Severe Weather Watcher Handbook/Manuel de l'observateur de temps violent - Severe Weather Watcher Handbook - [Meteorological Service of Canada

How outflow shapes storm type and evolution

Initially, with only clouds or showers (A), there is no outflow. As a storm begins to regenerate (B), outflow from the rain core initiates each new cell. Since multicell storms are not very intense, outflow is weak and does not spread far from the rain. When the storm is sever (C) its taller updraft/downdraft brings a constant supply of colder air down inside the storm, and outflow spreads out as a cold pool in the system's wake. Outflow is held in check at the front as long as intensity is maintained. A steady-state LP or classic supercell eventually weakens (E) into a forward-propagating system (worst-first). It may first evelve into an HP supercell (D) as huge amounts of rain and outflow are balanced against inflow during the most intense period. Outflow always wins in the end, and it, too, will evelve to forward propagation until it winds down. The common tendency for storms to begin as worst-last but end as worst-first is caused by outflow becoming dominant. Backside updrafts are cut off by SE-moving outflow from the rain core and a new updraft location forms along the forward side. This new core region is then continually shifted forward by the advancing gust front. When a storm system wanes (F), the rain core lightens up and inflow is replaced by a large, sprawling outflow pool that warms up slowly. The forward edge of the outflow will drift ahead as a detached gust front, which may rebuild into new storms later.