High Plains Deep Convection: The Forecast Challenges of Predicting Diurnally Varying Mesoscale and Synoptic Scale Interactions over Complex Terrain

Many forecasters (including the lead author) have chased their tails trying to predict correctly the initiation, location, duration and intensity of diurnally varying warm season deep convection over the complex terrain of the High Plains. The Mesoscale Predictability Experiment (MPEX), to be conducted from 15 May to 15 June 2013, is designed to test whether enhanced early morning synoptic and subsynoptic observations over the Intermountain region and their assimilation into convection-allowing models will lead to improved forecasts of convective initiation and afternoon convective mode. This presentation will focus on two MPEX-related case studies of deep convection to illustrate: 1) the forecast challenges arising from diurnally varying synoptic and mesoscale flow interactions over complex terrain, and 2) the physical processes that can contribute to mesoscale forecast uncertainty. A suitable case study from MPEX may also be included in the presentation.

A common attribute of both aforementioned case studies was an upstream subsynoptic-scale upper-tropospheric potential vorticity (PV) anomaly over the southern Rockies. In the first case (24 May 2011), an early morning mesoscale PV anomaly located ahead of the upstream PV anomaly triggered disorganized morning convection over eastern Colorado and western Nebraska. As this convection moved eastward and better organized, it produced a southward-propagating outflow boundary that became the focus of new convection over Oklahoma where it intersected the dryline. This Oklahoma convection became severe as the aforementioned subsynoptic-scale PV anomaly reached the High Plains.

In the second case (19 June 2011), a mesoscale PV anomaly crossed the southern Rockies, intercepted a flow of moisture from the Gulf of Mexico, triggered mesoscale frontogenesis, warm-air advection, and ascent, and enabled mesoscale convective system (MCS) development to occur in moist upslope flow over southwestern Nebraska, northwestern Kansas, and northeastern Colorado. The resulting MCS produced two separate high-impact weather events: 1) a stratiform precipitation-dominated heavy rainstorm with embedded convective elements in southwestern Nebraska, and 2) a convection-dominated severe weather episode with isolated supercells to the east along the Nebraska-Kansas border. Neither event was anticipated nor forecast very well by the available operational models.