In late-June 2017 a record breaking spell of hot weather a short wave trough crossed the UK and moved east over western Europe setting of a series of large and long-lived thunderstorms over Denmark, Germany and Poland.
By Ian Mills (EUMETSAT) and Djordje Gencic (RHMSS)
On the animation of the Meteosat-10 Severe Convection RGB imagery, animation from 22 June 06:00–18:00 UTC (MP4, MB) and Figure 1 (top right, click to expand), the first of these storms which developed over the North Sea (yellow area) could be seen moving across Denmark and northern Germany.
A second band of storms formed over southeast England and moved across the North Sea before dissipating as it reached Denmark. The bright yellow colours turn more orange as the convective activity dissipates and the ice crystals in the top of the cloud become larger.
However, convergence ahead of this storm set off further storms in eastern parts of Germany. Meanwhile in southwest Germany further storms were generated as surface temperatures rose. As the afternoon progressed the storm cloud tops covered virtually the whole of Germany.
The upper air sounding for Essen at 12:00 UTC on 22 June (Figure 2) showed extremely unstable air with cloud tops extending to around 40,000 FT (12,000 m). Cloud top temperatures would have been around -65 C, with the overshooting tops being even colder.
The animation of the Meteosat-10 infrared images, also from 06:00–18:00 UTC (MP4, 2 MB) , showed the coldest tops, as would be expected, in the range of -65 to -70 C. With cloud tops this high and this cold it was not surprising that the storms were so severe.
The Meteosat-10 HRV animation (MP4, 2 MB) from the same period, showed the overshooting tops as frequent small domes pushing out above the main cirrus anvil produced as the cloud spread out under the tropopause. These overshooting tops were frequent and numerous — once again pointing to the severe nature of the convection in these storms.
Very intensive developments can also be seen along the cold front boundary, but most of them were more short-lived.
As already mentioned above, the convective storms on this day formed on two fronts: first on the warm front (already in the morning hours, as elevated thunderstorms), which is the most important and persisting event of the day, and then also on the cold front (further to the west) in the afternoon hours.
Even though a warm front is not a first place where forecasters expect rapid, strong thunderstorm development, compared to a cold front, sometimes, during summer months when there are favorable conditions, intensive thunderstorm development can happen on a warm front boundary.
Specifically, most often, development starts on a warm front, close to what is known as a 'triple point' — a place inside the theoretical structure of a mid-latitude cyclone where three fronts (cold, warm and occlusion) meet. Near the triple point there is often enough dynamic uplift both in lower and higher levels of atmosphere. These conditions are very important conditions for long-lived, vigorous thunderstorms to form.
Also, low level convergence occurs near the fronts. This convergence provides the initial lift for the formation of storm clouds especially in an area of moist unstable air.
This combination of conditions existed inside the system which travelled very fast during the two-day period over central and southeastern Europe.
On the Meteosat-10 enhanced IR 10.8 animation, 22 June 00:00 UTC–23 June 23:30 UTC (MP4, 8 MB) that very persistent mechanism was present most of the time in the area of triple point, generating very intensive storm system starting over Northern Germany all the way to Romania and Black Sea coast.
Also, because of the very intensive streaming of air inside this small scale cyclone, main severe weather event was strong wind and, to a lesser extent, hail. This can be clearly seen on the European Severe Weather Database reports (Figure 3).