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

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

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

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

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

Weather Forecasting Tools

Weather Forecasting Tools

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

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

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)

Satellites and Weather Forecasting

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 www.thomsonedu.com/login. Fig. 9-9a, p. 242

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 www.thomsonedu.com/login. Fig. 9-9b, p. 242

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 www.thomsonedu.com/login. Fig. 9-10, p. 243

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.

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

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

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

Other Forecasting Methods

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.

Types of Forecasts

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

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

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

Predicting the Weather From Local Signs

Table 9-2, p. 254

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

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

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