Beyond the nowcast range: 24-h forecast guidance concerning convective initiation and mode for the Great Plains tornado outbreak of 3 May 1999

The 3 May 1999 tornadic outbreak did not conform to conceptions of the classical, synoptically evident outbreak (e.g. progressive, developing cyclone, advection of high equivalent potential temperature air in the boundary layer, well-developed mesoscale boundaries in the form of drylines/fronts, strong vertical wind shear). Although forecasters were well aware of the potential for severe weather 24 hours in advance and outlook risks were upgraded through the course of the day, the initiation/timing, form, and location of the convection were not readily anticipated. In particular, one issue was whether isolated supercells or a single squall line would move across Oklahoma, if convection initiated.

We argue that very high resolution regional models such as the PSU/NCAR MM5 can provide key information about these questions in forecast mode. The 0000 UTC 3 May 1999 forecast cycle of the National Centers for Environmental Prediction (NCEP) aviation (AVN) run of the Global Spectral Model (GSM) was used to provide cold-start initial conditions and lateral boundary conditions for the outermost domain of a quadruply nested (54, 18, 6 and 2 km), 27-h integration. We will show how the model forecast, in combination with available observations, could have been used by forecasters to help develop and, later update, expectations concerning the storms that day.