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I I I I I I I I I I I I I I I I I I I Convective Scale Interaction is a term given to a very important process set into motion when a collapsing storm produces precipitation. As the downdraft air, associated with the falling precipitation, nears the ground, "ground effect" turns the rushing air into a horizontal flow of air called a gust front, or outflow boundary. The gust front fans out below its cloud base usually undercutting relatively warm, moist air and other clouds, nearby and distant. If moisture is sufficient in the undercut air being lifted above the gust front, and if it rises into an unstable atmosphere, a cloud can grow very rapidly to become another severe storm which, in turn, quickly reaches maturity and collapses to produce another strong downdraft helping reinforce the original gust front or creating a new one, etc., repeating the earlier sequence. This can happen repeatedly. Single outflow boundaries have been known to be strong enough to travel I 00-200 miles, or much further, from its parent storm. Satellite views of clouds forming along these moving gust fronts often show them aligning into a semi-circular, fan-shape orientation which are called arc- clouds. Some of these clouds, by themselves can develop into large, severe convective storm systems. Single storms, bow-echo formations, multi-celled storms and supercells all have been identified as forming along these gust fronts. Earlier research in the southeastern part of the U.S.A. estimated 60%-75% of the storms existing in late afternoon on a typical storm day were caused by this convective scale interaction. Here in Western Kansas we have also identified considerable convective scale interaction effects produced by severe storms. Pilots usually are the first to see them and call them in to the radar meteorologist so tracking them can begin. These outflow boundaries have become the single- most important phenomena to monitor on a continuous basis during operational periods since we obtained the capability to track them. Gust fronts very often provide the operational intelligence that allows us high-predictive capability as to where, in advance, new severe storms are likely to form and/or where will be the most intense area in which to concentrate seeding efforts. More about gust fronts: Sudden severe new storm growth frequently develops in weak, old non-hail bearing precipitation areas which are being undercut by gust fronts. Satellite imagery can also give advance warning about subsequent new storm development potential which can't be seen immediately on radar or by pilots. Two, or more, colliding gust fronts frequently create extremely severe storms in between them, although the severe storms are relatively short-lived---having a lifetime of a few tens of minutes. Severe aircraft turbulence is frequently found in gust front air between the parent storm and the leading edge of the gust front. Whereas, in front of a gust front the air is generally smooth (except near the ground where terrain effects dominate). When gust front-air drops out of high-based clouds, micro-burst activity occurs which can flatten buildings, crops and cause aircraft accidents during landing and take-off. Gust fronts are variable in updraft velocity at its leading edge. Under some conditions rainfall augmentation over large areas has been performed by seeding atop the leading edge of a gust front which was able only to lift the air over it to create small cumuliform clouds with tops to 18,000 - 25,000 feet. Updrafts found above such gust fronts generally might vary around 100 - 200 feet per minute instead of a more typical 1,000- 2,000 feet per minute, or more. If this particular condition occurs at night with little threat of hail developing from new storm growth and weak updrafts are prevalent, rainfall stimulation seeding II