About Severe Thunderstorms
Literally thousands of thunderstorms occur each year in NSW and the ACT. Although all thunderstorms produce lightning, that is a danger itself, not all of them are "severe" or likely to produce damage. In Australia, Severe Thunderstorms are defined as those that produce any of the following:
Each year, upwards of 100 thunderstorms in NSW and the ACT are strong enough to produce one or more of these effects. The chart below shows the number of reported severe thunderstorms from the period July 1999 through to June2009. Statistics are shown in Financial years (i.e. from 1 July to 30 June the following year) because the peak months for storms occurrence are December and January, the middle of the Financial year. The differences between years are due to two factors. Firstly, the broad weather patterns that cause storms vary from year-to-year resulting in some seasons being more active. And secondly, despite the Bureau's extensive network of observation sites and a storm spotter network of around 1,300 individuals, severe thunderstorms still go unreported in the State, particularly over more remote areas.
Monthly Distribution of Severe Thunderstorms
Although severe thunderstorms can occur at any time, the distribution of events shows a marked pattern throughout the year. The graph below shows the percentage frequency of severe thunderstorms for each month of the year for a fourteen year period. There is a marked tendency for severe thunderstorms (indeed all thunderstorms) to occur during the months September through to March. This period is normally referred to as the "Severe Thunderstorm Season" in NSW. The increase in storms during this period is primarily due to the increase in energy provided by the sun during the warmer Spring and Summer months, coupled with Spring and Summer weather patterns that are favourable for storm growth.
Hourly distribution of Severe Thunderstorms
Just as there is a marked monthly distribution of storms, there is also a marked daily distribution, peaking between 2pm and 6pm during the afternoon. Again, this is primarily due to the daily heating of the earth's surface by the sun, which is a maximum during the afternoon.
Why do we get Severe Thunderstorms?
Thunderstorms require 3 main ingredients; a source of moist air, an unstable atmosphere and a mechanism to initiate their development. Moist air is important because when it condenses to form cloud, heat energy is released making the rising air more buoyant and "fueling" further cloud growth. An unstable atmosphere is necessary so that developing cloud is able to rise freely to great heights in the atmosphere. And initiating mechanisms are important as they serve as a focus for storm development. Typical mechanisms that initiate thunderstorms are fronts, troughs and regions of low pressure. Features of topography such as hills and mountains may also enhance storm development.
The severity of any subsequent thunderstorms will depend largely on the buoyancy of the rising air within the storm and the structure of the wind within the atmosphere. Wind direction and speed is rarely constant, and generally tends to increase in speed and turn anti-clockwise (in the southern hemisphere) with increasing altitude. The change in wind direction and speed as you move upwards through the atmosphere is known as "wind shear". Non-severe thunderstorms generally occur within environments possessing only low to moderate instability and minimal wind shear.
If the atmosphere is very unstable with light winds and little shear, storms develop a "pulse-like" character, rising strongly upwards then collapsing over the period of half to one hour. Severe thunderstorms in these conditions may produce large hail and strong bursts of wind but rarely produce widespread damage.
An increase in wind shear produces storms with additional opportunities for regeneration. This allows several storm "cells" at different stages of their lifecycle to be found within the one storm system, increasing their overall lifetime and the area they may affect. These "multicellular" thunderstorms may produce severe hail and wind, with the added possibility of flash flooding and weak tornadoes.
In some storms, the balance between buoyancy and wind shear approaches an optimum, leading to the development of long-lived thunderstorms with strong rotation within their cores. These storms are known as "Supercells" and are responsible for the majority of damage caused by severe thunderstorms. Supercell thunderstorms may produce very large hail, extraordinary wind gusts, powerful tornadoes and heavy rainfall.
The radar loop below shows a variety of thunderstorms occurring over the NE corner of NSW on the afternoon of 18 December 1998. The large storm that moves northwards along the coastline from Yamba is a Supercell storm that was accompanied by extreme winds, very heavy rainfall and hail up to 7cm diameter. The storms further inland to the left of the supercell are most likely multicells.
Thunderstorms may at times be arranged in lines 100's of kilometres long or in large circular clusters. These large storm systems are known as "squall lines" and "mesoscale convective systems" respectively. When severe weather is associated with these systems, it may occur over large areas.