18th August 2004: 06UTC
With exception of the Bay of Biscay which has medium clouds and weak negative vorticity advection, the deep convective clouds from West France to Northern Spain are super-imposed by positive vorticity advection (continuous lines) east of a short wave trough.
18th August 2004: 12UTC
The bright cloud band associated with the developing squall line is super-imposed by two centres of positive vorticity advection and are related to two short wave troughs.
18th August 2004: 18UTC
Strong positive vorticity advection east of the trough can be seen over Central France. This pattern relates well with the bright cloud complex where convection can be identified. However, the squall line over Western Germany was super-imposed only by weak vorticity advection (negative and positive) that is in a good correspondence with the weakening of the squall line.
19th August 2004: 00UTC
The rest of the squall line (North Germany and North Sea to Baltic Sea) are mainly in related to negative vorticity advection. there was warm advection over these areas (sub-chapter about surface pressure and geopotential in 300 hPa) resulting in stabilisation. Therefore, mainly stratiform rain was observed in that area.
