Sensitivity of northern great plains convection forecasts to upstream and downstream forecast errors

The role of earlier forecast errors on subsequent convection forecasts is evaluated for a northern Great Plains severe convective event on 11-12 June 2013 during the Mesoscale Predictability Experiment (MPEX) by applying the ensemble-based sensitivity technique to Weather Research and Forecasting (WRF) Model ensemble forecasts with explicit convection. This case was characterized by two distinct modes of convection located 150 km apart in western Nebraska and South Dakota, which formed on either side of an axis of high, lower-tropospheric equivalent potential temperature. Convection forecasts over both regions are found to be sensitive to the position of this θ_e axis. The convection in Nebraska is sensitive to the position of the western edge of the θ_e axis near an upstream dryline, which modulates the preconvective θ_e prior to the diurnal maximum. In contrast, the convection in South Dakota is sensitive to the position of the eastern edge of the θ_e axis near a cold front, which also modulates the preconvective θ_e in that location. The position of the θ_e axis is modulated by the positions of both upstream and downstream mid- to upper-tropospheric potential vorticity anomalies, and can be traced backward in time to the initial conditions. Dropsondes sampling the region prior to convective initiation indicate that ensemble members with better representations of upstream conditions in sensitive regions are associated with better convective forecasts over Nebraska.