Thunderstorms, while highly variable from year to year, are generally experienced from late April to the end of September in southern Saskatchewan, and from late May to the end of August in the north. During late fall and winter the weather situations conducive to the formation of thunderstorms occur only rarely in the province, and even then only in the south. The core of a thunderstorm is a convection current of relatively warm, moist air ascending rapidly and forming cumulus clouds of great vertical extent. In these clouds, rain drops and ice crystals are often mixed. Electrical gradients are produced between different parts of the cloud dominated by positive and negative charges respectively, and within-cloud lightning strokes result. The earth's surface having a positive charge and the bottom of the cloud being negatively charged, cloud-to-ground lightning strikes ensue.
Rain from thunderstorms is usually short-lived but intense. Occasional thunderstorms give rise to localized flash flooding, as near Parry, Saskatchewan, on June 19, 1999, when 92 mm of rain in four hours washed out the railway line and caused Long Creek to flood for three days through Dunnet Regional Park near Avonlea. Besides lightning and heavy rain, the tall cumulonimbus clouds of well-developed thunderstorms often produce hail and strong gusty winds; the most severe cases very occasionally generate a tornado. Strong wind gusts accompanying the arrival of a thunderstorm are known as downbursts or microbursts, and can do substantial damage. In the Oxbow area on the night of July 29, 1995, downburst winds exceeded 100 km/hr and created havoc, scattering lightly constructed buildings like chaff.