Thunderstorm Frequency Distributions - Winnipeg, MB
Day(s) with the highest frequency of thunderstorms = June 27 and July 11 (22 years out of 68)
The above graph shows that, for example, ~25% (25th percentile) of thunderstorm days occur on and before June 18, ~50% (median) occur on and before July 14 and ~75% (75th percentile) occur on and before August 15.