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Forecasting

Forecasting. ATS 113. Forecasts made by PEOPLE. Folklore: Groundhog Day Fuzzy caterpillars Walnut shells Farmers Almanac. Forecasts made by PEOPLE. Persistence: Assumes that the current weather is going to continue. Forecasts made by PEOPLE. Climatology

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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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