Fig. 3.1. Severe convective storms producing weather pattern in the US.

Figures 3.1 and 3.2 show typical severe weather patterns over three different regions. In the US (fig. 3.1) severe convection is likely to occur when a plume of warm and moist air pushes north hand in hand with a low-level jet and ultimately meets the east-west oriented frontal zone. Moreover, moist boundary layer air is quite often capped by dry and relatively warm air. A short upper-level trough may also approach from west. This pattern results in veering wind profiles and strong deep-layer shear with several thousand J/kg of CAPE.

Fig 3.2. Synoptic weather associated with severe thunderstorms in western continental Europe (left) and in Finland (right).

Fig 3.3. Typical Spanish Plume sounding with an elevated mixed layer and a cap. Source: University of Wyoming.

The weather pattern conducive to severe convection in Western Europe is presented in figure 3.2. The so-called 'Spanish Plume' pattern brings an elevated layer of mixed air from southwestern to western Europe ahead of a pronounced upper-level trough. If moist boundary layer air is present, these two layers of air together make a significant amount CAPE possible to form (fig. 3.3.).

Due to the jet streak and the strong upper-level trough deep layer shear increases in warm sector and provides favourable environment for severe thunderstorms. Additionally, minor low pressure centres and pre-frontal convergence zones may result in local southeasterly or easterly surface winds. This situation is ideal for the formation of significant amount of storm-relative helicity and this way for the development of severe thunderstorms with rotating updrafts.

In Finland, severe thunderstorms are likely to occur under strong southerly or southeasterly flow ahead of an elongated upper-level trough. A tongue of high equivalent potential temperature values streams from the Baltic States or Russia to Finland hand in hand with a low-level jet. A north-south oriented undulating frontal zone is also present in most cases. It is noteworthy that the basic ingredients for the severe thunderstorm occurrence are similar in the US, Western Europe and Finland but the orientations of the weather patterns are different.