Don't confuse this (internal process) with propagation along boundary layer convergence zones (external process)-discussed next. Cold pool strength is dependent upon cloud base height, subcloud & midtropospheric relative humidity, and mid- to upper-level storm-relative flow (shear)-how many "chilled" hydrometeors are falling near the updraft. Especially important when the midlevel shear is weak, and/or dry air exists below cloud base. Can produce dramatically different propagation for gust front out ahead of storm (faster) vs. gust front as a flanking line (slower) - also see Corfidi (2003). If the shear is confined to the low levels, the supercell may become outflow-dominated.