Convection » Heavy Precip./Flash Floods Forecasting Flash Floods
Flash floods generally occur where the precipitation rate is heaviest for the longest period. Flash flood-producing systems include: strong multicellular storms, supercells, mesoscale convective systems, and tropical disturbances (waves, depressions, and cyclones).
Flash floods will occur when a rain-producing system becomes quasi-stationary over a drainage basin or when successive convective systems pass over the same drainage area. New convective development is influenced strongly by the outflow boundary produced from existing (and previous) cells. Regeneration or "training" of cells in the same location is associated with a slow-moving low-level boundary, a low-level jet, upper-level divergence, and weak vertical wind shear.
This radar animation shows a persistent heavy rain event over an area just east of the Rocky Mountains in Fort Collins, Colorado, that produced a large flash flood.