Variability of central United States April-May tornado day likelihood by phase of the Madden-Julian Oscillation

April-May tornado day likelihood from 1990 to 2011 was calculated for the central United States for phases of the Madden-Julian Oscillation (MJO). An April tornado day was found more likely during MJO phases 6 and 8 and less likely during phases 3, 4, and 7. A May tornado day was found more likely during phases 5 and 8 and less likely in phases 2 and 3. During phases with above-normal tornado day likelihoods, positive anomalies of convective available potential energy, bulk vertical wind shear, and storm-relative helicity were found in the central United States. Negative anomalies were found during phases with below-normal tornado day likelihoods. Anomalies of such environmental parameters were connected to the MJO via variability in tropospheric circulation. These results provide an important starting point for extended range prediction of U.S. tornado activity.