Surface prognostic charts, or forecasts of the surface pressure, surface wind, and frontal depiction, are produced every 12 hours, beginning at 0000Z. Surface-pressure prognoses are available out to 120 hours (5 days). Figure 4-2-4 shows a typical surface-wind and surface-pressure prog-nosis. Winds and pressure are depicted the same as on the analysis charts. Frontal depiction prog-nosis charts are produced to match the area and map projection of the surface prognosis charts.

This polar stereographic map projection example (fig. 4-2-4) is typical of most of the charts produced for the mid-latitude users. The winds are plotted on a grid with spacing between grid points of about 5 degrees latitude at 15N, or about 300 nmi. As you look farther north, you

may notice that the same grid spacing is equivalent to about 7 degrees latitude at 50N, or about 420 nmi. This means that at 15N the wind plot represents an average wind for a 90,000-square-mile area, while at 50N the wind plot represents an average wind for a 176,400-square-mile area, or about one-half the resolution. You must keep that in mind when using these charts to make your forecast. Sometimes it is preferable to use a geostrophic wind scale and isobar spacing to determine forecast winds, instead of the winds plotted on the chart.

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