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The objective of this work is to study the atmospheric conditions during cloud-to-ground (CG) lightning activity and the associated meteorological systems. A more comprehensive study of the CG lightning characteristics may lead to a better understanding of the thunderstorm severity. For these developments, it was selected two cases of severe thunderstorms that occur in Southeastern Brazil. The synoptic and mesoscale conditions previous to and during the CG lightning activity was analyzed. Furthermore, the number of CG strokes and its polarity are also evaluated in time in order to verify the electrical behavior of the thunderstorm. As a main result, it was observed that some patterns in CG lightning activity might indicate some level of severity in such thunderstorms. A storm occurred on November 17, 2008 has its genesis around 12:00 Z on the state of Sao Paulo. Its a gradual evolution over time, but since its inception has caused heavy rain over the state, and this accompanied by lightning occurrence. The number of negative flashes during the whole time the storm was higher than the positive flashes, indicating a not so intense severity. But on June 27, 2009 a storm reaches State of the Sao Paulo near 12:00 Z, isnt a gradual evolution over time. The number of negative flashes is much higher than the positive in a few hours. But at other times, the number of positive flashes is closer to the number of negative flashes, particularly from 21:00 Z to 22:00 Z, where both number are almost equal. This behavior indicates a much higher severity in this case compared to the previous case. Among the results, considering storms with similar characteristics, not necessarily they present the same severity. Thus, the study of the lightning occurrence is important for the assessment a storm severity. As a future proposal other cases of severe storms including a more detailed study of lightning will be studied. The idea is to create a severity rating from the set of meteorological data based also in the lightning characteristics.