Intraseasonal variability of severe hail and wind: connection to the Madden-Julian Oscillation

The Madden-Julian Oscillation (MJO) is the leading mode of intraseasonal variability of the tropical atmosphere. Through teleconnections, the MJO has been shown to modulate a host of global weather phenoma throughout the year, including rainfall, circulation, and temperature. In the U.S., upper-level geopotential heights, surface temperature, convective rainfall, and cloud-to-ground lightning have been found to vary by phase of the MJO. However, until recently, very little has been known about the relationship between the MJO and severe weather events, despite known variability in large-scale patterns known to affect tornado activity. Recent work has found that April and May tornado-day likelihood in the central U.S., supported by concurrent variability in convective available potential energy (CAPE), bulk wind shear, and storm-relative helicity, also changed depending on MJO phase. Given that tornadoes are not the only hazard from severe convective storms, it is likely that hail and wind likelihood also vary by phase of the MJO.

The primary hypothesis of this study is that deep convection associated with the MJO generates Rossby waves that propagate eastward and northward, and, upon reaching the U.S., drive quantifiable intraseasonal variability in the synoptic and storm-scale conditions that support severe convective storms. Part of this hypothesis was proven by a recent study, and this current work extends their findings by examining metrics of severe activity created from the hail, and wind reports in the NOAA storm event database from 1990-2012. The specific objectives of this study are to (1) quantify the variability of U.S. severe wind and hail events by phase of the MJO; (2) quantify the intraseasonal variability in tropospheric variables known to be physically associated with severe wind and hail; and (3) quantify the intraseasonal variability by season, geographic region, and phase of other climate oscillations.