METEOROLOGIST JEFF HABY
Three characteristics that determine how severe a warm season thunderstorm will be are
low level speed and directional wind shear,
the upper level wind,
and instability (amount of positive buoyancy).
Strong surface wind adds fuel to a storm just as an influx of oxygen adds fuel to a fire. Strong
surface winds transport moisture
and momentum into a developing storm. Terms such as
moisture convergence and surface inflow describe this process.
Directional shear causes low level wind rotations which can cause
tornadic conditions to be more favorable. The
upper level wind has many effects on a thunderstorm, which include: a sheared anvil, and upper level divergence
(mass is removed from the top of the storm which intensifies the upward pressure gradient within a storm). Upper
level winds (between 500 and 300 mb level) of greater than 100 knots is considered significant. The amount of
buoyancy determines the updraft strength. The warmer the air within the storm is (compared to the surrounding
environmental air), the faster the air will rise within the storm.
CAPE values of greater than 1,500 J/kg and
LI values less than -5 indicates a very buoyant troposphere. All these factors and others combine together to
determine the severity of a thunderstorm.
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, instability and wind shear. The most common type of trigger
mechanism that will be referenced are lifting mechanisms such as fronts and other
low level convergence boundaries. Severe
weather situations will often have several triggers. The trigger
that initially sets the storms into motion will often be referenced as the trigger for the severe
storm event.
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