The Role of Elevated Thunderstorms in the Poleward Progression of Warm Fronts

Analysis of two elevated convection cases that occurred during the Program for Research on Elevated Convection with Intense Precipitation (PRECIP) is presented. Both of these cases occurred north of a θ_e boundary and above a surface stable layer. In one of the cases, which occurred 17 July 2014 over southern Oklahoma and northern Texas, the boundary stopped poleward progression after convection initiated; the boundary retrograded over 100 km in 12 hours as a result of elevated convection. This event was not well forecasted in terms of precipitation amounts or locations. Rainfall amounts in excess of 200 mm in 6 hr were reported in north-central Texas as a result of the boundary being forced farther to the south into a more moisture laden environment. Neither raw numerical model output, nor PRECIP or National Weather Service (NWS) forecasters were anticipating this to occur, and a flash flood watch was not in place for the affected area before heavy rainfall was observed. In another case, which occurred 02 – 04 April 2014 over central Missouri, the boundary continued to advance poleward, even after elevated convection initiated. This case was better forecasted in terms of precipitation amounts and location by models and forecasters, and amounts of >125 mm were observed. Public awareness was high for this event as a flash flood watch was initiated before heavy rain was observed. The current thinking is that the presence of a dynamic upper level jet streak, state of the surface low pressure system and orientation of Corfidi vectors play a role in how elevated thunderstorms and boundaries react. These three parameters will be highlighted, as well as analyses of low-level theta-e, temperature, 10 m wind analyses, frontogenesis (frontolysis). Additionally, work into determining storm propagation is currently ongoing, and will be presented. The goal of these analyses is to help determine why some boundaries continue to advance poleward, while others retreat after elevated convection initiates. As seen in the 17 July 2014, the inability to recognize when boundaries are retrograding can have big impacts and can result in poor situational awareness, and poor advanced warning.