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

Acquisition of Weather Information World Meteorological Organization National Centers for Environmental Prediction Watches: favorable condition for potential hazardous weather; what you need to do: pay attention Warnings: hazardous weather is imminent or actually occurring; what you need to do: take action 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

Weather Forecasting Tools 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

Near Orlando, FL Hail size > 3 inch 100% probability Figure 9.1: The AWIPS computer work station provides various weather maps and overlays on different screens. Near Orlando, FL Hail size > 3 inch 100% probability Fig. 9-1, p. 238

Satellites and Weather Forecasting 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?

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 Figure 9.5: The geostationary satellite moves through space at the same rate that the earth rotates, so it remains above a fixed spot on the equator and monitors one area constantly. Fig. 9-5, p. 240

Figure 9. 7: Generally, the lower the cloud, the warmer its top Figure 9.7: Generally, the lower the cloud, the warmer its top. Warm objects emit more infrared energy than do cold objects. Thus, an infrared satellite picture can distinguish warm, low (gray) clouds from cold, high (white) clouds. 3-D infrared imagery

Visible Infrared Enhanced Infrared 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.

Infrared water vapor channel 3-D TRMM satellite image Figure 9.11: Infrared water vapor image. The darker areas represent dry air aloft; the brighter the gray, the more moist the air in the middle or upper troposphere. Bright white areas represent dense cirrus clouds or the tops of thunderstorms. The area in color represents the coldest cloud tops. The swirl of moisture off the West Coast represents a well-developed mid-latitude cyclonic storm. Go to: www.weather.gov for real-time satellite information. Use 24 hr loop

The Computer and Weather Forecasting: Numerical Weather Prediction 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

Q: which model forecasts the low off the west coast better? 48-hr NWP forecast of 500 mb height 12km vs 60km grid spacing Q: which model forecasts the low off the west coast better? a) 12 km regional model; b) 60 km global model Figure 9.12: Two 500-mb progs for 7 P.M. EST, July 12, 2006 — 48 hours into the future. Prog (a) is the WRF/NAM model, with a resolution (grid spacing) of 12 km, whereas prog (b) is the GFS model with a resolution of 60 km. Solid lines of each map are height contours, where 570 equals 5700 meters. Notice how the two progs (models) agree on the atmosphere’s large scale circulation. The main difference between the progs is in the way the models handle the low off the west coast of North America. Model (a) predicts that the low will dig deeper along the coast, while model (b) predicts a more elongated west-to-east (zonal) low. analysis

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) incomplete data coverage (e.g., over remote regions) model deficiencies due to subgrid processes (e.g., clouds, land surface) 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

Why NWS Forecasts go Awry and Steps to Improve Them 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

Other Forecasting Methods 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

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

Probability for a `white Christmas’ – 1 inch or more of snow Figure 9.15: Probability of a “White Christmas”—one inch or more of snow on the ground—based on a 30-year average. The probabilities do not include all of the mountainous areas in the western United States. 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

Types of Forecasts 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.

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 Figure 9.17: The 90-day outlook for (a) precipitation and (b) temperature for February, March, and April, 1999. For precipitation (a), the darker the green color the greater the probability of precipitation being above normal, whereas the deeper the red color the greater the probability of precipitation being below normal. For temperature (b), the darker the orange/red colors the greater the probability of temperatures being above normal, whereas the darker the blue color, the greater the probability of temperatures being below normal. (National Weather Service/NOAA)

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

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

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

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 using the surface chart 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

Current front and front 6 hr ago 500 mb height Estimate for the next 24 hr observation for 24 hr later Figure 9.20: A 500-mb chart for 6:00 A.M. Tuesday, showing wind flow. The light orange L represents the position of the surface low. The winds aloft tend to steer surface pressure systems along and, therefore, indicate that the surface low should move northeastward at about half the speed of the winds at this level, or 25 knots. Solid lines are contours in meters above sea level.