The Atlantic Ocean was pretty quiet this July, not even giving us so much as a belch of tropical winds since June. Don’t let the quiet fool you—it's now August, and we’re getting closer to the peak of hurricane season with each passing summer afternoon. As the tropics start to wake up, one of the most common weather images you’ll see as storms begin to form is a graphic called a “spaghetti model.” A spaghetti model (sometimes called a “spaghetti plot”) is a valuable product generated by weather models. But it can be easily misunderstood if you don’t know how to read it. 

Weather models are complex computer programs that combine our knowledge of how the atmosphere works with past and current weather information in order to predict what the weather will do over the next week or two. This guidance slowly grows less accurate as time goes on, with seven days usually being the outer limit for confidently predicting specific weather events.

A weather model forecast from July 29, 2016, showing six-hour rainfall amounts and sea level pressure over the Caribbean Sea on August 3, 2016. | Image credit: Tropical Tidbits

Meteorologists don’t use just one computer model to predict the weather. They use a wide array of different models to produce their forecasts, as each model has its own strengths and weaknesses. Looking at several different models can help you avoid making an error based on one model’s biases—for example, if you look at a model that has a “wet bias,” using that model exclusively to predict rainfall amounts could lead to you calling for a deluge when the town only sees some drizzle. 

Each major weather model produces one run that most people focus on, called the “operational” run. If you were to open up any website that provides weather model images, odds are that you’re looking at the operational model. These model runs are useful, but they don’t show you the whole picture. This is where “ensemble” weather models come in, and where we get to spaghetti models.

An ensemble weather model is one where they run the same model dozens of times for the same time period, starting each run with slightly different initial conditions. Starting each model run with a different initial environment—say, one scenario starts with strong winds, another starts with light winds, the next run begins with storm nearby—seems negligible at first, but it can have dramatic consequences on the model’s predicted strength and track of a storm over the next few days. One environment with certain levels of moisture, heat, and winds could let a storm explode into a major hurricane, while a slightly different environment could cause that same storm to fall apart and dissipate. The ensemble weather models account for that. It’s the butterfly effect in real life.

A spaghetti model plot showing potential tracks for Hurricane Joaquin in October 2015. The consensus of the models shows the storm moving out to sea, which is the track Joaquin ultimately took. | Image credit: NCAR

When meteorologists run these ensemble weather models to predict the track of a hurricane, each run will produce a slightly different outcome based on the initial conditions they started with. The end result looks like spaghetti thrown on a map, with dozens of different tracks zigzagging here and there. This allows forecasters to cover their bases, giving them an idea of where a hurricane could travel based on lots of different scenarios so that if one thing changes, like a shift in prevailing winds, that change is already factored into their forecast.

The greatest strength in social media’s ability to rapidly spread weather information is also its greatest weakness. Weather hoaxes are the most dangerous when they go viral, but weather models provided without context can also go viral, leading people to believe that they’re looking at a quality-controlled forecast when they’re not. Spaghetti models are one of the most common pieces of weather information that quickly go viral, giving meteorologists a headache when people freak out over them.

Spaghetti models are only guidance that help trained forecasters predict where the center of a tropical storm or hurricane will travel. Usually the spaghetti models point you in the right direction, but in complicated situations, they can be completely wrong.

If you come across spaghetti model images this summer, remember to look at the overall pattern in the lines and not any specific track that might bring doom and gloom to your area. It’s best to focus on forecasts produced by the experts at the National Hurricane Center—they know what they’re doing, and if they get it wrong, the models probably didn’t get it right in the first place.