Where do you get the information for the turbulence layers?

The US and Global Turbulence map layers use different modeling techniques which results in different forecasts. The US model is based on NOAA's Graphical Turbulence Guidance. 

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Image 1. Turbulence (US)

The Global turbulence layer is derived from the Global Forecast System (GFS). This layer differs from the US layer in that the Global turbulence layer forecasts clear air turbulence and not mountain wave turbulence.  

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Image 2. Turbulence (Global)

More information regarding the turbulence layers can be found here: More Turbulence is Better