Forecasting day-to-day weather and predicting what climate will be five years or 50 years from now have very different outcomes. Both processes use most of the same land, water, and sea parameters, but the range for climate forecasts can also vary due to human activity. For climate forecasts, scientists may use the word "scenario" instead.

In today’s world, predicting temperature and precipitation for a local area is pretty accurate for at least a few days out. However, with any forecast, the farther out the prediction, the more variation that comes into play. Meteorologists now can take variability into account in longer-term forecasts, helping to provide more accuracy than was possible 40 years ago.

Short-term weather forecasting

Short-term forecasts typically use only one forecast model that can be very accurate, even on a relatively small geographic scale. Forecast models take a lot of computer capacity to run. Even though these are very accurate, there is always some uncertainty in weather conditions since there is no way to know exactly what is going on in the atmosphere, on land, and at sea every minute of every day. These slight inaccuracies can be amplified as the forecast reaches out multiple days.

Using several forecast models together can help to alleviate uncertainty. Each model starts out with slightly different weather conditions to produce a range of forecasts that include multiple possibilities. This also reduces calculation imperfections in the models that approximate physical conditions on earth.

When all the models are examined, the average of all models is used as a starting point. This is called an ensemble forecast. Using the ensemble forecast has increased the accuracy of longer-range forecasts compared to several decades ago.

Predicting climate scenarios 

With climate prediction, ensemble forecasting is used extensively. Climate models also must consider how future human activity may change over time. For example, will carbon dioxide emissions increase, stay the same, or decrease? These differences can make a big difference in what the prediction becomes for decades out.

For that reason, instead of using one average forecast, a range of scenarios is used. For example, multiple scenarios may predict both high and low emissions of carbon dioxide and other greenhouse gases throughout the forecast period. Scenarios can provide information that allows governments, businesses, and others a chance to make decisions now that can influence future conditions.

ABOUT THE BLOG: All About Weather explores the environment, climate, and weather topics for Illinois. Get in-depth information about things your weather app doesn't cover from summer droughts to shifting weather patterns. Never miss a new post! Sign up for our email list.

MEET THE AUTHOR: Duane Friend is the Illinois Master Naturalist Coordinator and Climate Specialist with University of Illinois Extension, serving the organization in many roles since 1993. Duane provides information and educational programs to adult and youth audiences in the areas of soil quality, weather and climate, energy conservation, and disaster preparedness. These programs provide practical solutions for families, farms, and communities.  He assists families in creating a household emergency plan, farmers with the implementation of soil management and conservation practices, and local government officials and business owners with energy conservation techniques.