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”

9. Forecasts made by COMPUTERS Assimilation of observations

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

12. Forecasts made by COMPUTERS Seven “Primitive Equations”: –The Ideal Gas Law

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

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

15. 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

16. 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.

17. Forecasts made by COMPUTERS 1.Take the current OBSERVATIONS. 2.ASSIMILATE the observations into the model grid. 3.Apply the 7 equations to every gridpoint to get a forecast for about 1 second later. 4.Repeat step 3 until you have a forecast for 12, 24, 36, 72, 120 hours into the future!

18. Forecasts made by COMPUTERS Resolution versus domain:

24. Current ETA forecast: 12 hours

25. Current ETA forecast: 24 hours

26. Current ETA forecast: 36 hours

27. Current ETA forecast: 48 hours

28. What’s going on in the weather?

29. 48 hr ETA forecast: Pressure

30. 48 hr ETA forecast: Temp.

31. 48 hr ETA forecast: Precip

32. 48 hr ETA forecast: Snow?

33. What do the other models say?

34. NGM: No snow

35. AVN: Lots of snow in MO!

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

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