|WHAT IS A TRIGGER MECHANISM?
METEOROLOGIST JEFF HABY
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.