Derechos and flash flooding
On occasion, on the rear flank of a convective system responsible for a derecho, a nearly stationary band of thunderstorms may form. If such a band persists for an extended period, and/or if the line forms in a region of rocky or steeply-sloped terrain, flash flooding may follow. Situations of this type are not common, but when they do occur, the results may be devastating as the convective system’s high winds are followed by the ravages of flooding. The “Ohio Fireworks” derecho-flash flood of July 4, 1969 is an example of what might be termed a concurrent derecho-flash flood convective system. Eighteen people died, many others were injured, and widespread destruction occurred as derecho winds swept across Michigan, Ohio, Pennsylvania, and adjacent Lake Erie. A few hours later, more than two dozen people perished from subsequent flash flooding in Ohio. The Johnstown, Pennsylvania flood of July 20, 1977 was associated with a similar convective system that produced both flash flooding and high winds.
Paradoxically, the same unidirectional wind profile that is conducive to the downwind (or forward) propagation of convective systems — that is, the repetitive, downwind development of new thunderstorms that can yield a derecho — also is favorable for repetitive storm development in the upwind direction. When the direction of the environmental flow around a convective system varies little with height, the system’s collective outflow of cool, surface air — the cold pool — over time necessarily elongates downstream, in the direction of the mean flow. The top part of the illustration below is a plan-view schematic of cold pool elongation for a case of unidirectional westerly flow, where the cold pool is shaded grey and “T” refers to “time.” As noted in Derecho Development, the leading edge of the cold pool, or gust front (here depicted by weather map frontal symbols), is the main site of new thunderstorm formation. If thermodynamic conditions are favorable for repetitive storm development along the progressive or downwind-moving part of the boundary, that part of the elongating cold pool / gust front potentially could yield a derecho-producing convective system.
As a cold pool in unidirectional environmental flow further elongates, the trailing, upwind flank of its gust front often becomes stationary or nearly so. This is shown in the right-most plan-view of the illustration above and in the plan-view inset of the figure below by the green-colored part of the outflow boundary, and by the alternating warm and cold frontal pips. Trailing gust front boundaries are especially likely to become stationary when the atmosphere is moist through a very deep layer, and the potential for strong, evaporatively cooled storm downdrafts is, therefore, comparatively limited. The persistent source of low-level uplift provided by the stalled gust front can then serve as the seat of repetitive thunderstorm development in the upstream direction.
As individual storms grow and mature, they move parallel to the boundary, causing multiple episodes of heavy rain at locations along the line. Such convective evolution is known as echo training. Prolonged echo-training in a moisture-rich environment nearly always results in excessive rainfall or flash flooding. Smaller scale or more intermittent episodes of echo training frequently occur on the rear flanks of derechos, and may cause localized flooding in the wake of a derecho’s high winds. In addition, back-building lines of storms sometimes form atop the cold pool left in the wake of a derecho; the combined derecho-heavy rain producing storm structure is then known as a bow-arrow convective system.