Reasons for thunderstorms BESIDES drylines
Sources of Lift (upward)
Typically, for a thunderstorm to develop, there needs to be a mechanism which initiates the upward motion, something that will give the air a nudge upward. This is done by several methods.
Differential Heating
This heating of the ground and lower atmosphere is not uniform. For example, a grassy field will heat at a slower rate than a paved street. The warmest air, called thermals, tends to rise. In the image (right) a wildfire provided the differential heating for a cumulus cloud to form over the smoke plum.
Fronts, Drylines and Outflow Boundaries
Fronts are the boundary between two air masses of different temperatures. Fronts lift warm moist air. Cold fronts lift air the most abruptly. If the air is moist and unstable thunderstorms will form along the cold front.
Drylines are the boundary between two air masses of different moisture content and separate warm moist air from hot dry air. While the temperature may be different across the dryline, the main difference is the rapid decrease in moisture behind the dryline. It is the lack of moisture which allows the temperatures to occasionally be higher than ahead of the dryline. However, the result is the same as the warm moist air is lifted along the dryline forming thunderstorms. This is common over the plains in the spring and early summer.
Outflow boundaries are a result of the rush of cold air as a thunderstorm moves overhead. The rain-cooled air acts as a "mini cold front", called an outflow boundary. Like fronts, this boundary lifts warm moist air and can cause new thunderstorms to form.
Terrain
As air encounters a mountain it is forced up the slope of the terrain. Upslope thunderstorms are common in the Rocky Mountain west during the summer.