There could be a major snowstorm along the East Coast this weekend. Of course, snowstorms are common in the United States in January. But decent thumps of wintry precipitation along the East Coast always seem to be more high-maintenance than snow in other parts of the country. These systems are notoriously hard to predict more than a day or two in advance—an uncertainty that tends to drive people crazy, especially in an era when we expect (and often get) instant answers. 

GFS model forecast from January 18, 2016, showing surface pressure (mb) and wind speed (kt) for January 23, 2016. Image credit: Pivotal Weather

If there’s one thing we humans don’t like, it’s uncertainty. Telephone psychics, fortune cookies, and novelty toys like the Magic 8-Ball thrive on our undying desire to know what the future holds. Political junkies live and breathe opinion polls to know what voters will decide before they cast their ballots. Sports analysts spend hours trying to predict the outcome of games that haven’t yet started.

But unlike the outcome of an election or sporting event, we need to know what the weather will be like in order to live and survive. Meteorologists have it harder than these forecasters because the atmosphere is unlike humans in that we have no control over what it does. We can’t directly influence a storm to move one way or another. The weather is destined to happen as it happens, and it’s a meteorologist’s seemingly impossible task to figure out what it’s going to do long before a cloud forms.

The Northeast Megalopolis has about the same population as 12 Midwestern states combined. |mage credit: Dennis Mersereau

Predicting the whereabouts and actions of a snow or ice storm on the East Coast is an especially high-stakes exercise that requires an attention to science and skill. The Interstate 95 (I-95) corridor between Richmond, Virginia, and Boston, Massachusetts, is home to more than 50 million people; this strip of land, known as the Northeast Megalopolis, is home to about as many people as 12 Midwestern states combined. New York City alone has a greater population (about 8.4 million people) than 39 of the 50 states.

Given the great amount of people who live there, many of whom are packed tightly together, snow and ice that would be seen as a simple nuisance in colder and snowier places has the potential to turn into a disaster along the I-95 corridor, crippling ground and air travel, severing power to millions, and disrupting schools and businesses for a week or more. The freak-out before a storm makes this region the butt of jokes for its reaction, but whether or not the societal chaos that unfolds is justified, a lot of this anxiety stems from uncertainty, and East Coast storms are a hard nut to crack. 

But why? All of the big, historic snows that live in the record books in cities like Washington D.C. and New York were produced by a unique kind of East Coast storm known as a “nor’easter,” so called because the storm produces strong northeasterly winds along the coast. Nor’easters form when the dynamics in the upper levels of the atmosphere come together just right to form a low-pressure system at the surface that eventually tracks off the coast of the Mid-Atlantic—think North Carolina and Virginia—and moves parallel to the coast as it heads toward New England and eventually Canada.

Nor’easters can grow into very powerful storms, sometimes the strength and size of a hurricane. The strong winds wrapping around the low-pressure system often drag bitterly cold air from the west and warm, moist air from the south. The varying temperatures through the storm usually lead to the whole spectrum of precipitation, including snow, sleet, freezing rain, and regular rain. The temperature gradient can be so sharp that two neighboring towns can see completely different weather conditions, with one hit by heavy snow while the other gets ice or rain.

When you have such dramatic differences in weather over such short distances, the track of the storm is everything when it comes to determining who will see the worst snow and who will see a cold rain—and this is usually where the greatest uncertainty lies. It takes a very specific path and combination of atmospheric ingredients in order to produce feet of snow along the I-95 corridor. It’s challenging to determine the exact track of a low-pressure system—if you’ve ever followed a hurricane drawing closer to the coast, you know that the center wobbles and shifts and can sometimes go far off the track that meteorologists predicted it would follow. Nor’easters are similar in this regard. If a storm shifts a few dozen miles to the east or west, it can result in a city that expected rain to see all snow, or a city that braced for a blizzard to wake up to clear skies.

Weather radar from January 27, 2015, showing the heaviest snow staying just east of New York City. | Source: Intellicast.com

New York City grew intimately familiar with the perils of predicting East Coast snowstorms in January 2015, when forecasters expected a powerful blizzard to bring 2 feet of snow to the Big Apple. The nor’easter tracked a few dozen miles farther out to sea than they anticipated, and most of the city ultimately got less than 10 inches of snow, while Long Island got pounded with more than 2 feet of snow.

Weather is hard. Despite the difficulty, meteorologists usually manage to predict the weather with stunning accuracy—a level of accuracy they only dreamed of just a few decades ago. We’ve gotten really good at figuring out what the weather will do in the future, but there are still some limits. Determining the exact track—down to the mile—of a large snowstorm with lots of variables pushing and pulling and swirling about is very hard. It’s doable, but in many cases it’s tough, and there’s always inherent uncertainty built into even the best forecast. When your friendly neighborhood meteorologist calls for a large range of snow totals or says that they’re not quite sure what could happen yet, just prepare for the worst and hope for the best.