Here is the difference between a forward flank and rear flank downdraft. The forward flank downdraft results from the evaporational cooling of air that spills to the earth's surface. It spreads out in all directions under a storm's downdraft but spreads more quickly in the direction the PBL winds are blowing. The forward flank downdraft (ffd) represents relatively cool and moist air. The fact that the ffd represents dense air is due more to the fact that the air is cold rather than being moist. Moist air is less dense than dry air but temperature is a larger air density contribution than moisture content.

The rear flank downdraft (rfd) represents relatively warm and dry air which is forced down from the mid-levels of the atmosphere. A thunderstorm represents a barrier to airflow. Some air, as it collides into the side of a thunderstorm, is pushed down toward the surface. This air is warmed adiabatically and was previously (before descent) fairly dry. When this air reaches the surface it is warmed adiabatically and has an even lower relative humidity. The air associated with the forward flank downdraft is warmed adiabatically upon descent BUT the ffd starts out at a much colder temperature than the rfd. The rfd being warm has a relatively lower density than the ffd. Density circulations (mesoscale baroclinic circulations) are generated due to the differences in density between the two small scale air masses. The interaction between the ffd and rfd generates tubes of horizontal vorticity and is important to the generation of the low level circulation necessary for tornadogenesis. If the PBL winds are strong, large values of helicity will be produced. Helicity is storm relative inflow times horizontal vorticity generated by speed and directional wind shear. The interaction between the ffd and rfd increases the amount of low level shear.