Numerical simulations and forecasting potential for extreme rainfall events in the southern United States

The southern United States is occasionally subject to rainfall events that last one to four days and produce widespread precipitation exceeding 300 mm, with point totals exceeding 600 mm. We are investigating these exceptional events using the PSU/NCAR MM5 numerical model, at resolutions adequate to explicitly resolve the convective precipitation.

The MM5 is able to produce remarkably realistic simulations of many of these events, allowing us to investigate the structure and dynamics of these unusual systems. Research on two of these events is ongoing; preliminary results show that both events lack a mesoscale focusing mechanism and do not easily fit into the classical paradigms for sustained, heavy precipitation.

The implications of these results will be discussed in the context of forecasting these events at a range of 12-48 hours. The success of the MM5 in simulating many of these events without special observations or subjective enhancement of initial conditions suggests that these events may be predictable from the larger-scale state of the atmosphere, and that very-high-resolution targeted forecasts can provide valuable advance warning of the potential for extreme rainfall.