Severe Storm

According to the BOM Climate Education website

Severe thunderstorms

Hot, humid, unsettled weather conditions. An approaching cold front or trough. The ideal conditions for severe thunderstorms, and their progeny of flash flooding, large hail, destructive wind gusts, and even tornadoes. Most Australians can vividly recall at least one major thunder-, wind- or hailstorm in their area, and occasionally these storms are of such note to be talked about for many years afterwards. In Melbourne, the February 1972 flash flood was such a storm; in Brisbane, the tornado of November 1973, in Sydney, the 1990 and 1999 hailstorms. Residents of Yaamba in northern Queensland would not quickly forget the 480mm that cascaded down in six hours in April 1996.

Storm damage

According to Emergency Management Australia, severe thunderstorms cause more damage in Australia each year than any other natural hazard, and the damage bill in individual cases has gone into the hundreds of millions of dollars. Hail causes the greatest proportion of the damage, accounting for nearly half the total losses from severe storms. The 1990 storm in Sydney inflicted about $380 million damage - a record figure for a hailstorm, until dramatically surpassed by the storm of 14 April 1999. In that storm, insurance losses exceeded $1.5 billion - with total losses considerably higher.

Australia does not experience the extreme temperature contrasts over short distances which characterise some weather patterns in the United States of America, and partly for this reason severe storms are not generally as frequent or severe as in, say, the Midwest of that country. To take the example of tornadoes: the United States generally has at least 10 tornadoes per year ranked as F4 or F5 on the Fujita scale (see Table below), and therefore capable of producing great destruction, whereas only one storm in Australia's history has (officially) been ranked this high. While it is probable that many more such tornadoes go unnoticed in Australia because of our much lower population density, this is unlikely to explain the difference entirely.

When are severe storms most likely?

All areas of Australia experience severe thunderstorms, but their preferred time of occurrence varies. In most of northern Australia, the "transition seasons" (autumn and spring) are the most likely time, in eastern Australia it is the late spring/summer period. In southwestern Australia, severe thunderstorms are most common in winter in areas nearer the coast (but in spring-early summer further inland). As a general rule, severe storms are most common in New South Wales, Queensland and parts of Western Australia, and least common in Tasmania.

Though individual severe storms, which last for mere minutes or hours, have little climatic significance in themselves, they do in a sense define the extremes of that area's climate. Many people would probably be surprised to learn that Melbourne's climate history includes a tornado (Brighton, 1918) powerful enough to rank with some of the stronger storms over the plains of Kansas and Nebraska in the American midwest!

Several examples of each of the main severe thunderstorm types - tornadoes, hailstorms, flash floods - along with instances of the gale-force winds over the southern States that accompany strong midlatitude low pressure systems, may be found under the appropriate heading. The link to dust-storms leads to a separate description of these sometimes dramatic events.

The Fujita scale of tornado intensity

Rating	Wind speed km/h	Expected damage
F0	64-116	Damage to chimneys and windows. Branches torn off trees.
F1	117-180	Roof peels off. Caravans bowled over. Cars pushed off roads.
F2	181-253	Roofs torn off houses; large trees snapped or uprooted
F3	254-332	Roofs, some walls torn from well constructed houses; most trees in forest uprooted.
F4	333-418	Well-constructed buildings levelled. Cars thrown, large missiles generated.
F5 over	418	Strong frame houses lifted, carried and disintegrated; Steel-reinforced buildings severely damaged.