Why are atmospheric instability sometimes dangerous?

Instability may become dangerous in some areas, resulting in the formation of flash floods, due to large amounts of rain falling in a narrow range, and in a short period of time, especially in rocky areas, or mountainous areas that flow rainwater. In it from the top to the valley, and the presence of previous dry seasons helps in the occurrence of torrents because rain water does not find channels for it to be absorbed into the ground, so the water remains accumulated on the surface of the earth, it started to run very quickly in the torrent valleys or the torrents that dug it previously, so It is necessary to monitor the streams of torrents, and to stay away from valleys, low areas and streams of torrents when there is a state of atmospheric instability, or atmospheric depressions.