MCSs frequently develop at the leading edge of frontal cloud bands. This is most likely the situation where there is a specific vertical distribution of air mass and conveyor belts: a warm conveyor belt at low levels in front of the front and a dry intrusion in the layers above.

Such a situation develops when a jet streak crosses the front; dry air on the cyclonic side of the jet axis overruns the frontal cloud band and reaches its leading edge. Whilst the relative streams of the dry intrusion are sinking behind the frontal cloud band, they change to rising during the crossing of the front.

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

• frontal conditions indicated by TFP, a zone of high thickness gradients, a downward inclined gradient of isentropes, a frontal circulation created through convergence and upward vertical motion, etc.
• a rising warm conveyor belt in low to middle layers along the leading edge of the frontal cloud band
• a (rising) dry intrusion in middle to high layers along the same leading edge of the frontal cloud band
• the effect of both both conveyor belts lying on top of each other leads to the unstable stratification of the troposphere

The satellite image shows a cold front cloud band which extends from Tunisia across the Mediterranean Sea, the Adriatic Sea, east Austria to north-east Poland. Along the leading edge of the cloud band MCSs have developed, especially over the Mediterranean Sea south of Sicily (approximately 38N/12E) the Tyrrhenian Sea (approximately 40N/13E) and the Adriatic Sea (approximately 42N/16E).
The first image (top left) has the thermal front parameter and the temperature advection field superimposed. The MCSs have developed ahead of the surface cold front, which is situated where the thermal front parameter has its greatest values.
The cross section (top right) crosses the Cold Front above the Tyrrhenian Sea (bottom left).
The relative streams on the isentropic surface of 314K (bottom left) show that the MCS are under the influence of a warm conveyor belt, which is a moist relative sream.
On the higher isentropic surface at 318K (bottom right) the whole area of the MCS is under the influence of a relative stream of dry intrusion. This vertical distribution of conveyor belts, together with upward motion, leads to the development of a conditionally unstable stratified troposphere. The satellite image (top left and right) shows the development of MCSs near the coast of Tunisia (approximately 38N/09E).
The MCS development is also situated within the area of the left exit region of the jet streak which extends from the south coast of Ireland (approximately 51N/09W), across the Bay of Biscay and the Mediterranean Sea to east Algeria (approximately 34N/07E) (top left). The left exit region is accompanied by a pronounced PVA maximum at 300 hPa.
The WV image clearly shows the jet axis (dry air along the cyclonic side and moist air along the anticyclonic side of the jet axis) following the line 50N/04W - 45N/00E. Above north-east Algeria (approximately 37N/08E) and the Mediterranean Sea (approximately 38N/07E) some low level cloudiness develops along the leading edge of the dry air which is situated along the cyclonic side of the jet. The MCS cloudiness itself is characterized (in WV) by a black area around each MCS which is typical for such systems (see Cloud structure in satellite images).
Six hours later, the MCSs within the area of the left exit can be found above the Tyrrhenian Sea (approximately 40N/14E) (bottom left). Other MCSs have developed downstream above Italy and the Adriatic Sea.
The WV image again very clearly shows the jet axis (bottom right). The jet axis can be found at approximately 50N/01W - 45N/02E - 42N/04E. The cloudiness along the cyclonic side of the jet axis, in the leading part of the dry air, can be found above the Mediterranean Sea, south-west of Sardinia (approximately 39N/08E). The cloud tops along the cyclonic side are continously increasing downstream which can be explained by the supply of more humid air in upper levels. The surroundings of the MCS cloudiness are again characterized by this typical black (dry) area around of the cell (see Cloud structure in satellite images).

MCSs develop in connection with warm air, therefore they can be found in front of or within the leading parts of Cold Fronts. Cbs develop at the rear parts of Cold Fronts or in the cold air, have different synoptic environments and are descibed in Cb Cluster, EC and Comma (see Cumulonimbus Cluster , Enhanced Cumulus and Comma ).