A typical area with a high probability for the initiation of convection is the area of the warm sector in front of and some distance
away from the frontal cloud band. Initiation of convection only happens in cases where the troposphere has a specific structure.
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.
The troposphere under such conditions is characterized by :
- a tongue of high values of equivalent potential temperature at a low levels (usually at 850 hPa) representing moist warm air
- warm advection in lower and middle troposphere and cold advection aloft resulting in potentially unstable vertical stratification
- significant 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 the troposphere
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MCSs develop in connection with warm air, therefore they can be found in front of or within the leading parts of cold fronts.
References