The two most important mechanisms for producing most thunderstorms are layers of instability - a region of the atmosphere with a tendency to promote upward air motions - and strong dynamic lifting - factors other than buoyancy that move air upward. Working in tandem, these two mechanisms provide that lift necessary to turn a precipitating cumulus cloud into a towering, lightning-producing cumulonimbus. In a warm-season, airmass thunderstorm, the atmospheric instability needed to start the process begins at the sun-heated ground. The moist air warmed at the surface becomes less dense (more buoyant) and begins to rise high into the sky, a process known as surface convection. The less dense the air is relative to the surrounding air, the higher and faster it will rise; in such cases we have a deep layer of surface instability.
But in the cold season, and particularly in air conducive to snowfall, the lower troposphere is dry and cold which makes it relatively stable and inhibits parcels of surface air from rising. However, the troposphere can have many layers of distinct airs with differing vertical thermal gradients (called lapse rates), and when a layer of warmer, moist air wedges its way between a surface cold layer and colder air above, a region of elevated instability forms that may give rise to elevated convection, the convective rise of air beginning at an altitude above the surface. The effect is enhanced when there is cooling in the middle troposphere layers above the warm air wedge to enhance the convection potential.
An example of elevated convection at an occluded front resulting in thundersnow.
There are several mechanisms that can cause strong dynamic lifting of air (which I look at in more detail in What Goes Up: Introduction to Updrafts ). When one of these occurs in an atmosphere that has layers of instability, it becomes a trigger for air to begin rising and continue to do so. In this manner, strong updrafts are formed. The lifting can work on elevated layers as well as surface layers and thus produce elevated convection. In the most intense cases of dynamic lifting, such as air forced over a mountain range, the resulting cumulus clouds have all the characteristics of purely convection cloud formation.
In late fall and winter, when cold air moves across the relatively warm waters of the Great Lakes and other large water bodies, intense convective storms often develop that drop snow upon reaching the downwind shores (see Great Lakes Snow Storms on this site). When the waters are significantly warmer than the air, they act similar to the solar-heated ground in the summer and form surface convection. Under these conditions, intense cumulonimbus cloud development may arise that induces lightning flashes. When these snow squalls strike with heavy snows and gusty winds, they are punctuated with flashing lightning and booming thunder, i.e., thundersnow.
