Slow movement of fronts or tropical systems that have heavy rain already associated with them will continue to produce heavy rain over the same areas. Trigger mechanisms such as fronts will begin to stall if upper level winds are nearly parallel to the surface trigger mechanism. The lifting of a deep layer of moisture has the potential to produce heavy rain. In the US, this can occur from a deep layer of Gulf moisture banking up against a slow moving trigger mechanism. Soils can be saturated from rain in a previous storm system or a prolonged rain event from one storm system.
Snow melt adds to the amount of runoff.
Heavy rain combined with rapid snowmelt is conducive to flash flooding. Frozen soil acts like saturated soil. All rainfall will immediately runoff. The pore spaces in very dry soil do not allow water to penetrate at a quick rate. Very heavy rain on bone dry soil can caused flooding.