous cold snaps in the central U.S. This is due to regional temperature differences forming atmospheric waves, which can reflect upward and disturb the shape of the polar vortex.
Cohen’s team have trained a machine learning algorithm to identify these disturbances and provide potential early warning for polar vortex events. This algorithm is unique in that it is used to predict cold snaps several weeks in advance, a much longer timescale than standard ten-day forecasts. Last December, this model was able to predict a smaller-scale polar vortex event — which caused temperatures to plunge the week before MIT’s fall semester finals — earlier than traditional forecasting methods. In the short term, Cohen’s forecast indicates that the rest of this winter will be very cold. Even if there is not a full polar vortex collapse like some meteorologists predict, more stretching like what has already been seen is very likely. One can only hope that Punxsutawney Phil was wrong again this year, but all signs point to his wintry six-week prediction being painfully accurate.