Why is There Uncertianty in Weather Forecasts

Weather Forecast Uncertainty
National Weather Service Lincoln IL
(Modified from original explanation put together by National Weather Service Louisville KY)
Why is there uncertainty in forecasts?
·      Uncertainty can arise due to the complex nature of our atmosphere, which is sensitive to initial weather conditions (i.e. what is happening right now).
·      A slightly inaccurate initial state of the atmosphere may result in uncertainty with time in computer model forecasts that are used by forecasters.
·      Models are a simulation of the atmosphere, and their accuracy is limited by how precisely they can represent complex atmospheric processes. Some processes are difficult to simulate perfectly in a computer which can lead to error.
·      NWS forecasters evaluate model output, incorporating knowledge of model strengths and weaknesses to prepare official NWS forecasts, which can reduce some uncertainty.
·      Ensemble forecasting has been implemented recently, where a series of slightly different initial weather conditions are inputted to a model to determine various possible outcomes.  If different ensemble model members show much different results in time, then forecast uncertainty is higher for a given situation. However, if ensemble model members agree well, then uncertainty is reduced and NWS meteorologists can be more confident in their official forecasts.  


Overall, forecast confidence decreases in time:

Forecast accuracy can be limited for a number of reasons:

  • Limitations in observations
  • Data errors/inconsistencies
  • Insufficient data at the times and locations needed (particularly over oceans)
  • Incomplete understanding of complexities and interrelations between atmospheric physics and chemistry
  • The need to approximate complex atmospheric processes
Where does forecast uncertainty come from?

How to deal with uncertainty?

  • Monitor the weather on a daily basis, including forecasts on the NWS Lincoln website
  • Avoid being complacent when the long-range forecast indicates benign weather
  • Be cognizant when conditions are changing rapidly
  • Provide the NWS with reliable weather reports through social media on rapidly evolving situations
  • Understand how to interpret probabilities
  • Think about potential weather impacts and have a preparedness plan in place


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