Weather Maps, Forecasts, and Data

Abstract

Weather maps prepared by the National Weather Service summarize and synthesize weather data to provide a comprehensive picture of weather conditions at a given time. They are the basis of weather maps used on television to show precipitation, high and low pressure centers, and fronts. Weather maps are produced using both surface data and data from specified pressure levels. Data are plotted and contoured by computer, and analysts use satellite photos, satellite video loops, weather forecast models, and extrapolations from previous frontal and pressure system analyses to locate fronts and pressure centers. An example of a surface weather chart is presented in figure 9.1. A 500-mb chart for the same date and time was presented in figure 5.1. Symbols are used on weather maps to indicate synoptic-scale features. High and low pressure weather systems (highs and lows) are indicated by the letters H and L, with isobars labeled in millibars. Lines indicating frontal positions (section 6.2) represent the position on the ground of boundaries between air masses. Additional meteorological variables, such as temperature, are often analyzed on the same map using dashed or colored lines. Pressure, temperature, and other data from the reporting stations are plotted in coded form at the station locations. A station model specifies the positions in which different types of data are plotted relative to the station location. Figure 9.1 used an abbreviated station model. A complete station model is shown in figure 9.2. Figures 9.3 — 9.7 show additional symbols used in station models to indicate total sky cover, winds, pressure tendency, cloud types, and present weather types, respectively. A surface weather chart, with the symbols indicating fronts and high and low pressure centers and the information included in the station model, provides a snapshot of synoptic-scale conditions at ground level. By overlaying charts for several pressure levels (section 5.1.3), changes with altitude can be identified and the three-dimensional structure of the atmosphere at a given point in time can be visualized. By comparing consecutive charts, the rate of movement of fronts and the rates of development of high and low pressure centers can be determined.