A trigger mechanism is any process that initiates precipitation or storm development. It is in reference to a process that causes a precipitation or storm event and without this process precipitation or storms would not have occurred. Common trigger mechanism examples are lifting mechanisms, increase of low level moisture, daytime heating, upper level synamics, instability and wind shear. The most common type of trigger mechanism that will be referenced in forecast discussions are lifting mechanisms such as fronts and other low level convergence boundaries. Below are some examples of trigger mechanisms that can make the difference between precipitation and no precipitation. The last example can mean the difference between tornadic and non-tornadic storms:

Daytime heating: once temperature at the surface warms high enough, then instability is released causing storms

Cold front, dryline, warm front, outflow boundary: The low level convergence forces air to rise and the lifting is strong enough that it generates precipitation or instability release that creates storms

Influx of low level moisture: Higher dewpoint values will increase instability and can increase instability enough that storms are generated

Upper level Dynamics: lifting initiated by positive vorticity advection or a jet streak

Wind shear: wind shear can mean the difference between tornadic and non-tornadic storms. Strong low level speed and directional wind shear helps support a rotating updraft that can generate a tornado.