 Organized areas of thunderstorm activity not only reinforce pre @-@ existing frontal zones , but can outrun cold fronts in a pattern where the upper level jet splits apart into two streams , with the resultant Mesoscale Convective System ( MCS ) forming at the point of the upper level split in the wind pattern running southeast into the warm sector parallel to low @-@ level thickness lines . When the convection is strong and linear or curved , the MCS is called a squall line , with the feature placed at the leading edge of the significant wind shift and pressure rise . Even weaker and less organized areas of thunderstorms lead to locally cooler air and higher pressures , and outflow boundaries exist ahead of this type of activity , which can act as foci for additional thunderstorm activity later in the day . 
