I guess I'll take a stab at this one. Often a subjective assessment, a storm which is described as outflow dominant is typically a storm where the gust front has completely undercut the storm's updraft. The 'cold pool' (rain-cooled air behind the gust front) of an outflow dominant storm often is much colder than storms that do not become outflow dominant, as a deep and large cold pool can often expand faster than the storm updraft, leading to an undercut storm. Additional factors at play that come to mind are the deep layer shear, which dictates the storm motion and spacing between the updraft and the main precipitation core; the height of the cloud base, as higher cloud base means lower relative humidity, more evaporational cooling and a stronger cold pool; low-level shear, as the surface winds, when opposed to the cold pool, can slow the rate of cold pool expansion (so you would want strong surface winds opposite the storm motion). While an outflow dominant storm can still produce significant severe weather, there is a lower probability of a tornado as the gust front surges well ahead of the updraft. As cells get older, the cold pools generally get stronger with time, so many storms will become outflow dominant later in life as the storm updraft starts to ingest more and more of it's own exhaust, and you may want to start looking for a new storm to chase.
Glen
