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Air-mass storms can transition into a line containing multiple cells exhibiting some characteristics similar to that of a squall line. During the development of these multi-cellular lines cloud base updrafts frequently shift location although still found around several of the individual cells comprising it and can make proper cloud base treatment difficult. The location of the important updrafts on these type of cloud systems usually develop along a line in its frontal portion and can be from a few miles to many miles in length. Other times, the best seeding area may be around only one end of the line. Multi-celled lines may also appear as a remnant of a weakened squall line or as part of a line of storms associated with fronts, surface troughs and thunderstorm outflows. The cloud system known as the squall line is an organized line of cumulonimbus clouds with its important updraft area found along its leading edge. Updrafts pertinent to the precipitation process (and hail) are seldom found along the trailing edges of squall lines except at the end of a line or at significant breaks within the line. Squall lines can be extensive, crossing a few counties in a state, or cross even more than one state; frequently they are associated with surface troughing or frontal passages. Updrafts can exceed 2000 feet per minute, often producing "scud" clouds visible nearly to the ground. Inflow areas are usually smooth ahead of the line; however, when storms collapse within the line strong outflows are pushed out rapidly ahead of the line and can cause severe turbulence. Convective Scale Interaction is a term describing a process in which a collapsing storm produces precipitation and associated downdrafts strong enough to create a gust front, also called an outflow boundary, which acts to initiate new storm growth at some distance from its' "parent storm", even for a hundred miles or more. A considerable volume o~ air is thrust below cloud base which often undercuts warm, moist air as it fans out below the cloud base. When this occurs, the gust front lifts the warm, moist air ahead of it into unstable air in which new storms can grow rapidly, mature, collapse, produce its own gust front. and repeat the same process. Satelli te views of clouds seen forming along these moving gust fronts which align themselves in a semi-circular, fan-shaped orientation are called "arc-clouds" and can develop into large, severe convective storm systems. Single storms, multiple storms and supercells all have been identified as forming along these gust fronts. One study in the southeas"Cern part of the U.S.A. estimated 60% - 75% of the storms existing in late afternoon on a typical storm day were caused by scale interaction. Two colliding gust fronts, or more, frequently create extremely severe storms. However, despite the severe turbulence found beneath each of them, the air in front of and just above the gust fronts is usually very smooth. Scale interaction as described here occurs frequently on the WKWM Program and when identified on radar or reported by pilots, its occurrence and direction of movement is monitored carefully. 11