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Utility of Ensemble Techniques: 7-day forecast example

This press release discusses the value of ensemble techniques in weather forecasting, illustrated through two case studies on predicting heavy rains. It delves into evaluating forecast probabilities, ROC analysis, skill scores, and the way forward for improving forecast guidance and efficiency.

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Utility of Ensemble Techniques: 7-day forecast example

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  1. Utility of Ensemble Techniques:7-day forecast example

  2. Two Case Studies • Press release of expected first summer rains around 7 October 2003 • 7/8-day forecast • Lessons • Heavy rain over NW province and Gauteng 19 October 2003 • Predictability of cut-off low event • Pin-pointing area of heavy rain

  3. Evaluating Probability Forecasts • Calculate hit rates and false alarm rates for the various forecast probabilities • Relative Operating Characteristic (ROC) is widely used for this purpose • Skill score ranges from –100% to 100% • Perfect score means no false alarms • Zero score when FAR and HR equal

  4. ROCSS for DJF months 2001-2003

  5. ROCSS for DJF months 2001-2003

  6. ROCSS as a function of lead time

  7. Frequency of forecast probabilities by observationDJF in Northern Freestate

  8. Frequency of forecast probabilities by observationJJA in Southwestern Cape

  9. Ensemble Spread as a Predictor of Skill

  10. Ensemble Spread as a Predictor of Skill 99.9% 99% 95% 90%

  11. Way Forward • Idea is to provide useful forecast guidance so that the forecaster can work smarter – i.e. better forecasts with less work • Prediction research undertakes to assist forecasters where possible by: • Providing useful products (smarter not more) • Maintaining forecast techniques on a par with other centres around the world • Handling requests speedily

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