There are typical situations in the thickness ridge ahead of Cold Fronts where MCSs develop quickly in the afternoon. In low and middle layers moist warm air is advected from the south or south-west which, together with the drier and colder air at upper levels, leads to potentially unstable stratification of the troposphere.

If such a situation occurs close to the Cold Front the effect is often increased by dry cold air from behind the Cold Front overrunning the frontal zone at higher levels (see Thickness Ridge Cloudiness ).

The troposphere under such conditions is characterized by (see Key parameters ):

• a tongue of high values of equivalent potential temperature at a low level (usually at 850 hPa) representing moist, warm air
• warm advection in the lower and middle layers and cold advection above resulting in potentially unstable vertical stratification of the troposphere
• notable convergence in the lower layers (for instance from the surface up to 700 - 600 hPa), which is one process causing upward motion
• a rising warm conveyor belt throughout a thick layer of the troposphere

The satellite images show a Cold Front extending from Algeria (approximately 33N/00E) across the Mediterranean Sea and Switzerland to the border of France and Germany (approximately 49N/08E). The leading part of the Cold Front lies over Corsica, the northern Tyrrhenian Sea and northern Italy (approximately 45N/10E).
The above left image shows a pronounced TFP indicating the surface front within the leading area of the Cold Front. The MCSs situated ahead of the surface cold front have developed along the warmer side of the high gradient of equivalent thickness.
The above right image shows that the MCSs have developed within an area of WA within the lower and middle levels of the troposphere.
The relative streams are calculated on the isentropic surface of 312K.
The isentropic analysis shows that the MCSs have developed within the ascending relative stream (called upper relative stream). This relative stream rises from approximately 500 hPa to 400 hPa within the area of MCS development. The vertical cross section shows a typical vertical distribution of the divergence field: there is pronounced convergence in front of the Cold Front in the lower levels of the troposphere up to approximately 600 hPa, and pronounced divergence above. The images above show the development of MCSs above France (approximately 48N/01E and 50N/03E). Other MCSs have developed above the Iberian Peninsula. The MCSs are situated ahead of a well-developed Cold Front.
In the above left image, the Cold Front is characterized by a pronounced TFP close to the leading edge of the front and a high gradient of equivalent thickness within the cloud band (compare Cold Front). The MCSs have developed in the tongue of warm air ahead of the surface front (maximum values of TFP) which is also indicated by the model by high values of equivalent thickness.
In the above right image, the temperature advection field mostly shows weak WA where the MCSs have developed.
The equivalent potential temperature at 850 hPa has a pronounced gradient within the Cold Front zone (lower left image). The MCSs can be found along the warmer side of this high gradient where the equivalent potential temperature has a maximum. The above diagrams show a cross section across the frontal cloud band and the MCSs which have developed ahead of the cloud band.
In the cross section the MCSs are characterized by high pixel values in the IR as well as in the WV image (grid point 28/23).
The isentropes in this area show an unstable stratified troposphere (decreasing values of equivalent potential temperature with height) within the the lower layers of the troposphere up to approximately 700 hPa. WA can be found in the lower levels and CA in the upper levels of the troposphere (above right image).
The field of divergence (lower left image) does not correspond to the ideal vertical distribution of divergence: In this case, the area of the MCS is indicated by weak divergence at lower levels and weak convergence at middle levels of the troposphere.
The stability analysis of radiosonde stations (especially 06476 - St. Hubert) show an unstable stratified troposphere in the lowest layers up to approximately 850 hPa (red). Between approximately 850 hPa and 600 hPa the troposphere is conditionally unstable. Above 600hPa the radiosonde show layers of stable and conditionally unstable stratifications. The appearance of this column is typical for an atmosphere where Cb cloudiness or MCSs develop.