|HABYTIME MINI LECTURE 73:|
THUNDERSTORM INITIATION POINT FACTORS
METEOROLOGIST JEFF HABY
In a thunderstorm situation, why is it that storms develop in the places that they do? What are the special considerations that cause a storm to
develop at a particular location. This writing addresses these questions.
There are several factors that combine together to determine where a storm initially develops. The location with the best combinations of these
factors along with other potential and subtle factors influences where a storm first develops.
Heat: Many thunderstorms are formed from air rising from near the surface. As air warms it becomes less dense and can be more buoyant. Thus, one
factor that influences where a storm first develops is the ground temperature and the temperature of the air near the ground. Warmer air is more
favorable to thunderstorm initiation as compared to cooler air. A term that is used in meteorology for a region with warmer surface temperatures
is termed a thermal ridge.
Moisture: Along with rising temperature, increasing moisture (higher dewpoints) also makes air more buoyant. A term used in meteorology for a
region of higher dewpoints is termed a moisture axis. Heat and moisture can be combined together to produce a thermodynamic value called
Theta-E. A Theta-E axis is a region with a higher combination of heat and moisture. These locations can be favored locations for
Convergence: Convergence is a region where air flow comes together. A front is a classic region in which convergence occurs. In a region of
convergence, moisture piles together increasing the amount of moisture and the depth of moisture. A temperature discontinuity typically
develops at a convergence boundary. These varying densities next to each other make rising air more conducive. Thus, the piling of moisture
and extra impetus of rising along a convergence axis contributes to an increase likelihood of thunderstorms first developing along that axis.
Less Updraft Resistance: This factor can cause storms to first develop at locations that initially may not be seen as the best place
for thunderstorm initiation when only examining the surface conditions. This decreased resistance to updraft motion can be caused by:
a) region of strongest upper level divergence (PVA, Jet Streak)
b) region with a relatively weaker cap
c) region where trough axis aloft is stronger
When forecasting thunderstorms, these factors discussed in this writing will be examining to help determine where thunderstorms are most likely
to develop and where they are most likely to develop first.