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Forecasting

Weather forecasting has evolved, utilizing both traditional methods and advanced computer models. Folklore techniques, such as Groundhog Day predictions or the Farmers’ Almanac, rely on historical weather patterns. In contrast, computer-generated forecasts, known as Numerical Weather Prediction (NWP), leverage detailed observations and complex mathematical equations to predict future conditions. By assimilating real-time data, these models apply fundamental laws of physics to forecast hourly to several days ahead. Dive into the fascinating world of forecasting, where human intuition meets computational power.

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Forecasting

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  1. Forecasting ATS 113

  2. Forecasts made by PEOPLE • Folklore: • Groundhog Day • Fuzzy caterpillars • Walnut shells • Farmers Almanac

  3. Forecasts made by PEOPLE • Persistence: • Assumes that the current weather is going to continue.

  4. Forecasts made by PEOPLE • Climatology • Assumes that the weather in the near future will be a lot like the mean weather of the last 30 years.

  5. Forecasts made by PEOPLE • Analog forecasts: • Find times in the past in which the weather was just like the current weather • Assumes that the weather in the near future will be just like the weather was after these analogous times

  6. Forecasts made by COMPUTERS • A.K.A. “Numerical Weather Prediction”, NWP • A.K.A. “Modeling”

  7. Forecasts made by COMPUTERS • Starts with good OBSERVATIONS OF THE CURRENT WEATHER • Weather prediction is an “initial value problem”

  8. Forecasts made by COMPUTERS • Assimilation of observations

  9. Forecasts made by COMPUTERS • Seven “Primitive Equations”: • Three equations about forces: gravity, pressure gradient force, coriolis force, etc.

  10. Forecasts made by COMPUTERS • Seven “Primitive Equations”: • The Ideal Gas Law

  11. Forecasts made by COMPUTERS • Seven “Primitive Equations”: • The First Law of Thermodynamics: about moving heat around (radiation, conduction, convection, etc.)

  12. Forecasts made by COMPUTERS • Seven “Primitive Equations”: • The Continuity Equation (about how convergence and divergence lead to rising and sinking motion)

  13. Forecasts made by COMPUTERS • Seven “Primitive Equations”: • Conservation of Water – about how evaporation and condensation affect the amount of water vapor in an air parcel

  14. Forecasts made by COMPUTERS • The Seven Primitive Equations are prognostic—they make a forecast of the temperature, pressure, winds, humidity, etc. for some time in the future. • About ONE SECOND in the future.

  15. Forecasts made by COMPUTERS • Take the current OBSERVATIONS. • ASSIMILATE the observations into the model grid. • Apply the 7 equations to every gridpoint to get a forecast for about 1 second later. • Repeat step 3 until you have a forecast for 12, 24, 36, 72, 120 hours into the future!

  16. Forecasts made by COMPUTERS • Resolution versus domain:

  17. Current ETA forecast: 12 hours

  18. Current ETA forecast: 24 hours

  19. Current ETA forecast: 36 hours

  20. Current ETA forecast: 48 hours

  21. What’s going on in the weather?

  22. 48 hr ETA forecast: Pressure

  23. 48 hr ETA forecast: Temp.

  24. 48 hr ETA forecast: Precip

  25. 48 hr ETA forecast: Snow?

  26. What do the other models say?

  27. NGM: No snow

  28. AVN: Lots of snow in MO!

  29. Why the difference? • Different resolutions • Different approximations to the 7 primitive equations • Different errors in the model

  30. Ensemble Forecasting • Using MANY models of the weather to reach a consensus forecast • Typically at least a dozen models!

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