Sensitivity of Central Oklahoma Convection Forecasts to Upstream Potential Vorticity Anomalies during Two Strongly Forced Cases during MPEX

The role of upstream subsynoptic forecast errors on forecasts of two different central Oklahoma severe convection events (19 and 31 May 2013) characterized by strong synoptic forcing during the Mesoscale Predictability Experiment (MPEX) are evaluated by applying the ensemble-based sensitivity technique to WRF ensemble forecasts with explicit convection. During both cases, the forecast of the timing and intensity of convection over central Oklahoma is modulated by the southward extent of upstream midtropospheric potential vorticity anomalies that are moving through the base of a larger-scale upstream trough but pass by central Oklahoma prior to convective initiation. In addition, the convection forecasts are also sensitive to the position of lower-tropospheric boundaries, such that moving the boundaries in a manner that would lead to increased equivalent potential temperature over central Oklahoma prior to convective initiation leads to more precipitation. Statistical PV inversion and correlation calculations suggest that the midtropospheric PV and near-surface boundary sensitivities are not independent; the winds associated with the PV error can modulate the position of the lower-tropospheric boundary through advection in a manner consistent with the implied sensitivity. As a consequence, it appears that reducing the uncertainty in specific upstream subsynoptic features prior to convective initiation could improve subsequent forecasts of severe convection.