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I have the following questions on Significant Weather Prognostic Charts: 1) What does the valid time mean? If a chart shows a valid time of 18Z, does it mean the chart is valid till 18Z or it is valid for 12/24 hours from 18Z? Does the same apply to other charts such as a TAF for example? 2)Why are the tops of moderate/greater turbulence shown and not the base? 3) Why does it contour the height of the highest freezing level in 4000 ft increments and not the lowest?

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4 Answers

  1. Lucas on Oct 15, 2014

    Here is a video on the FAA questions on the Prog Chart. Maybe it can help answer some of your questions


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  2. psequeira on Oct 21, 2014

    Thank you Lucas

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  3. psequeira on Oct 22, 2014

    I still did not find my answer through the video Lucas

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  4. Lucas on Oct 29, 2014

    The valid time in a Prog Chart is when the forecast starts so it would be a 12 hour forecast starting at 18Z.

    A TAF displays both the issue time and valid time:
    KEWR 291121Z 2912/3018
    For Newark issued the 29 of October at 1121 Zulu and valid from the 29th at 12Z to the 30th at 18Z.

    The bases of the turbulence are also shown at times the number on top shows the TOPs and the number underneath shows the BASEs of the turbulence.

    for the freezing levels take a look at this:

    The surface freezing level is depicted by a zigzag line and labeled “SFC.” The surface freezing level separates above-freezing from below-freezing temperatures at the Earth’s surface. Freezing levels aloft are depicted by thin, short dashed lines. Lines are drawn at 4,000-foot intervals beginning at 4,000 feet and labeled in hundreds of feet. For example, “80” identifies the 8,000-foot contour. Freezing level heights are referenced to MSL. The lines are discontinued where they intersect corresponding altitudes of the Rocky Mountains. The freezing level values for locations between lines is determined by linear interpolation. For example, the freezing level midway between the 4,000 and 8,000 foot lines is 6,000
    feet. Areas with forecast multiple freezing levels have lines drawn to the highest freezing level. For example, with freezing levels forecast at 2,000, 6,000, and 8,000 feet, the analysis is drawn to the 8,000 foot value. Notice that not all freezing levels are identified with a multiple freezing level event.
    Information about the 2,000- and 6,000-foot freezing levels in this example would not be displayed. Surface-based multiple freezing levels are located over areas which have below-freezing temperatures at the surface and above-freezing temperatures within at least one layer aloft. Freezing rain and freezing drizzle (freezing precipitation) are associated with surface-based multiple freezing levels. The 11-5 intersection of the surface freezing level line and freezing level contours encloses an area with surface based multiple freezing levels.


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