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Low Level Significant Weather Charts

Asked by: 3239 views Weather

I'm confused how to use the Low Level SigWx charts.

At AWC there is a drop down menu offering four different choice of chart combinations.  On the lower left of each chart, there is a VT (valid time) and also annotation denoting the duration.  The left side is a 12HR chart and the right side is a 24HR chart representing what I assume is the duration of the forecast.

Let's say a user selects from the drop down menu charting with a VT of 06Z for the 12HR chart and VT 18Z for the 24HR chart.  It's my understanding that those two charts would be used to show the weather beginning at 06Z lasting for 12 hours on the left chart and then at the conclusion of that 12 hours (18Z) the weather for the next 24 hours on the right chart.  In total, it is depicting the weather for 36 hours into the future beginning at 06Z.  Is this correct?  From the drop down menu there are a series of four forecasts using the same 12/24 hour forecast period.

Why then, is there little resemblance between a 12HR chart with a VT of 18Z to a 24HR chat with a VT of 18Z?  If you randomly examine 12HR and 24HR charts with the same valid time, there is usually no resemblance between the two charts.

Given the dynamic nature weather, how useful is a chart purporting to show MVFR, IFR, turbulence for a 24 hour period?  In a 24HR chart, is the weather the worst that can be expected, the average?

I can't believe I am so confused on how to actually use this chart.  If someone could use a flight planning scenario to walk me though using the charts, specifically the VT and the duration, I would be much obliged.

 

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1 Answers



  1. Craig on Feb 03, 2020

    As I understand it, per the ADDS website, “Low altitude Significant Weather charts are issued four times daily and are valid at fixed times: 0000, 0600, 1200, and 1800 UTC.” aviationweather.gov/progchart/help?page=low

    That would NOT mean the 12 hr chart is depicting the forecast weather beginning at 06Z lasting for 12 hours, but rather it depicts the forecast weather at 06Z. The 24HR chart would depict the forecast weather at 18Z, in your example. That’s similar to a TAF for an airport. So in between those times, you’d have to expect the forecast weather would be somewhere between what is depicted in the 12HR and 24 HR charts, even though we’re told not to try to interpolate.

    You can sort of see the forecast change graphically if you click the right and left arrow buttons b/w “Time” and “B&W” aviationweather.gov/progchart/low

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