Role of mesoscale factors at the onset of deep convection on hailstorm days and their relation to the synoptic patterns

Hailstorm forecast is a challenging task. The development of convective events is related to the synoptic environment as well as to the mesoscale factors, whose characteristics and evolution depend greatly on the topography of the study zone. This paper studies the mesoscale factors conditioning the beginning of deep convection in the 12 main events of severe hail in the Middle Ebro Valley, Spain, between 2001 and 2008. Previous results have enabled to classify the hailstorm days into five clusters, according to the atmospheric situation at a synoptic scale at low and mid‐tropospheric levels. Mesoscale factors that enhance the development of convection, such us convective instability, water vapor convergence and the low-level wind, have been studied using the Mesoscale Model MM5. The synoptic anomalies observed on the study days have been related to the mesoscale features responsible for the beginning of deep convection. The results show well-defined patterns with clear differences between the days belonging to the different clusters. The weight and the spatial patterns of the three selected factors are distinct in days of different clusters and lead to particular spatial precipitation patterns. This mesoscale analysis and its bidirectional relation to the synoptic anomalies and the spatial distribution of hail precipitation constitute a potentially useful tool for forecasting convective events in the study area.

► Synoptic patterns for severe hailstorm days show deep and well-defined anomalies. ► Mesoscale analysis is essential for identification where deep convection is initiated. ► The selected mesoscale fields allow to identify where the first hailstorms develop. ► The topography of the area plays an essential role in the beginning deep convection.