When an outflow boundary forms due to a shallow layer of rain-cooled air spreading out near ground level from the parent thunderstorm , both speed and directional wind shear can result at the leading edge of the three dimensional boundary .

When an outflow boundary moves away from a thunderstorm due to a shallow layer of rain-cooled air that spreads out at ground level , both speed and directional wind shear can result at the leading edge of the three dimensional boundary .
