Question submitted by Jay
(Grand Forks, BC)
Ok, Jay, first let's denounce the misconception that thunderstorms only occur when it's "warm". As Einstein would say "it's all relative!" Thunderstorms can occur whenever a pocket of RELATIVELY cooler air is introduced to an airmass - at a RELATIVELY warmer temperature - and "lift" or a upward forcing trigger is applied to that airmass. On the date in question, a nasty squally cold front swept through the region.. introducing an area of cooler temperatures aloft to an area that was relatively cool, but much wamer than the air moving into the region. That together with the lift provided by the cold front, set the thunderstorms off.
Also Grand Forks - topographically speaking - is in a bit of a low land. The mountains surrounding the area often provide lift to incoming weather systems.. thereby generating their own thunderstorms (in situ so to speak) relatively easily.
Hope this answers your great question.