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 1

Acquisition of Weather Information n n World Meteorological Organization National Centers for Environmental Prediction n Watches: favorable condition for potential hazardous weather; what you need to do: pay attention n Warnings: hazardous weather is imminent or actually occurring; what you need to do: take action n Advisories: like warning but for less hazardous weather, what you need to do: take action if necessary • The internet plays a crucial role in the global communication of weather information. • Go to: http: //www. weather. gov 2

Weather Forecasting Tools n n AWIPS: Advanced Weather Interactive Processing System used by forecasters and can process satellite, radar, surface observation, radiosonde data and weather forecasting model output Sounding Meteorgram Q: Is there fog based on the sounding? a) yes, b) no 3

Near Orlando, FL Hail size > 3 inch 100% probability 4 Fig. 9 -1, p. 238

Satellites and Weather Forecasting n n n n Geostationary satellites Geostationary Operational Environmental Satellite (GOES) Polar orbiting satellites infrared images: estimate height; 3 -D image visible images: not useful at night water vapor images: particularly useful for clear sky Satellite sounder data of temperature and humidity Q: Can a geostationary satellite cover polar regions? a) yes, b) no Q: Can a polar-orbiting satellite cover the whole earth? a) yes, b) no 5

Geostationary Satellite: cannot see polar areas; 36, 000 km above equator at fixed location; Same speed as earth’s rotation Polar-orbiting Satellite: 850 km above surface; nearly follow the meridion; cover the whole earth 6 Fig. 9 -5, p. 240

3 -D infrared imagery 7

Visible Infrared Enhanced Infrared 8

Infrared water vapor channel 3 -D TRMM satellite image Go to: www. weather. gov for real-time satellite information. Use 24 hr loop 9

The Computer and Weather Forecasting: Numerical Weather Prediction n n Analysis: final chart using available data at present numerical weather prediction (NWP): based on computer models, starting from initial time with “analysis” data atmospheric models: fluid dynamics and atmospheric physics Progs: prosnostic chart for weather forecast for a specific future period • One of the world’s first computers was built for the specific purpose of performing weather forecasts. Q: Which one is for the current condition? a) analysis, b) prog, c) both 10

48 -hr NWP forecast of 500 mb height 12 km vs 60 km grid spacing Q: which model forecasts the low (L) off the west coast better? a) 12 km regional model; b) 60 km global model analysis 11

Why NWS Forecasts go Awry and Steps to Improve Them grid spacing: models cannot resolve features within a grid cell (e. g. , 40 km for global models) n incomplete data coverage (e. g. , over remote regions) n model deficiencies due to subgrid processes (e. g. , clouds, land surface) n Chaos: 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. This is the reason that we cannot predict the weather 1 month or 1 year from the forecasting day. Q: For global model with a grid spacing of 40 km, can you see the Mt. Lemmon or Tucson in the model? a) yes, b) no n 12

Why NWS Forecasts go Awry and Steps to Improve Them n ensemble forecasting: spaghetti plot to indicate the robustness of forecast Q: where do you have more confidence in the forecasting? a) Northeastern Pacific b) Northwestern Atlantic 13

Other Forecasting Methods n n n persistence forecast: using current state to predict future; 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)--correct known model errors probability forecast: particularly for precipitation climatological forecast: using climatology to predict future; good for Tucson rainfall in June 14

Q: What does it mean by `chance of (steady) rain is 60% for one area’? a) It will rain over 60% of the area b) 60% chance that any random location in the area will receive measurable rainfall Q: Your friend claims that the forecast of 50% chance of rainfall is meaningless as it is the same chance for head in coin tossing. How do you respond? a) agree, b) disagree, c) don’t know Pay attention to the last note 15

Probability for a `white Christmas’ – 1 inch or more of snow Q: The probability for a white Christmas is 20% in Northern Arizona. It means a white Christmas would occur: a) once every 5 years, b) once every 20 years 16

Types of Forecasts n n n very short range forecast or nowcast: 0 -6 hr short range forecast: 6 hr – 2. 5 days Medium-range (or extended) forecast: 3 -8. 5 days long range forecast: 8. 5 days – 2 weeks Monthly and seasonal outlooks: above, near, or below normal conditions • Long-range forecasts are less specific than short range forecasts. 17

Use this figure to answer the two questions below: Q: Precipitation outlook (left panel) for Arizona is a) above normal, b) near normal, c) below normal Q: Temperature outlook (right panel) for Arizona is a) above normal, b) near normal, c) below normal 18

Accuracy and Skill in Forecasting Forecasts show skill only when they are more accurate than a straightforward forecast (e. g. , only using persistence or climatology) • Both persistence and climatology are surprisingly accurate forecasting methods sometimes. Q: If you forecast clear sky for the next three days in mid. June for Tucson, and those forecasts turn out to be accurate. Does it means that your forecasts have skill? a) absolutely yes, b) absolutely no, c) not necessarily yes Q: Do we have forecasting skills in predicting the weather at noon 2 weeks from the forecasting day? a) yes, b) no 19

Predicting the Weather from Local Signs Halo: `a halo around the moon portends rain’ (folklore) n • To see a halo: block out the sun with your hand look at the cirrostratus clouds. Wear polarized sunglasses if possible. Use typical changes of wind, T, Td, clouds, and precipitation associated with cold fronts 20

Use typical changes of wind, T, Td, clouds, and precipitation associated with warm fronts Using information related to longwave cooling to predict nighttime temperature: Q: if you predict a clear and dry night, the nighttime temperature would be: a) relatively cold; b) relatively warm Q: if you predict cloudy night, the nighttime temperature would be: a) relatively cold; b) relatively warm 21

Determining the Movement of Weather Systems 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 n • Internet now provides much of the weather information http: //www. weather. gov (briefly discuss “warnings & forecasts”, “graphical foreasts”, …) http: //www. atmo. arizona. edu http: //www. wrh. noaa. gov/twc 22

Current front and front 6 hr ago Estimate for the next 24 hr 500 mb height observation for 24 hr later 23