In this part of the country severe weather can and does occur at any time of year. But it is the springtime that brings the greatest threat of thunderstorms and tornadoes.
It is common knowledge that thunderstorms are fueled by warm humid air. Yet the hottest days of summer actually have fewer severe thunderstorms than the cooler spring months. That's because the ingredients for the massive storms that bring hail, high winds, and tornadoes actually come together most often in the spring, when the atmosphere is transitioning from winter to summer.
Instability - When the atmosphere is ready to rumble: Think of a marble at the bottom of a mixing bowl. The marble is in what we call a "stable" situation. Try to move it, and it will roll right back to the bottom of the bowl. Turn the bowl over now, and balance the marble on top. A little nudge could send it rolling faster and faster off the bowl and probably off the table. In this case the marble is in an "unstable" situation.
In weather, our 'marble' is a parcel of warm air. When warm air aloft overlies cold air at the surface, the atmosphere is stable. But when our warm air parcel finds itself beneath colder air aloft it wants to rise and we say that the atmosphere is unstable. A nudge from a front or other disturbance can set our 'marble' rolling. This kind of instability occurs most often in the spring, when the sunshine is strong, but higher levels of the atmosphere are still cold due to the retreating winter. Add humidity and the atmosphere becomes even more unstable. This is because when moist air condenses into the water droplets that clouds are made of it releases heat, making the warm air even more buoyant.
Wind Shear - Taking storms out for a spin: On every weather segment there is a report of the current wind speed and direction. And most viewers probably assume that the wind is pretty much the same 'all the way up' in the atmosphere. In fact, wind speed and direction can chance a great deal with increasing altitude - a phenomenon know as "wind shear". Study the sky in the springtime and you will often see low clouds scooting in one direction, mid-level clouds moving in another direction, and perhaps high thin clouds drifting in yet a third direction.
The proper type of wind shear can turn a run-of-the-mill thunderstorm into a self-sustaining severe thunderstorm, with warm humid air feeding in from one direction, cool dry air coming in from another direction, and a rotating core spinning out hail or even tornadoes. Severe thunderstorms in the Heartland are usually associated with humid southeast to southwest winds at the surface and strong southwest to northwest jet stream winds aloft.
Fronts, Dry Lines, and Outflow - Pulling the trigger: Even when all the ingredients seem to be in place, the atmosphere usually needs a final 'trigger' to set things in motion. There's often a layer of warm air aloft which acts as a sort of lid, keeping clouds from growing beyond a certain height. At this point, forecasters watch for something that would force the atmosphere to rise past this lid, allowing clouds to build into giant thunderheads. Common forcing mechanisms include 'dry lines' on the high plains and 'sea breezes' near coastlines. In our part of the country, a pressure trough or cold front can often act as a trigger. Once the lid is broken, outrushing winds from one thunderstorm can set a whole complex of severe thunderstorms into motion. The transition from sunny pleasant weather to severe storms can occur very rapidly- a very dangerous situation for anyone unaware of the weather forecast.