How do they make those hurricane modeling charts? And what do they predict for Lee? (2024)

As Hurricane Lee continues to roar across the Atlantic with sustained winds of 155 mph, people along the East Coast of the U.S. are naturally growing more curious about the storm's future path.

The storm was moving at 13 mph Friday and was expected to slow in the coming days, and wouldn't reach the Northeast for more than a week. So forecasters can't yet describe with confidence what the storm's path will be.

But the ensemble computer modeling systems that the National Oceanic and Atmospheric Administration and the European Centre for Medium-Range Weather Forecasts can give some clues. NorthJersey.com spoke with Steven Decker, an expert on weather analysis and forecasting at Rutgers University and John Scala, a certified consulting meteorologist and instructor for the Federal Emergency Management Agency, to help readers understand how to interpret those modeling charts and how they are made.

The mathematical premise of chaos theory

The modeling is built on the mathematic premise of chaos theory — very tiny changes to the initial data when running the models can change the longer term forecast completely, Decker said.

At the start of each model run, the modelers take atmospheric data from around the world on wind speeds, air temperatures and other measurements. When hurricane hunters take measurements inside a hurricane, there's still some uncertainty in their observations.

So the modelers make plausible slight variations, say, to the temperature in one location at a particular point in time, and that could create a hurricane path that varies a bit from the others.

In other words, each of the lines showing the potential path of a hurricane in these models starts from a slightly different but equally plausible set of data. "So each should be equally likely" to occur, Decker said.

More: Hurricane Lee remains a major Category 4 storm with 155 mph winds. Will it hit NJ?

But if you run 20 or 50 different scenarios and most of the paths turn out to be similar, the likelihood that an aberrant path will be the one a hurricane actually takes is quite low, Scala said. "When you see an outlier that's showing the outside envelope of what could happen," he said. "We don't completely discount it, but it's a very low probability."

In some cases, the outlier might be driven by climatology rather than what's actually happening in the upper levels of the atmosphere, Scala said — prior storms in this current position and strength have historically made landfall on the East Coast.

So what do the models for Hurricane Lee show?

Let's look at the latest modeling on Hurricane Lee. The NOAA modeling system is called the Global Ensemble Forecast System. The ensemble produced Friday morning shows most possible paths to be quite similar — Lee, currently heading northwest, with eventually turn north, and then northeast, putting Bermuda in potential harm and eventually making landfall in the Maritime provinces of eastern Canada, but missing the East Coast of the U.S.

One of the paths has Lee making landfall at Montauk on the eastern tip of Long Island. But that's just one path of many in the ensemble, so the chances are low that would happen, Decker said.

The European models run Thursday show a much wider spread among the potential paths. But again, they show Lee heading northeast away from the East Coast, except for one path that had Lee making landfall in the middle of Long Island — again, at this point a low probability.

Obviously, as modelers receive new data on Lee's actual behavior, they update their models and the possible storm paths could change. For instance, Friday's European models showed no potential paths for Lee hitting Long Island.

The National Center for Atmospheric research runs the Tropical Cyclone Guidance Project, which also produces hurricane modeling maps. They too so far show Lee most likely heading northeast toward Canada and avoiding the East Coast, though one path runs northwest across Long Island and New York City and on into upstate New York.

Why is Lee likely to shift its path

So why do most of the scenarios show Lee eventually shifting its path to the north and then the northeast?Decker and Scala said that a trough, or upper level disturbance, is expected to arrive, which would help the mid-latitude westerly winds push Hurricane Lee away from the East Coast.

So the models are trying to account for the trough's future arrival by showing Lee's path shift north and then northeast, Scala said.

How quickly the trough arrives and how strong it is will help determine Lee's ultimate path, Decker said.

Why do the storm paths have different colors?

The varying colors of the storm paths on the modeling charts indicate expected minimum pressure for the hurricane in each scenario.

The NOAA paths for Lee are mostly red, indicating very low pressure and a strong storm. Some of the European paths from Thursday's modeling were orange, yellow or even green, showing higher pressure. But that modeling from Thursday did not reflect how quickly Lee intensified into a major hurricane, Decker said. The Friday European ensemble showed more paths in red.

To check daily updates to both the National Weather Service and the European models, visit tropicaltidbits.com.