Chapter 9 Weather Forecasting Acquisition of weather information

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

Acquisition of Weather Information n n World Meteorological Organization National Centers for Environmental Prediction Watches: favorable condition for potential hazardous weather Warnings: hazardous weather is imminent or actually occurring Advisories: like warning but for less hazardous weather • The internet plays a crucial role in the global communication of weather information. • Go to: http: //www. weather. gov

Weather Forecasting Tools n AWIPS: Advanced Weather Interactive Processing System used by forecasters and can process satellite, radar, surface observation, radiosonde data and weather forecasting model output n n meteogram sounding

Near Orlando, FL Hail size > 3 inch 100% probability Fig. 9 -1, p. 238

Satellites and Weather Forecasting n n n n geostationary satellites polar orbiting satellites Geostationary Operational Environmental Satellite infrared images: estimate height; 3 -D image visible images: not useful at night water vapor images: particularly useful for clear sky http: //www. goes. noaa. gov

Cannot see polar areas 850 km altitude Fig. 9 -5, p. 240

Fig. 9 -7, p. 241

Visible Infrared Enhanced Infrared Fig. 9 -9 a, p. 242

Fig. 9 -11, p. 243

The Computer and Weather Forecasting: Numerical Weather Prediction n Analysis: final chart using available data n numerical weather prediction (NWP): based on models n atmospheric models: fluid dynamics and atmospheric physics n Progs: prosnostic chart for weather forecast • One of the world’s first computers was built for the specific purpose of performing weather forecasts.

48 -hr NWP forecast of 500 mb Height 12 km vs 60 km Resolution refers to grid spacing Fig. 9 -12 a, p. 245

Why NWS Forecasts go Awry and Steps to Improve Them n n n grid spacing incomplete data coverage model deficiencies due to subgrid processes chaos ensemble forecasting: spaghetti plot to indicate the robustness of forecast • Chaos theory tells us that weather forecasts are highly sensitive to our ability to observe the weather. Since it is impossible to observe the weather at all places at all times, weather forecasts will never be perfect.

Other Forecasting Methods n n persistence forecast: not bad for Tucson in June trend forecast: assuming constant change rate analogue method: search for similar chart in history statistical forecast: routinely used; Model Output Statistics (MOS) n n probability forecast: particularly for precipitation climatological forecast: good for Tucson rainfall in June

What does it mean by `chance of rain is 50% for one area’? Choose the right answer a. It will rain over 50% of the area b. 50% chance that rainfall will not occur in the whole area c. 50% change that any random location in the area will receive measurable rainfall in the given period Table 9 -1, p. 249

Probability for a `white Christmas’ – 1 inch or more of snow Fig. 9 -15, p. 250

Types of Forecasts n n n very short range forecast or nowcast: 0 -6 hr short range forecast: 6 hr – 3 days Mediate-range forecast: 3 -8. 5 days long range forecast: 8. 5 days – 2 weeks Monthly and seasonal outlooks • Long-range forecasts are less specific than short range forecasts.

Fig. 9 -17, p. 251

Accuracy and Skill in Forecasting persistence n climatology n • Both persistence and climatology are surprisingly accurate forecasting methods.

Predicting the Weather from Local Signs n Halo: `a halo around the moon portends rain’ (foldlore) • To see a halo: block out the sun with your hand look at the cirrostratus clouds. Wear polarized sunglasses if possible.

Determining the Movement of Weather Systems n forecasting rules of thumb: surface pressure systems tend to move in the same direction as the 500 mb wind; the speed at which surface systems move is about half the wind speed at 500 mb n using the surface chart • Internet now provides much of the weather information http: //www. weather. gov http: //www. atmo. arizona. edu http: //www. wrh. noaa. gov/twc

Current front and front 6 hr ago Estimate for the next 24 hr 500 mb height observation for 24 hr later Fig. 9 -20, p. 256