"If you don't know what happened yesterday and why, then the chances of you being able to forecast the future are less," AccuWeather Meteorologist Bernie Rayno said.

Long-Range Forecasting Takes More Than a Crystal Ball

Meteorologists look at observations around the Western Hemisphere, like temperature, pressure, wind speed and precipitation. They start by looking at patterns, which show the major characteristics of the atmosphere.

Like peeling an onion from the outside in, the forecasters start with the biggest movements in the atmosphere before looking at the smaller details. To analyze the weather, they use tools like satellite and radar.

One tool that mets use that doesn't get much attention is the surface map. Surface maps give meteorologists an idea into why weather is happening. When analyzing a surface map, meteorologists can see where some of the key players in the weather are located, like high pressure, low pressure, cold fronts, warm fronts, cloud cover, wind and precipitation.

"Storm systems move. They're like cars. So if you know what [weather conditions are] associated with the storm or this car, as it moves east tomorrow, you can kind of guess, 'Well, if this storm system is doing this here, it may be doing this in the location where I think it's going to be tomorrow.'"

When looking at a surface map, meteorologists can generally locate storm systems by looking for areas of low pressure. High pressure generally indicates fair weather.

Using the Models

"Meteorologists have computer models which model the atmosphere using parameters such as barometric pressure, temperature, humidity and a plethora more of complicated mathematical equations," AccuWeather Meteorologist Erik Pindrock wrote, in an article about Exponential Changes in Forecasting Technology.

Scientists constantly update the physics of the models so a weakness of a model today might not be tomorrow.

"Here in the 21st century," Pindrock wrote, "meteorologists have several dozen computer models to look at. Within some of these individual computer models are other model forecasts with a slightly different output. These other model forecasts are referred to as a model ensemble. In other words, you start out with a single model and you perturb or alter the initial atmospheric conditions of that model. The end result is five, 10 or maybe 20 or more different outputs of one particular computer model. So in reality, meteorologists have hundreds of computer weather models to choose from!"

Collaboration

For example, one meteorologist might see a 3- to 6-inch snowfall from a storm that another meteorologist sees as leaving just a coating. When they compromise on their forecast, they're more likely to hit on what will actually happen.

"As in any field," AccuWeather Chief Meteorologist Elliot Abrams said, "where there may be differing opinions about things, and people have different levels of expertise, sharing and the give-and-take of ideas often lead to better product. Specifically, one person may be looking at a certain area and assessing one aspect of a weather forecast while a colleague may be looking at a different area. Working together they may think of things they hadn't thought of before for the forecast that they're working on."

The Sixth Sense

Lastly, some meteorologists say, tongue in cheek, that forecasting takes a sixth sense.

"I'm not saying that I can look into a crystal ball and say 'Aha! This is what's going to happen!' but the sixth sense [of forecasting] is used in conjunction with everything else: understanding the past, understanding the present," AccuWeather senior meteorologist Bernie Rayno said.

A forecaster's "sixth sense" develops over time. The "sixth sense" that meteorologists use is pattern recognition that comes from years of experience with forecasting. Weather cycles repeat, and when experienced meteorologists have seen certain cycles before, they use their gut feeling to guide their forecast.

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