<P> As supercells and multi-cell thunderstorms dissipate due to a weak shear force or poor lifting mechanisms, (e.g. considerable terrain or lack of daytime heating) the squall line or gust front associated with them may outrun the squall line itself and the synoptic scale area of low pressure may then infill, leading to a weakening of the cold front; essentially, the thunderstorm has exhausted its updrafts, becoming purely a downdraft dominated system . The areas of dissipating squall line thunderstorms may be regions of low CAPE, low humidity, insufficient wind shear, or poor synoptic dynamics (e.g. an upper level low filling) leading to frontolysis . </P> <P> From here, a general thinning of a squall line will occur: with winds decaying over time, outflow boundaries weakening updrafts substantially and clouds losing their thickness . </P> <P> Shelf clouds and roll clouds are usually seen above the leading edge of a squall, also known as a thunderstorm's gust front . From the time these low cloud features appear in the sky, one can expect a sudden increase in the wind in less than 15 minutes . </P> <P> Tropical cyclones normally have squalls coincident with spiral bands of greater curvature than many mid-latitude systems due to their smaller size . These squalls can harbor waterspouts and tornadoes due to the significant vertical wind shear which exists in the vicinity of a tropical cyclone's outer bands . </P>

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