IR-imagery, surface measurement of pseudopotential temperature and wind

18th August 2004: 06UTC
The cloud band extended from western France to the Bay of Biscay. The pseudopotential temperature (tpspot) in combination with the wind barbs exhibit values between about 50 and 60°C, indicating that thunderstorms are possible. Maximum values are connected with a confluence near the Atlantic coast of France. This gives the first hints that thunderstorms are likely to develop.

18th August 2004: 07UTC
We see a displacement to the East and an increasing tpspot to about 60°C.

18th August 2004: 08UTC
At the leading edge of the developing squall line tpspot increased to values between 60 and 65°C indicating a higher risk of thunderstorms.

18th August 2004: 09UTC
At the leading edge of the developing squall line tpspot increased to values between 65 and 70°C. This is a clear hint that many thunderstorms may develop and some may be severe.

18th August 2004: 10UTC
At the leading edge of the developing squall line tpspot increased to values around 70°C (severe thunderstorms). At this time we recognise a confluence near the highest values of tpspot. This is typical before the onset of convection.

18th August 2004: 11UTC
At the leading edge of the developing squall line tpspot increased to values of nearly 70°C further to the South values were over 70°C. At this time we recognise a confluence near the highest values of tpspot near the eastern edge of the cloud band. It is likely that new cells will develop causing a sudden displacement of the squall line to the East.

18th August 2004: 12UTC
At the leading edge of the developing squall line tpspot increased to values just over 70°C. At the eastern edge of the developing squall line the confluent streaming intensified.

18th August 2004: 13UTC
At the leading edge of the developing squall line tpspot increased to over 70°C. Near Paris ("P") deep convection is occurring (small blue area).

18th August 2004: 14UTC
At the leading edge of the developing squall line tpspot increased to values just over 70°C. Near Paris ("P") deep convection is occurring (small blue areas). A more pronounced convergence zone can be seen in parts of the frontal edge of the cloud area.

18th August 2004: 15UTC
This image shows the typical structure of an intensifying squall line. Hot and humid air (tpospot about 70°C) at the leading edge of the squall. In the frontal part we find deep convective cells (cloud top temperature below -60°C) ordered as a line and combined with a confluence and tpspot of about 65°C (reduced air temperature, high dew points). At the rear we observe air of lower tpspot (colder and more stable).

18th August 2004: 16UTC
The patterns described at 15 UTC are more pronounced. The satellite image exhibits a chain of deep convective cells and a well developed convergence near-by. Tpspot between 65 and 70°C at the leading edge of the squall line indicates a risk of thunderstorms some of which may be severe.

18th August 2004: 17UTC
A large cell with cloud top temperatures below -60°C in combination with a strong convergence zone developed over North East France / Luxemburg.

18th August 2004: 18UTC
The cloud band and the convergence zone moved quickly east and influenced West Germany. According to the diurnal cycle tpspot decreased to values of about 65°C in the leading edge of the squall line. We observe the typical fluctuation of cloud tops due to decaying and developing of cells. We find only a few small cells with top temperatures below -60°C.

18th August 2004: 19UTC
New convective clusters developed over the Rhine valley. Even by 19 UTC we still see tpspot of more than 65°C, indicating high potential for convection triggered by a pronounced convergence at the surface.

18th August 2004: 20UTC
The squall line moved quickly east whereas the most intense convection happened over the Western and Northwestern parts of Germany. However, we see that the confluence is in the central part of the cloud band. The squall line started weakening as can be seen from the decaying blue coloured areas.

18th August 2004: 21UTC
The 21 UTC image shows some small areas with cloud top temperatures below -60°C typical structure of a weakening squall line. We find a confluence in the central to the rear of the cloud band and tpspot of 50 to 55°C indicating only a low risk of thunderstorms. First small blue coloured pixels developed over the North Sea. Here, we have higher tpspot in combination with confluence. That may cause some convection over the North Sea later on.

18th August 2004: 22UTC
With the exception of the Western Alps we find only a few small areas of deep convective clouds. However, tpspot of more than 60°C over the North Sea in combination with the confluence could be an indicator of the development of new convective cells.

18th August 2004: 23UTC
The blue areas are distributed over Northwest Germany, the Eastern North Sea and Western Baltic Sea. The satellite image shows convective structures (not very deep) over many parts of Germany that are related to some thunderstorms. Several confluences (e.g., over South Germany) can be seen. However, they do not exhibit a line structure.

19th August 2004: 00UTC
At 00 UTC the image shows deep convective clouds over South Eastern France and between the Northern parts of Germany and Southern Scandinavia. Over the North Sea and Southern Sweden cloud top temperatures below -60°C were detected and there was a weak convergence extending from the North Sea to Central Germany. However, no thunderstorms were observed at 00 UTC.