Weather Depiction Chart  Significant Weather Prognostic Charts  A weather depiction chart details surface conditions as  derived from METAR and other surface observations. The  weather depiction chart is prepared and transmitted by  computer every 3 hours beginning at 0100Z time and is valid  data for the forecast period. It is designed to be used for flight  planning by giving an overall picture of the weather across  the United States.  [Figure 13-12]  The weather depiction chart also provides a graphic display  of IFR, VFR, and marginal VFR (MVFR) weather. Areas of  IFR conditions (ceilings less than 1,000 feet and visibility  less than three miles) are shown by a hatched area outlined  by a smooth line. MVFR regions (ceilings 1,000 to 3,000  feet, visibility 3 to 5 miles) are shown by a nonhatched area  outlined by a smooth line. Areas of VFR (no ceiling or ceiling  greater than 3,000 feet and visibility greater than five miles)  are not outlined. Also plotted are fronts, troughs, and squall  lines from the previous hours surface analysis chart.  ceiling height, weather, and obstructions to visibility, but  does not include winds or pressure readings like the surface  analysis chart. A bracket ( ] ) symbol to the right of the station  indicates the observation was made by an automated station.  Significant weather prognostic charts are available for low- level significant weather from the surface to FL 240 (24,000  feet), also referred to as the 400 mb level and high-level  significant weather from FL 250 to FL 630 (25,000 to 63,000  feet). The primary concern of this discussion is the low-level  significant weather prognostic chart.  The low-level chart is is a forecast of aviation weather  hazards, primarily intended to be used as a guidance product  for briefing the VFR pilot. The forecast domain covers the 48  contiguous states, southern Canada and the coastal waters for  altitudes below 24,000 ft. Low altitude Significant Weather  charts are issued four times daily and are valid at fixed times:  0000, 0600, 1200, and 1800 UTC. Each chart is divided on  the left and right into 12 and 24 hour forecast intervals (based  on the current NAM model available).  Weather depiction charts show a modified station model  that provides sky conditions in the form of total sky cover,  Figure 13-12.  Weather depiction chart.  13-15 