Precipitation - Rain and or hail. The record for Minneapolis is 12 inches of rain from a thunderstorm in less than three hours. In most cases rain or hail is an inconvenience, however, one should consider what would happen to your snug harbor if there were a flash flood, because of massive rainfall upriver. Fifty to sixty knot winds are not uncommon. Lightning - Without a doubt lightning is the most dangerous aspect of a thunderstorm.
More people die annually in the U.S. from lightning strikes than any other weather phenomena.
While this may be the most frightening aspect of a thunderstorm, in truth it may be the most helpful. Since lightning is the real killer, thunder can help you tell how close a lightning flash is to you. This is based on the concept that light travels at 186,000 miles per second and sound travels at a little over 1,000 feet in a second, or about five seconds to cover a mile. When you see a particularly vivid flash you begin counting one thousand one, one thousand two, etc. If you get to five before you hear the thunder, the lightning strike is about a mile away.