Geopotential height and Relative Humidity 700 hPa

18 September 2007, 00 UTC

Geopotential height in black lines and relative humidity (>80%) in cyan shading are superimposed to the IR 10.8 image.

A cold front is approaching the Alps from the west. The frontal clouds are located on the leading edge of a very deep trough at 700 hPa. ECMWF humidity field corresponds to the cloud band.

18 September 2007, 06 UTC

The through propagates to the east and is now crossing France. The onset of the prefrontal convection is seen in the Alpine region. The humidity field corresponds nicely to the cloudband of the cold front, but in the region of prefrontal development correpondence is not that good.

18 September 2007, 12 UTC

The trough, together with the frontal cloud band is moving further eastwards. At this point in time there is a lot of convective activity in the southwesterly stream on the leading side of the trough. Notice a small trough appearing over the western edge of the Alps, over northern Italy.

18 September 2007, 18 UTC

The front is passing over the Alps. There is still convective activity ahead of it. The small trough above the Alps gets deeper. The humidity field at this point corresponds more to the leading edge of the frontal cloud band.

19 September 2007, 00 UTC

The front has moved further east and the convective activity ahead of it still occurs in eastern Europe.