Chapter 9: Weather Forecasting

1. Chapter 9: Weather Forecasting • Acquisition of weather information • Weather forecasting tools • Weather forecasting methods • Weather forecasting using surface charts

2. Acquisition of Weather Information • To make accurate forecasts, we need to know what the weather is right NOW • A network of 10,000 land and sea based weather stations report data at least 4 times a day • World Meteorological Organization (WMO) • Responsible for keeping weather data from 175 nations in the same format • Example : Baghdad History

3. Acquisition of Weather Information • Observations come to National Center for Environmental Prediction (NCEP) • Analyze the data, then transmit to local public and private offices via the National Weather Service (NWS) • All forecasters in the country use information provided by NCEP and NWS • Watch - issued when hazardous weather is expected for a specific period of time • Warning – indicates hazardous weather is imminent of occurring already

4. Acquisition of Weather Information • Advisory – to inform public is less hazardous conditions • For example: High wind warning is issued when sustained winds are at least 40 mph of gusts have reaches 57 mph. • Severe thunderstorm watches are issued thunderstorms with strong winds and large hail are possible • Full list of watches and warnings on page 237.

5. Weather Forecasting Tools • Advanced Weather Interactive Processing System (AWIPS) • Allows forecasters to put all tools in one program • Can look at observations, radar, maps, etc. • Meteogram – a charge that shows how variables change for a given time • Sounding – two-dimensional vertical profile of temperature, dew point, and winds

6. Weather Forecasting Tools

7. Weather Forecasting Tools

8. Satellites and Weather Forecasting • Before satellites, tropical storms and hurricane went undetected. Now, they provide advanced warning • Geostationary satellites – orbit at the same rate as the Earth is spinning. Example : Here

9. Satellites and Weather Forecasting • Polar orbiting satellites – parallel median lines and move from pole to pole • Orbit at a lower altitude (greater detail) • Can get the poles with no distortion

10. Satellites and Weather Forecasting • Geostationary operational environmental satellite (GOES) • New generation satellite • Can detect amount of radiation coming from clouds • Can obtain info about moisture at different levels • Infrared images – shows how much radiation clouds are emitting. (Diff between high and low clouds?) • Visible images – Detect radiation that is reflected to satellite (like a photograph)

11. Satellites and Weather Forecasting

12. Fig. 9-9a, p. 242

13. Fig. 9-9b, p. 242

14. Fig. 9-10, p. 243

15. The Computer and Weather Forecasting: Numerical Weather Prediction • Analysis – a final interpretation of a weather chart after human correction • Numerical weather prediction – the use of computers to predict the weather using complex math • Atmospheric models – mathematical models that create a picture of the current atmosphere • Progs (prognostic chart) – a chart weather forecast for a time in the future • One of the world’s first computers was built for thespecific purpose of performing weather forecasts.

16. Why NWS Forecasts Go Awry and Steps to Improve Them • Bad assumptions by a model (sometimes good and sometimes bad) • Grid spacing varies for each model • Chaos – small, unpredictable changes • Ensemble forecasting • Running multiple models to obtain a consensus

17. Other Forecasting Methods • Persistence forecast – forecast for tomorrow will be what happened today • Trend forecast – atmos movements will remain the same in the future • Analogue method – current patterns will provide similar weather to similar patterns in the past • Statistical forecast – forecasts weighted and made by a computer

18. Other Forecasting Methods • Probability forecast – gives a percent change of something (what does 40% change of rain mean?) • Climatological forecast – forecast based on climatology

19. Other Forecasting Methods

20. Types of Forecasts • Very short range forecast – forecast for up to few hours (nowcast) • Short range forecast – generally a forecast for up to a few days • Medium range forecast – 3 to 8 days out • Long range forecast – beyond 8 days out • Long-range forecasts are less specific than short range forecasts.

21. Types of Forecasts

22. Accuracy and Skill in Forecasting • Forecasts of a few hours to a few days are good • Forecast of a few days are better than “flipping a coin” • Long range forecasts are generally very bad • What is a bad forecast? Is a forecast off by a few degrees bad? • If I say that it will not rain in the summer in San Jose, does that show skill? • The skillful forecast would be to say when it WOULD rain in San Jose in the summer

23. Accuracy and Skill in Forecasting • Forecasts have gotten better over the past 15 years due to computer models • 7-day forecast now are as skillful as 3-day forecast 10 years ago • Forecasting thunderstorms and tornadoes are extremely difficult

24. Predicting the Weather From Local Signs • Book of Signs was written in 300 B.C. to try to predict the weather • Used color and shape of clouds. Also intensity of which a fly bites • Halo around the sun – a halo around the sun could mean rain is on the way. Halo made by bending light through cirrostratus

25. Predicting the Weather From Local Signs

26. Table 9-2, p. 254

27. Determining the Movement of Weather Systems • Forecasting rules of thumb • Storms are fronts tend to move in the same direction with the same speed as previous 6 hours • Low pressure systems move parallel to isobars in warm air region • Lows move where pressure drops fast, highs where pressure rises fast • Systems move in same direction as wind in 500-mb level

28. Determining the Movement of Weather Systems • forecasting rules of thumb • using the surface chart

29. A Forecast for Six Cities • Augusta, Georgia • Washington, DC: rain or snow? • Chicago: major snowstorm • Memphis: mixed bag • Dallas: cold wave • Denver: clear and cold

30. Stepped Art Fig. 9-21, p. 256