18th August 2004: 12UTC
The squall line extends from the English Channel to the Bay of Biscay and is just beginning to develop. We see the first lightning detections (11:45 to 12:00 UTC) at its leading edge where bright clouds appear. However, the tops are still warmer than -60°C.
18th August 2004: 13UTC
The squall intensified and we find many lightning detections near the deep cold clouds (blue areas).
18th August 2004: 14UTC
This image shows that the squall line is an organised band of cloud with a chain of thunderstorms at the leading edge. Lots of lightning was detected near the clusters with the lowest cloud top temperatures. Additionally, some lightning occurred further to the south where we can see deep convective clouds (bright areas).
18th August 2004: 15UTC
The squall line intensified quickly. It exhibits a clear structure of thunderstorms at its leading edge and high lightning activity near the highest cloud tops.
18th August 2004: 16UTC
The squall line continued to intensify. With the exception of the south-western part (however, here the DWD-lightning detection system exhibits a rate of only about 50%!) the image shows a high density of lightning, mainly near the highest clouds. Within the whole picture 1410 reports of lightning in only 15 minutes were detected. In the northern part of the squall line the maximum activity of lightning is more to the centre and rear of the deep convective clouds.
18th August 2004: 17UTC
The lightning activity decreased. We can still observe a well structured line of lightning extending from Brussels to eastern France. There is not a strict correlation between lightning activity and cloud top temperature. The most active zone is moving more and more to the rear of the broadening cloud band.
18th August 2004: 18UTC
The amount of lightning decreased further. However, the squall line kept its line structure.
18th August 2004: 19UTC
A strong cluster with lots of lightning developed over Western Germany. The highest cloud tops are observed there.
18th August 2004: 20UTC
The whole system moved quickly to the east. The squall weakened in its southern part however an intensification (with some lightning) can be seen over northwest Germany.
18th August 2004: 21UTC
The squall weakened and only 720 lightning strikes were detected. Areas of cloud top temperatures below -60°C are no longer concentrated at the leading edge.
18th August 2004: 22UTC
The lightning activity over Germany weakened. However, over the North Sea we can see an intensification. The low cloud top temperatures do not mean we can automatically assume severe thunderstorms, especially during the decaying phase of a cell.
18th August 2004: 23UTC
The reduction of lightning strikes over Germany continued. However, from the tpspot-values and the confluence over the North Sea (see corresponding links) we observe increased lightning activity in that region.
19th August 2004: 00UTC
Almost no lightning was detected over Germany. The rest of the squall line shows mainly stratiform rain (see link to sigwx). Over the North Sea we see some lightning. However, further to the east the satellite image also shows cloud top temperature below -60°C. This is cirrus clouds.
