Weather Forecasting Chapter 9 Dr Craig Clements SJSU Met 10 How to prepare a Weather Forecast Acquisition of weather information Weather forecasting tools Weather forecasting methods Forecasts provide information for the public advisories This information can be a critical safety issue such as thunderstorms fire weather conditions etc Flags indicating advisories and warnings in maritime areas Figure 1 p 237 Weather data acquisition NOAA Weather Radar Doppler radar data from Melbourne Florida on March 25 1992 during the time of a severe hailstorm that caused 60 million in damages in the Orlando area In the table near the top of the display the hail algorithm determined that there was 100 percent probability that the storm was producing hail and severe hail The algorithm also estimated the maximum size of the hailstones to be greater than 3 inches A forecaster can project the movement of the storm and adequately warn those areas in the immediate path of severe weather Fig 9 2 p 238 A Meteogram illustrating predicted weather elements Fig 9 3 p 239 A sounding of air temperature dew point and winds Fig 9 4 p 239 geostationary satellite 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 Polar orbiting satellites Polar orbiting satellites scan from north to south and on each successive orbit the satellite scans an area farther to the west Fig 9 6 p 241 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 Three Satellite Products tools for the forecaster Visible satellite images Infrared images Water vapor images A visible image of the eastern Pacific Ocean Clouds in the visible image appear white Fig 9 9a p 242 Infrared satellite image of the eastern Pacific Ocean Notice that the low clouds in the infrared image appear in various shades of gray Fig 9 9b p 242 An enhanced infrared image Fig 9 10 p 243 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 welldeveloped mid latitude cyclonic storm Fig 9 11 p 243 AWIPS computer work station provides various weather maps Fig 9 1 p 238 Numerical Weather Models A forecast chart is produced by computers solving the equations of motion and heat for the atmosphere These charts are called prognostic charts of progs These computer simulations solve the equations more quickly than can be done by hand For example to produce a 24 hr forecast chart for the Northern Hemisphere it requires hundreds of millions of mathematical calculations It would take meteorologists working non stop with hand calculators years to produce a single chart 500 mb progs for 7 P M EST July 12 2006 48 hours into the future A resolution grid spacing of 12 km Fig 9 12a p 245 500 mb progs for 7 P M EST July 12 2006 48 hours into the future GFS model with a resolution of 60 km Fig 9 12b p 245 The 500 mb analysis for 7 P M EST July 12 2006 Fig 9 13 p 245 Ensemble 500 mb forecast chart for July 21 2005 48 hours into the future The chart is constructed by running the model 15 different times each time beginning with a slightly different initial condition The blue lines represent the 5790 meter contour line the red lines the 5940 meter contour line and the green line the 500 mb 25 year average called climatology Fig 9 14 p 247 Figure 2 p 248 Figure 3 p 248 Fig 9 16 p 250 The 90 day outlook for a precipitation and b temperature for February March and April 1999 Fig 9 17 p 251 A halo around the sun or moon means that rain is on the way Fig 9 18 p 253