Thunderstorms

Severe thunderstorms are the most common and most damaging types of storms in New South Wales, accounting for the great bulk of the total cost of damage. They are small scale systems, with damage often only affecting areas a few kilometres across, and they have short life spans ranging from tens of minutes up to several hours. One class of thunderstorms known as a ‘supercell’ is particularly severe with life spans up to six hours or so and extensive damage tracks. The severe thunderstorm of 21 January 1991, for example, had a hail and damage path around 10km wide and over 40 kilometres long.

Although severe thunderstorms can occur at any time, the distribution of events by season shows a definite pattern. There is a marked tendency for severe thunderstorms (indeed all thunderstorms) to occur during the months from October through to March. This period is normally referred to as the ‘Severe Thunderstorm Season’ in New South Wales. The increase in storm frequency 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.

The products of severe thunderstorms may be very strong winds (at least 90 km/h and sometimes greater than 200 km/h), tornadoes (see separate heading), large hail (at least 2cm in diameter and sometimes exceeding the size of cricket balls) and very heavy rain (leading to flash flooding, especially in urban areas when artificial drainage systems surcharge). Of these products, hail has been most damaging to property whilst strong winds, flash flooding and tornadoes pose the greatest threat to life. Severe thunderstorms can cause severe damage to buildings, especially to roofs and windows, and may demolish whole buildings as a result of trees being blown over or having their trunks snapped. Damage also is caused to cars parked on streets and in commercial car yards, especially as a result of hail, and downed power and telephone lines are common. In some cases, people have been without electricity and telephone services for several days.

Tornadoes

Tornadoes are extremely damaging weather phenomena that occur in conjunction with some severe thunderstorms. A tornado itself is an intense, localised, funnel-shaped vortex that extends from the thunderstorm cloud base to the ground. Tornadoes range in size from a few tens of metres across up to around one kilometre in diameter. Because of their relatively small size, damage is normally restricted to a small area but it can be very intense and may include the complete destruction of buildings.

Most tornadoes in New South Wales occur in late spring and summer but they have been known to occur at all times of the year. Tornadoes have been reported across NSW including Sydney, Bulahdelah, Port Macquarie, Cobar, Gilgandra, Dubbo, Moree, Tumbarumba, Merimbula, Pambula and Tucabia. In some of these events, buildings have been demolished by the extreme winds which can range from 120 km/h to above 400 km/h in very rare cases.

Tropical Cyclones and Ex-Tropical Cyclones

Tropical Cyclones develop over very warm tropical waters from pre-existing tropical weather disturbances. They have relatively long life cycles, of the order of up to about two weeks. Weather systems originating as tropical cyclones in the Coral Sea or the Gulf of Carpentaria do sometimes affect New South Wales bringing very strong winds, flooding rains, very high seas and storm surges. High seas and storm surges may cause erosion of sand dunes and in severe cases may expose landward areas to sea water inundation.

The northern parts of the state, especially the coastal areas, are the most affected, but the impacts of these systems can extend as far south as Sydney. Examples include ex-Tropical Cyclone Nancy, which crossed the New South Wales coast at Byron Bay in February 1990, and ex-Tropical Cyclone Zoe which crossed at Coolangatta/Tweed Heads in March 1974. Ex-Tropical Cyclone Violet caused very high seas between Coffs Harbour and Ballina in March 1996 and flash flooding in coastal areas. A cyclone in 1954 which crossed the NSW coast around Tweed Heads resulted in 26 deaths.

Within the Bureau of Meteorology, the Brisbane Tropical Cyclone Warning Centre maintains a cyclone warning service extended to around Taree (32o south) but may extend warnings further south in some situations.

Mid-Latitude Low-Pressure Systems (including East Coast Lows)

More frequent visitors to coastal New South Wales are intense low-pressure systems known as East Coast Low-Pressure Systems. As a rule, these storms generally have much shorter lifetimes than Tropical Cyclones, usually of the order of only a day or two.

They generally develop over the Tasman Sea close to the coast and often intensify dramatically overnight. They have a compact size and deep low-pressure centre, and like Tropical Cyclones can produce gale to storm-force winds, heavy rainfall and in some cases very high seas and storm surges. Erosion of sand dunes and the inundation of land by sea water may occur.

The storm of 7-8 August 1998 was an example of an East Coast Low-Pressure System. It produced considerable wind damage in Sydney and flooding to the west of the Great Divide on the upper Macquarie River. In August 1996, a severe wind event over much of eastern New South Wales was generated by a deep low-pressure system located south of Tasmania.

Due to their relative fast development and short life cycle, warnings issued for East Coast Lows will usually have less lead time than their tropical counterparts. Typically, a land gale warning will provide 6-24 hours lead time of winds expected to average at least 65 km/h over land or gust over 90km/h. Routine forecasts issued by the Bureau of Meteorology may also mention local heavy rainfall and strong winds and dangerous surf in the lead-up to a significant event.

More distant low pressure systems can create hazardous conditions along foreshores as large swells can extend many thousands of kilometres from their generation zone.

Low Pressure Troughs

Regions of low pressure that do not possess a closed circulation are known as low pressure troughs. Although they lack the strong winds typical of cyclones and east coast lows, these systems are often the focus for thunderstorms and rain.

Troughs on or near the east coast, combined with very strong onshore winds, have been responsible for very severe flash flooding events such as at Coffs Harbour in November 1996 and Wollongong in August 1998.

Cold Fronts and Southerly Busters

Frontal activity can produce strong winds that generally shift from the west or northwest around to the southwest as they pass a location. They are often the focus of thunderstorms. On hot, dry and windy days, fronts can pose serious problems in the control of bush fires as they sweep narrow fire flanks into raging fire fronts.

Southerly Busters produce shallower wind changes that mostly affect coastal locations and the eastern flanks of the Great Divide and are common during the warmer months. Winds following a southerly buster can be strong and gusty for several hours. In January, 2001 a strong southerly buster with gusts in excess of 90 km/h affected a large part of Sydney. The event resulted in one death and over 2000 SES requests for assistance.

Gale force westerly winds associated with Cold Fronts can occur across wide areas of NSW particularly in the winter months.

Cold Outbreaks

At times from late autumn through to early spring, significant outbreaks of cold air advance northwards over New South Wales resulting in unseasonably cold temperatures and snowfalls on the ranges, sometimes as far north as the Queensland border. These conditions produce snow and ice-related problems on roads over the ranges although they seldom result in deep snow conditions. Vehicles can be stranded on roads which are impassable and communities can be isolated for hours or days.

Widespread snowfalls and blizzard conditions are most commonly associated with persistent north westerly winds over the New South Wales Alps. These result in heavy accumulations of snow rather than mere showery falls that occur during a true cold outbreak. Areas in the Alps can be cut off for several days by heavy snowfalls. Examples of such events are July 1965 and August 2005.