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I 1 I I I I I I I I I I I Under some conditions multi-celled storms may become very large, developing several new growth areas simultaneously with distinct "cores" growing embedded in and around the periphery of' the cloud boundary while the cloud system is in transition from being a relatively small severe storm into a large supercell. One characteristic of a supercell is that a some point it exhibits a "right-turning" motion relative to the direction of the mean steering wind. Supercells can be quite dangerous as they are capable of ejecting hail long distances in different directions, occasionally throwing it into the flight paths of seeding aircraft both at cloud base and cloud top. Supercells produce the most destructive tornadoes, however, not all supercells produce tornadoes; some estimates have indicated about one- quarter of all supercells may be associated with tomadic development. The cloud system known as a squall line is an organized line of cumulonimbus clouds many miles in length. Important updraft areas are found along its advancing cloud edges. Updrafts important to the precipitation and hail processes are seldom found along the trailing edges of these lines except at its end, or at significant breaks within the line. Squall lines can be extensive, crossing a few counties within a state or crossing more than one state; frequently squall lines are associated with surface troughing ahead of frontal passages. Updrafts can easily exceed 2,000 feet per minute and produce "scud" clouds visible nearly to the ground. Ahead of the squall line, updraft areas are usually smooth. I I I I I I Convective Scale Interaction is a term. given. to the process in which a collapsing storm produces precipitation and downdrafts and promotes subsequent new cloud growth. The downdraft air, also called a gust front or outflow boundary, flms out below its cloud base undercutting relatively warmer and often more moist air. If moisture is sufficient in the air being lifted above the gust front, it can rise into an unstable atmosphere, growing rapidly into another new severe storm which quickly reaches maturity. Then, the storm collapses, producing its own downdraft, thereby repeating the whole sequence again and can do so over and over, again many times. Single outflow boundaries have been known to be strong enough to travel 100-200 miles, or more, 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, themselves can develop into large, severe convective storm systems. Single storms, multiple storms and supercells all have been identified as forming along these gust fronts. Previous research in the southeastern part of the U.S.A. has estimated 60%-75% of the storms existing in late afternoon on a typical storm day were caused by scale interaction. Two, or more, colliding gust fronts frequently create extremely severe storms, although the severe storms are often short-lived. Severe aircraft turbulence is frequently found in gust front air between the parent storm and the leading edge of the gust front. Ahead of the gust front the air is generally smooth. When gust fronts drop out of high-based clouds, micro-burst activity is prevalent which has been known to flatten b~ildings, crops and cause aircraft accidents while landing and taking-off. Convective Scale interaction as described here frequently occurs on the WKWMP and when identified on radar or reported by pilots, its occurrence and direction of movement is monitored carefully for subsequent new storm growth to develop above it. Also, it has been observed that severe new storm growth often develops in weak, old non-hail bearing precipitation areas, which are 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 visually by pilots. 9