Wx Watch: Forecasting By Bits and Bytes

Modeling the weather

December 1, 1998

No doubt you've heard television weatherpeople mention "models" or "forecast models" as part of their badinage. They're referring to the computer-generated imagery and numerical products that have come to rule the science of meteorology. From its modest beginnings in the early 1960s, computer-generated forecasting has taken over as the prime method of predicting synoptic weather. As computer power increases, forecast models may even deliver on the promise of pinpoint prediction of the formation, life cycles, and movements of ever-smaller-scale weather events, such as thunderstorms and tornados.

Modeling, as you might suspect, demands gobs of computer power. At the National Center for Environmental Prediction (NCEP) in Camp Springs, Maryland, there are at least four giant Cray supercomputers running night and day, chugging along at thousands upon thousands of gigaflops (billions of calculations) per second. They crunch numbers using observations, meteorological algorithms, and climatological data as fodder, then turn them into reams of numbers and thousands of images.

There are all sorts of forecast models. Their names describe their jobs. The Global Spectral Model scouts out major climatic changes that involve the entire world. The Nested Grid Model (NGM) lets forecasters "zoom in" on trouble spots, using a moveable mesh of weather data. The Medium Range Forecast and Eta models are for three- to six-day forecasting and are popular with television weathercasters. The RUC (Rapid Update Cycle) Model is a revved-up stream of data that updates at three-hour intervals and is meant for short-term forecasts. The Hurricane Model tries to predict where, when, and in what direction tropical storms and hurricanes will form and move.

The Aviation Model, of course, is the one we're interested in. To check it out online, call up the Web site ( http://wxp.atms.purdue.edu/aviation/). There are other Web sites that can give you access to the aviation model, but for the sake of simplicity, let's stick with this one for the time being. This Web site, operated by Purdue University, gives you not just the Aviation Model, but plenty to link to, including radar and satellite imagery, as well as other forecast models — such as the Eta and Nested Grid models.

Right off the bat, you'll see what are called four-panel forecast plots near the top of the site. They're called four-panel charts because they cover four forecast periods: 12, 24, 36, and 48 hours from the issue time, which is 5 p.m. Eastern time. Updates are twice a day, at 5 a.m. and 5 p.m. Eastern time. Six-panel forecast plots go out to 72 hours, or three days.

There are four-panel plots for the surface forecast, 1000-millibar pressure surface (which is very close to the surface, considering that standard sea-level pressure is 1013.2 millibars), plus plots of the 850-mb (approximately 5,000 feet), 700-mb (approximately 10,000 feet), 500-mb (approximately 18,000 feet), and 300-mb (approximately 30,000 feet) pressure surfaces.

The surface forecasts of pressure and precipitation are useful in checking out the forecast positions of any fronts or rain/snow at the surface. Be sure to note the valid times at the top of each chart.

The 1000-mb forecast charts are especially important in thunderstorm season. These charts can be a confusing jumble of colors and symbols. The big thing to look for here is shaded areas, which indicate converging air near the surface. Convergence means lifting, so shaded areas indicate where thunderstorms could form. Dewpoints and wind directions are also shown. Dewpoints above 20 degrees Celsius or 68 degrees Fahrenheit together with winds out of the south also mean trouble, because this is a combination of warm air and high moisture content.

Because the western United States has higher terrain elevations, surface pressures are lower than 1000 millibars, so these charts don't apply in those regions.

The 850-mb four-panel plots are comparatively easy to read. Temperatures aloft are shown in color contours, the height contours show lows and highs, and the winds aloft projected for this altitude (about 5,000 feet msl) follow the height contours. One interesting use of the 850-mb plot is to determine low-level precipitation type. If the temperature is above freezing and the surface plot shows a forecast of precipitation, then it should be in the form of rain. If the contours show below-freezing temperatures, there's a chance of snow — and icing in clouds.

The 700-mb (about 10,000 feet) plots, like the others, show the height contours of this pressure surface. As with surface isobars, the more closely spaced the height contours, the stronger the winds aloft. Vertical vorticity — the amount of storm-causing vertical rotational motion in the atmosphere — is also shown on the 700-mb panels. Positive vorticity (shown in green, yellow, and red) indicates upward motions and probable storminess. Negative vorticity (blue and magenta shades) implies sinking air, high pressure, and clearer skies.

Four-panel 500-mb plots are important because this level (about 18,000 feet) is called the "steering level." Lows and frontal complexes tend to follow the winds at this level, as shown by the height contours. Vorticity is also displayed, according to a legend at the edge of the vorticity plot. Values of 14 (green) or higher predict strong rotational motions, and eddy currents that can help to strengthen any low pressure at or near the surface.

The 300-mb (approximately 30,000-feet) plots predict the wind flows at the jet stream level. Any troughs at this level can produce low pressure systems beneath them, so checking the charts for any southward-trending dips at the 300-mb level can warn of deteriorating weather.

Finally, the Aviation Model provides a six-panel forecast of relative humidities and lifted indices out to 72 hours from the time of issue. The relative humidities are for between the 1000- and 500-mb pressure surfaces. If relative humidities top 60 percent (dark blue shading), then the advice to forecasters is to expect generally overcast or mostly cloudy skies. If they're above 80 percent (light blue shading), count on an overcast. The closer to 100 percent humidity, the greater the chance of precipitation — particularly if the surface plots agree on precipitation. Areas with lifted indices of less than minus 4 are given gray shading. These are zones where instability is greatest and where severe thunderstorms may crop up.

Together with the information gleaned from meteograms and other atmospheric soundings obtained by surface observations and weather balloons, plus satellite readouts, output from the Aviation Model is central to the methodology for generating the TAFs, winds aloft, and area forecasts that we all should check before each flight.

The Aviation Model is yet another useful tool that we can all use for both short- and long-term planning purposes. Spend some time with the Aviation and other models and you'll soon understand how your local weathercaster comes up with his "special" long-range forecast.

E-mail the author at tom.horne@aopa.org.