High Quality Content by WIKIPEDIA articles! A severe thunderstorm warning (SAME code: SVR) is issued when trained storm spotters or a Doppler weather radar indicate a strong thunderstorm is producing dangerously large hail or high winds, capable of causing significant damage. In the United States, it does not account for lightning, a significant hazard in any thunderstorm, or flooding caused by a severe thunderstorm`s extreme rainfall. (a Flood advisory or Flash flood warning is issued in this case). A similar warning is issued by Environment Canada`s Meteorological Service of Canada from their offices in Vancouver, Edmonton, Toronto, Montreal and Dartmouth. Skywarn issues the warnings for the United Kingdom.