Climatology of elevated thunderstorms in the western United States

Most thunderstorms in the western United States are rooted in the boundary layer and are dependent on surface or near-surface conditions. During the warm season, moist convection increases in coverage and frequency due to the greater instability and periodic moist monsoonal flow. Thunderstorms are most common across the higher terrain of the West where air parcels can more easily reach the level of free convection (LFC). Across the valleys and deserts there is often limited low-level moisture and available forcing is not sufficient to lift parcels to the observed high-based LFC. Inversion layers, subsidence and divergent low-level flow can also inhibit or limit convection. This research has focused on the exceptions to boundary layer influenced moist convection; which have been labeled as elevated (non surface-based) thunderstorms. Using model and observed soundings along with satellite and lightning data the study attempted to make a distinction between elevated moist convection and moist convection which was caused by surface insolation effects, propagation, advection or may have been forced from low level boundaries.

Identifying the potential for elevated thunderstorms and understanding the processes that are involved was found to be a weakness in operational forecasting. Despite thunderstorms being common during the warm season this convection is typically considered to be terrain-driven and cold pool outflow influenced with a strong diurnal tendency. Given the proper synoptic pattern there can be sufficient mid-level moisture, lift, and deep layer instability to produce thunderstorms which are not dependent on terrain, boundaries or surface diabatic effects. The elevated thunderstorms examined in this study displayed similar intensity whether they occurred nocturnally, over cooler waters, in the valleys and deserts, or across the mountainous terrain.

The consequences of these thunderstorm events can have major implications for fire danger, human activity and agriculture. During a few events the intensity of the storms caused severe weather and over 5000 cloud-to-ground lightning strikes were detected. This climatological study analyzed numerous elevated thunderstorm outbreaks across the western United States and offers forecasting applications using satellite and numerical model data. The study also used composite reanalysis for identifying synoptic patterns.