What’s the difference between a weather forecast and a climate scenario?

Predicting temperature and precipitation for a local area is 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 provide more accuracy than was possible 40 years ago.

Forecasting day to day weather and predicting what climate will be 5 or 50 years from now are very different in outcomes. Both processes use 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.
 

Forecasts

For very short-term forecasts, there is typically only one forecast model used, 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 every day. These slight inaccuracies can be amplified as the forecast reaches out multiple days.

To alleviate the uncertainty, several forecast models are used. Each model starts out with weather conditions that are slightly different, to try to mimic what may not be exactly known at the beginning. This also limits imperfections in the math used in the models that approximate physical conditions on earth.

Ensemble forecast

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 longer range forecasts compared to several decades ago.
 

Climate Scenarios

With climate prediction, this type of 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 using one average forecast, a range of scenarios are used. The scenarios range, for example, from high emissions of carbon dioxide and other greenhouse gases throughout the forecast period to lower emissions during that time. Scenarios can provide information that allow governments, businesses, and others a chance to make decisions now that can influence future conditions.