1. The Forecast Process

2. Objectives Develop a best practice scheme for the use of data in producing a forecast Produce a ‘flow chart’ illustrating this for reference purposes

3. Forecast Process We need to develop a best practice scheme for producing a forecast From earlier lectures we know we have to take the Strengths & Weaknesses of the NWP into account We will work through a case study to illustrate the forecast process

4. Forecast Process Analysis Verification Continuity Extrapolation Development & decay NWP Climatology and experience

5. Forecast Process ANALYSIS Verification Continuity Extrapolation Development & decay NWP Climatology and experience

6. ANALYSIS – getting the start right Surface synop, metar, ship and automatic observations Upper air observations – mainly tephigrams Satellite imagery Global sferic (lightning discharge) reports Radar rainfall imagery

7. VT 1200 Z 18/10/2006

9. Forecast Production process AnalysisAnalysis VERIFICATION Continuity Extrapolation Development & decay NWP Climatology and experience

10. VERIFICATION - checking the analysis against: Previous forecasts NWP And then note the differences i.e. Faster or slower Bigger or smaller Stronger or weaker

12. COTONOU VT 181200Z

13. Forecast Production process Analysis Verification CONTINUITY Extrapolation Development & decay NWP Climatology and experience

14. CONTINUITY – no change Important forecast tool in the Tropics  What happened yesterday is going to happen again today! Less so in high latitudes BUT….. Must be considered for NOWCASTING (0-6 hours ahead)  What is happening now will still be happening in 6 hours

15. Forecast Production process Analysis Verification Continuity EXTRAPOLATION Development & decay NWP Climatology and experience

16. EXTRAPOLATION – continuity of movement Thunderstorms 100 NM to east of station moving westwards at 25 KT Expected at station in 4 hours OK up to T+2 Poor forecasts by T+6

18. Forecast Production process Analysis Verification Continuity Extrapolation DEVELOPMENT & DECAY NWP Climatology and experience

19. DEVELOPMENT AND DECAY – getting worse or better? Theoretical knowledge:  Eg Upper trough approaching making showers heavier and more frequent  Strengths & Weaknesses of NWP Effect of hills and water Diurnal effects

20. It is 1200 Z – is the upwind shower likely to develop or decay?

21. ALAM Wind forecast valid 1800 Z Low level moisture Low level convergence

22. Forecast Production process Analysis Verification Continuity Extrapolation Development & decay NWP Climatology and experience

23. NWP – model generated forecast products  MUST BE VERIFIED AT ANALYSIS STAGE FIRST Height, pressure and wind fields Humidity and precipitation fields  Most models do not advect showers  Shower signal equates to the potential for shower development Thickness fields Forecast tephigrams Severe convection indices.

24. 1200 Z verification Good Poor

25. A

26. Decision time… Will there will a shower/thunderstorm at point A between 1200 and 1800? Extrapolation suggests that shower/thunderstorm will occur at 1600 Z Diurnal effects suggest that shower will develop NWP (verified well at 1200 Z) indicate potential for showers between 1200 Z and 1800 Z So what actually happened?

27. VT 1800 Z 18/10/2006

28. Poor

29. Forecast Production process Analysis Verification Continuity Extrapolation Development & decay NWP CLIMATOLOGY AND EXPERIENCE

30. CLIMATOLOGY & EXPERIENCE Climatology:  Snow in Bahamas in June?  Heatwave in Mongolia in January? Experience:  Geographical knowledge  Seen it all before!

31. NWP verification for forecast periods beyond T+36 ‘Poor man’s ensemble’  Compare model output from different centres  High forecast confidence if there is good inter- model consistency  Recent individual model ‘history’ EPS products  High forecast confidence if there is good inter- member consistency.

32. Any questions?