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The simple answer is to look at a picure of a cumulonimbus cloud, or a diagram may be better, you will notice that it is vaguely triangular. The rain is released at the middle and towards the rear of the cloud. So first you see the approach with its thunder and lightning then as the cloud passes over you, you have the rain.Edit:If you want to learn the basics then pop down to your library and find books by Alan Watts, 'weather wise' is a good enough place to start. Or listen to MPR (minnesota public radio, which is next to you...i think) they have a good meteorlogical segment on wednesdays or thursdays.you could even pop down to the University of Minnesota in St Paul, if you are thinking of meteorology as a career, for a look around the Dept of Soil, Water, and Climate
The thunderstorm life cycle:1) Towering cumulus stage - Imagine a parcel of air like a balloon. If the air in the balloon is warmer than the environment around it, it will rise. As the balloon (air parcel) rises, the air cools, eventually cooling to its condensation point. A cloud becomes visible. As the air condenses, heat is released which helps the air parcel remain warmer than its surrounding environment, and so, it continues to rise, building up speed. This rising air forms the updraft, a thermal. A towering cumulus cloud has grown with crisp, hard edges forming a puffy or cauliflower look to the cloud. The height of the cloud is usually equal to or greater than the width of the cloud's base.2) Mature thunderstorm stage - The warm air continues to rise until eventually it has cooled to that of its surrounding environment. This is often not until it hits the tropopause and the more stable air of the stratosphere. The storm may now have reached a height of 5 to 10 miles above the ground. The rising air has been moving at speeds near 40 mph. Now as it slows, the upper level winds begin to fan out the cloud forming the anvil. With strong winds aloft and longer lasting storms, anvils can spread 100 miles downwind.A thunderstorm's updraft can carry 8000 tons of water aloft per minute! The water vapor condenses to cloud droplets which collide and grow in the rising updraft. Eventually, the weight of the droplet overcomes the rising air and it falls. The falling rain droplets begin to drag the air down around them and a downdraft forms. The rain also is falling into unsaturated air and so some evaporation occurs. Evaporation is a cooling process (your body cools when sweat evaporates from your skin). This rain-cooled air is now cooler than its surrounding environment and it sinks, helping to form and intensify the downdraft. A thunderstorm with concurrent updrafts and downdrafts is considered mature. As little as 20 minutes has elapsed since the cloud began to form.3) Dissipating stage - As the downdraft hits the ground, the rain-cooled air begins to spread out in all directions. Eventually, this more stable air (since it is cool) chokes off the warm inflow that was driving the storm's updraft. With no new fuel to keep the storm alive, it dies. The downdraft dominates and the storm rains itself out. Sometimes, all that is left it the anvil.This entire thunderstorm life cycle from the growing cumulus cloud to the dissipated storm can take only 30 minutes. This is why thunderstorms can strike so quickly and with little if any warning. The National Weather Service predicts the likelihood of thunderstorms to develop, but does not warn for lightning nor general thunderstorms.