The thunderstorm potential graphic displays the following graphics, which when overlaid show areas favorable for thunderstorm development.  The graphics are based on the GFS model output at 40km resolution and therefore have all the uncertainty and limitations associated with raw model output.

Graphic 1 is a layer averaged relative humidity (RH) with a layer covering (xx-xx mb) or approximately (xx to xx feet ASL).  Green shading starts at a threshold of 40% RH and transitions to yellow then red as RH increases to a maximum of 100%.

Graphic 2 is the Lifted Index (LI) which is one indicator of atmospheric instability.  Green shading starts at Neutral/Slight Instability (0) and transitions to yellow then red as the degree of instability increases.

The resulting graphs are then “combined” or overlaid and areas of bright green to bright red indicate the best areas for potential thunderstorms.

1)  If the model solutions for moisture or instability are incorrect then the graphic will erroneously indicate thunderstorm potential where there is none OR not indicate thunderstorm potential where it exists.

2)  This graphic does not account for vertical lift (upward motion) in the atmosphere which is the third element needed for thunderstorm development.

3)  Vertical lift (upward motion) can overcome the lack of instability; therefore, thunderstorms may develop in regions where no instability is indicated by the graphic.