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

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. Figure 9.9a: A visible image of the eastern Pacific Ocean taken at just about the same time on the same day as the image in Fig. 9.9b. Notice that the clouds in the visible image appear white. Superimposed on the image is the cold, warm, and occluded fronts.
Watch this Active Figure on ThomsonNow website at
Fig. 9-9a, p. 242

13. Figure 9.9a: A visible image of the eastern Pacific Ocean taken at just about the same time on the same day as the image in Fig. 9.9b. Notice that the clouds in the visible image appear white. Superimposed on the image is the cold, warm, and occluded fronts.
Watch this Active Figure on ThomsonNow website at
Fig. 9-9b, p. 242

14. Figure 9.9a: A visible image of the eastern Pacific Ocean taken at just about the same time on the same day as the image in Fig. 9.9b. Notice that the clouds in the visible image appear white. Superimposed on the image is the cold, warm, and occluded fronts.
Watch this Active Figure on ThomsonNow website at
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 the specific 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. The forecast tomorrow is 75 degrees because the historical average for tomorrow is 75 degrees. What type of forecast is this?
Persistence
Climatological
Analogue
Probability

28. The forecast tomorrow is 60 degrees because the yesterday it was is 75 degrees. What type of forecast is this?
Persistence
Climatological
Analogue
Probability

29. There is a 40% chance of rain tomorrow. What type of forecast is this?
Persistence
Climatological
Analogue
Probability