Multi-scale characteristics of extreme precipitation episodes in the southern United States

Abstract:

The southern United States, with its proximity to the Gulf of Mexico, the subtropical jet, and common tropical cyclone pathways, frequently experiences widespread episodes of heavy rainfall. Within these climatologically frequent rainy periods, however, are truly extreme precipitation episodes. Using observations, analyses, operational ensemble forecasts, and numerical simulations, the multi-scale processes responsible for extreme rainfall in the southern US will be examined in this presentation. At the synoptic scale, these episodes are generally characterized by a pattern supporting persistent forcing for ascent, but mesoscale convective organization is crucial to the occurrence and distribution of extreme precipitation. Furthermore, in subtropical environments with modest vertical wind shear, the latent heating in mesoscale convective systems (MCSs) often produces vortices that outlive the MCS that produced them. Similarly, convective processes can support the maintenance of remnant tropical cyclones after landfall. These situations are particularly favorable for long-lived, widespread heavy precipitation. Ensemble-based diagnostics and numerical model experiments will be used to quantify the respective roles of large-scale, convective, and orographic processes, along with their predictability, in recent episodes of extreme precipitation in the southern US, including those occurring in Texas and Oklahoma in May and June of 2015.