3.3 Pattern recognition
3.3.1 Upper-level features

Where positive vorticity advection increases with height the omega equation predicts rising motion (neglecting all other terms in the equation). It was shown in Fig. 1.3. and 1.4. show that positive vorticity advection tends to occur ahead of upper-level troughs and in the left exit and right entrance regions of jet streaks. This large scale ascent may provide lift that is required for the initiation of deep convection.

Strong upper-level flow in the proximity of jet streams is associated with moderate or strong deep-layer shear (0-6 km AGL). Where moderate to high deep-layer shear and sufficient instability overlap, thunderstorms may become severe. When shear is stronger than around 20 m/s in the 0-6 km AGL layer, supercells become likely. Values around or above 20 m/s are favourable for severe bow echoes, too. As illustrated in Fig. 1.5, the ageostrophic circulations associated with jet streaks may lead to the backing of surface winds over the jet streak exit region. As a result, veering wind profiles and clockwise curved hodographs can be expected that are associated with significant amounts of storm-relative helicity. If thunderstorms develop in such an environment, forecasters should be on the alert for supercells.