People walk in the middle of a car-free Fifth Avenue in New York on Tuesday, Jan. 27, 2015. AP Photo/Seth Wenig

"If you asked me to concoct a situation with high potential for a busted forecast, I’d concoct the one we had yesterday."

New York City got a snow storm overnight, but "historic" it wasn't—unless you count the subway being shut down for the first time for that kind of weather* in all its 110 years. Despite forecasts calling for up to three feet of snow, New York's official snow accumulation tally was a decidedly more modest 10 inches or less.

While New York City Mayor Bill DeBlasio and state Governor Andrew Cuomo defended their widespread travel bans and transit shutdowns as "better safe than sorry," Gary Szatskowsi, meteorologist-in-charge at the Philadelphia/Mt. Holly National Weather Service Forecast Office, made a striking apology for his part in the forecast on Twitter.

Of course, plenty of places—including Boston, Long Island, and poor, submerged Nantucket—got legitimately dumped with snow last night, and are still getting walloped. But clearly, New York City's experience didn't come close to meeting expectations.

So what went wrong with this forecast? According to Jon Michael Nese, Associate Head of Penn State's Undergraduate Program in Meteorology, the storm's characteristics had "unpredictable" written all over them. "If you asked me to concoct a situation with high potential for a busted forecast, I’d concoct the one we had yesterday," he says.

Part of the trouble is that the storm developed in two parts. It started as a small clipper in the Midwest, where it dropped a few inches of snow. But as it moved east, it was pushed off the coast, which is where it strengthened rapidly (take a look at the NASA satellite video below for the blow-by-blow). Nese says these kinds of storms are notoriously hard to predict, compared to ones that, say, develop in the Gulf of Mexico or off North Carolina and then come up the coast after dumping rain and snow for several days.

"Those [latter] storms have existed for a while, so they're trackable," says Nese. "The ones that get strong right off the coast don't have a history, so it's particularly difficult to say what'll happen."

The fact that this storm gathered strength over the sea also hampered NOAA's ability to predict exactly where it would hit hardest—which depends tremendously on where it begins. Initial location data may have been off. "In the Northeast, 50 to 100 miles is going made a huge difference between who gets a few feet of snow and who gets a few inches," says Nese.

It's also difficult to make precise observations about things like air temperature, air pressure, air moisture, and wind when a storm develops off the coast. "We have some buoys out on the sea, but compared to the dense network of weather instruments we have on land, it's much harder to get good information," says Nese.

Without complete observations, the forecast models can't be as exact. And although on average, weather forecasts are more accurate than ever, Nese says it's unwise to predict specific snow amounts too far in advance—there are just too many moving parts.

*This article has been updated to reflect the fact that this was the first time the subway was shut down for a snowstorm, not ever.

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