Why is that? Baroclinic storms often have hot and cold air at the same altitude. (Readers from Colorado to Connecticut know that after a thunderstorm pushes through, it can sometimes cool off a warm day.) This difference in temperature leads to forward motion, pushing a storm through the atmosphere and driving weather across the planet.

“With a tropical cyclone, its very symmetric. The temperature differences driving it are vertical—there’s warm, wet air below, and cool, dry air up above—so there’s a vertical circulation, but it doesn’t move in a horizontal direction,” said Neal Dorst, a research meteorologist at NOAA’s Hurricane Research Division.

“There’s no impetus driving a tropical cyclone—it will have a tendency to stay where it is,” he said. If a hurricane appeared on a globe with no other weather systems, it would drift northerly only at a couple miles per hour, thanks to the Coriolis effect. It would not move in any other way, he told me.

The National Weather Service runs multiple weather models for each new forecast, which are issued four times a day.

Three different technological breakthroughs are key to the improved storm-track models, according to Kerry Emanuel, a professor of atmospheric science at the Massachusetts Institute of Technology. First, scientists now observe much more of a storm than they used to, through both constant satellite surveillance and data-rich aircraft flights through individual storms.

Second, meteorologists have improved their simulations of atmospheric processes, and they have better computers to resolve the math required by them. Thirty years ago, a high-quality atmospheric model might have had grid points 150 kilometers apart; now, they are five kilometers apart, he told me.

Finally—and most importantly, according to Emanuel—scientists are much better at “initializing” forecast models; that is, models now start their forecasts with a vision of reality that’s much closer to the real world. These initial states often start from a six- or 12-hour-old forecast, which the computer model then brings to a current state by assimilating new observations.

“The fact is that observations are few and far between—we’re certainly not measuring every molecule of the atmosphere,” said Emanuel.

About half of the computing power thrown at any hurricane forecast is spent entirely on describing this initial, current state, he said. The other half is spent projecting the future.

Yet those model runs alone do not determine what shows up in a broadcast or on a news site. (Unless someone has improperly tweeted a spaghetti plot.) Ultimately, the team at the National Hurricane Center might sit down with hundreds of simulated storm-track runs. Major models like the European or the U.S.-operated Global Forecast System also prepare dozens of “ensemble” runs, where they adjust small aspects of the weather’s initial state.