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The fourth nor'easter in just three weeks is pummeling the East Coast, bringing heavy snow and sleet to the most populated part of the U.S., from Washington, D.C. northeastward to Boston on Wednesday.

The storm formed overnight on Tuesday as a strong low pressure in the upper levels of the atmosphere approached the East Coast and helped spark a surface area of low pressure off the coast of the Carolinas and Mid-Atlantic. The low pressure area is intensifying as it moves slowly northeastward, spreading areas of heavy snow inland.

This is likely to be the biggest snowstorm for so late in the season on record in Washington, D.C., and it could be the biggest March snowstorm in Philadelphia, where about a foot of snow is expected. More than a foot of snow could pile up in New York City if conditions come together just right, potentially making it one of the top storms for so late in the season.

Further north in Boston it will probably just rank as a moderate to heavy spring snowstorm, but not a historic event.

The storm is snarling travel across the Northeast and Mid-Atlantic, with most flights in and out of New York's airports and Philadelphia canceled on Wednesday, and significant disruptions at Washington area airports as well.

One feature of the storm forecast has been a dichotomy between the National Weather Service's (NWS) snowfall predictions and those of private sector forecasters. The NWS has both Philadelphia and New York City in a zone of 12-to-18-inch accumulations, whereas reliable media outlets have been predicting less, on the order of 6-to-12 inches, based in part on the high March sun angle and likelihood of a mix between snow and sleet at times.

In addition, the precise location of the most intense bands of snow, known as "mesoscale bands" because of their small dimensions, is difficult to forecast beyond an hour or two in advance. It's the areas that experience these bands for the longest period of time that will wrack up the highest snow totals, while areas just a few miles away could see half as much snow. Thundersnow also typically occurs in these bands as well.

The forecast has been particularly challenging this time because of disagreements between leading computer models, as well. The European model -- statistically the most accurate global weather model for the medium time range -- suggested far lower snowfall amounts on Tuesday afternoon, which raised forecasters' blood pressure considerably. But other models held steady, predicting serious snow accumulation in big cities.

The Weather Service has been warning that this will be a high-impact storm, particularly for the Philadelphia area and New York City, where power outages and "impossible" travel conditions are expected by the evening commute, as narrow bands of very heavy snow pivot across this region on the northwest side of the low pressure area.

While it's unusual to see four nor'easters in a row, this is not unheard of. The Weather Prediction Center tweeted an example of such an occurrence in December and January of 1987.

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However, there is statistical evidence showing that slow-to-breakdown weather patterns that can lead to extreme outcomes, such as heat waves and heavy precipitation events, are becoming more common as the climate warms in response to human emissions of greenhouse gases from fossil fuels.

One study, published in the journal Nature Communicationson March 13, tied warming Arctic winters and sea ice loss to an increase in intense winter storms along the East Coast of the U.S.

However, the precise physical mechanisms that cause Arctic warming to alter weather patterns well outside the Far North have not yet been well-established in the scientific literature, and this is an area of active research and debate in the weather and climate communities.

A general explanation of what's been going on so far this March, however, is that the jet stream -- the highway of air moving from west-to-east at around 35,000 feet, which steers weather systems and helps energize them -- has been repeatedly taking big dips across the East Coast. Right now, the jet stream has a sharp bend to it, carving out a low pressure area in the middle of the curve, or trough.

Other factors have influenced the fusillade of nor'easters, including a pattern of air pressure over the North Atlantic, known as the North Atlantic Oscillation, as well as a strong blocking pattern of high pressure near Greenland that is holding up weather systems like a stop light at an intersection.

A Greenland block, which typically consists of a persistent area of high pressure, is considered a prerequisite to causing some of the biggest East Coast snowstorms on record, since it forces the jet stream to dive southward across the eastern U.S., and allows storms time to intensify.

A blocking pattern can also force storms to move close to the coast, and sometimes even forces them to move backwards, from east-to-west, prolonging heavy snow, strong winds, and coastal flooding.

The storm on Wednesday will move fast enough to avoid causing severe coastal flooding, but if the NWS is right, and a foot-and-a-half of snow falls in some parts of the Washington to Boston corridor, this event will still go down in history as one of the biggest snowstorms for so late in the season.

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