The five distinct regions of on-axis weak scintillation behavior exhibited by Gaussian beam propagation in the turbulent atmosphere are reviewed. One of the regions (D1) is unique, in that the random irradiance across much of the target plane is non log normal, scintillation predictions derived from the Rytov approximation are inaccurate, and no generally applicable and computationally efficient scintillation theory is currently available. The D1 region cannot be easily dismissed, since it includes horizontal path focused beams and ground to space collimated beams. A survey of the D1 region is provided, including physics insight from past publications, the role of the phase screen approximation, and the impact of distributed low order turbulence. An overview of a new approach that yields accurate, computationally efficient scintillation predictions within D1 is provided.