The 4 June 1999 derecho event: the ultimate challenge for numerical weather prediction?

Warm season convective system rainfall forecasts remain a particularly difficult forecast challenge. For these events, it is hoped that ensemble forecasts will provide helpful information unavailable in a single deterministic forecast. In this study, the intense derecho event of 4 June 1999 accompanied by a well-organized band of heavy rainfall is used to show that for some situations, the predictability even within a 12-24 hour period is so low that a wide range of simulations using different models (Eta, WRF, MM5), different physical parameterizations (boundary layer schemes, convective schemes, explicit microphysical schemes) and initial conditions (Eta and AVN at two times) all fail to provide even a small signal that the event will occur. The failure of a wide range of models and parameterizations to depict the event suggests inadequate representation of the initial conditions. However, a range of different initial conditions also failed to lead to a well-simulated event, suggesting some events are unlikely to be predictable with the current observational network, and ensemble guidance for such cases may provide limited additional information useful for a forecaster.