Variability of central U.S. April–May tornado day likelihood by phase of the Madden-Julian Oscillation
April–May tornado day likelihood from 1990–2011 was calculated for the central U.S. 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 U.S. 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.