Southerly buster

"Southerly" redirects here. For the musical act, see Southerly (band). For the Australian literary magazine, see Southerly (journal).

Arrival of a southerly buster in Sydney, over the Royal National Park.

A southerly buster is the colloquial name of an abrupt southerly wind change in the southern regions of New South Wales and Victoria, Australia, which approaches from the southeast, mainly on a hot day, bringing in cool, usually bringing severe weather and a dramatic temperature drop, thus ultimately replacing and relieving the prior hot conditions. Marking the boundary between hot and cool air masses, a southerly buster is sometimes represented by a roll-up cloud perpendicular to the coast, which appears from the south and coexists with the wind change, though sometimes there is little visual signal of the southerly's arrival.

Southerly busters occur about 32 times each year on the coast of southeastern Australia, with variable strength, usually in spring and summer. Although southerly busters are often associated with NSW and Victoria, they also occur on the east coast of Tasmania, New Zealand, and in Argentina and Chile. They are known as ‘backdoor fronts’ in North America, and ‘Spanish plumes’ in Europe.

History

On hot afternoons in the late nineteenth century, before the days of radio, the Sydney Observatory would fly a flag bearing the letters "JB" to indicate that the southerly buster had reached Jervis Bay, about 150 km to the south as the crow flies. The people of Sydney greeted the arrival of the southerly buster after a hot summer day. During the early days of the European settlement, Sydney's summer storms were accompanied by rolling red dust from the colony's brickworks. In 1879, George Herbert Gibson published a book called Southerly Busters, where a buster wreaks havoc near Hyde Park, Sydney. Author Ruth Park makes a reference to the southerly buster in her novel Poor Man's Orange (1949):

After an unbearably hot day, the old men on the balconies were sniffing the air and saying, 'Here she comes!' The Southerly Buster, the genie of Sydney, flapped its coarse wing over the city ... The women undid the fronts of their frocks and the little children lifted up their shirts and let it blow on their sweaty bottoms.

Formation

For a southerly buster to occur, a cold front approaching the southeast of Australia should lie between two high pressure systems; one in the Tasman Sea and the other in the Great Australian Bight.

On a hot day, a strong offsea gale develops from the south usually in the late afternoon and early evening, causing a rapid fall in temperature as it arrives, and sometimes a short rain and/or thunderstorm may accompany, especially if it's affiliated with a cold front coming from the south and a trough, with the strongest winds being at the leading edge of the buster. However, the southerly buster does not always create precipitation, aside from light drizzle and light rain, which tend to occur a day after the southerly buster's arrival as its effects may still persist for 24 hours. It is proposed that the southerly buster is basically a coastal gravity current that is held against the mountains by the Coriolis force and in transverse geostrophic balance, and is generated when a cold front is obstructed, experiencing anticyclonic distortion near the Great Dividing Range, spreading northward as a coastal trapped orographic jet.

The southerly buster is caused by its interaction with the Great Dividing Range, as the cool air becomes trapped against the ranges, oftentimes in the Gippsland area of Victoria, where the mountains create a channelling effect as the southerly winds move across the New South Wales coast. When the inland portion of the cold front is held against the mountains, the part over the sea proceeds to move along the shore, twisting the front into an 'S' shape. This activity continues on the southern New South Wales coast, while areas leading the front are still experiencing hot northwesterly winds.

Characteristics

The main distinguishing feature of a southerly buster is the sudden, squally southerly change in wind direction which replace the continental northwesterly winds. This is accompanied by a marked temperature drop and sea level pressure rise. Wind gusts in excess of 40 knots (74 km/hour) near ground level averages about three per year, which usually come about after very hot days and would tremendously ease within 30 to 60 minutes after the Buster's arrival, becoming rather light within a few hours. A regular southerly buster is between 20 and 60 nautical miles wide, with the heavier winds concentrated on the seaward strip, with its depth being around 1000m, restricted by the height of the mountains to west.

An orographic jet, the southerly buster is clearly a mesoscale phenomenon, developing on about one day. Because busters seldom keep a staunch speed while advancing along the coast, its arrival has always been difficult to foretell, though meteorologists nowadays have the gain of satellite imagery and weather radar to foresee it, with wind warning issued by the Bureau of Meteorology. Temperature changes can be dramatic, with falls of 10 °C (18 °F) to 15 °C (27 °F) often occurring in a few minutes. In extreme conditions, a southerly buster may lower the temperatures from 40 °C (104 °F) to 19 °C (66 °F). To note, some southerly busters can be mild and not very pronounced, where they would arrive on lukewarm days and even during sultry thunderstorm events, bringing in light, though still noticeably cooler winds in the evening, with its affects still remaining in the following few days as well in some cases.

Regions

• In New South Wales, southerly busters generally reach their maximum intensity between Nowra in the South Coast and Newcastle. Southerly busters rarely pass beyond the Mid North Coast or Port Macquarie – When they do they are usually strengthened by the presence of a tropical cyclone off the north coast of the state. Moreover, they are less pronounced in that region with narrower temperature changes. Not restricted to the immediate coast, southerly busters do generally impinge upon the inland regions of NSW as well, namely the regions in the southeastern block of the state, such as, the Greater Western Sydney area, eastern portion of the Central Tablelands region going eastwards from Bathurst, the lower Hunter Valley region, Albury in the South West Slopes in a few occasions, Cooma in the Monaro (New South Wales) to the south, Canberra in the ACT, Goulburn in the Southern Tablelands and the Snowy Mountains, with the latter regions having the most dramatic cool changes due to their southerly vicinity. Sydney receives an average of about five Southerly Busters a year, mostly in late spring and early summer, with the stronger ones generally reaching the city in the late afternoon or early evening on a hot day, though at times it would arrive after several days of hot weather. It is a crucial weather feature in the Sydney area, particularly for yachtsmen.
• In Victoria, most notably in Melbourne, southerly busters occur during the afternoons where the domineering heat, brought on by north-westerly winds from central Australia, suddenly gives way to a rapid drop in temperature, followed by rain, thunderstorms and a relatively cool night. They would reach as far inland as Swan Hill in the north and Omeo to the east, but would be less pronounced and intense as one moves more inland to the north. Temperature drops in these parts of Victoria are more dramatic than those in the east coast of New South Wales, where a 10°C (18°F) drop can occur within half an hour (part of the easily changeable weather). Southerly busters most emerge in spring, as the landmass northbound of Melbourne starts to warm up. Meanwhile, though, the Southern Ocean, which provides cool breezes to Victoria from the west, does not warm up as swiftly as the mainland. As such, the temperature difference between hot air from the north and cold from the ocean would be very great, thus providing good conditions for the formation of thunderstorms.
• In South Australia, Mount Gambier would the most affected by southerly busters in the state due to its southeasterly location. The buster may also reach Adelaide and Whyalla in some occasions.
• In Western Australia, the Buster occasionally reaches Eucla due to its location near the Southern Ocean in the warm months, where it can experience dramatic temperature drops.
• In Wellington, New Zealand these storms are normally short and frequently have winds gusting between 120 km/h and 160 km/h though higher speeds are known.
• In South America, these southerly fronts frequently encroach on the southern coast of Chile and Argentina and would then advance northwards on both sides of the Andes.
• In North America, the south moving 'back-door' cold front on the eastern side of the Alleghenies in the northeastern US is comparable to the southern buster of the southern hemisphere.

Records

The strongest recorded Southerly Buster fell around 4:30 pm on 12 March 2010 in Wellington, New Zealand, with a maximum gust recorded of 72 knots (133 km/h).

Incidents

On 21 November 2016, at around 6pm, a powerful southerly change occurred in Melbourne, which resulted in the death of 10 people, who were asthmatic and succumbed to respiratory failure. Thousands of others across the city experienced allergic reactions and asthma-like symptoms triggered by the storm. This was due to a stark southerly wind (60 km/hour) that distributed ryegrass pollen into the moist air, rupturing them into very fine specks, small enough particles to enter people's lungs, as they were sucked up into the warm updraft of air forming the storm cells, before they returned to earth in the storm's cool down-draft, spreading across the land in the storm's efflux area. Hospitals and medical centres in the city had to arduously manage 8,500 emergency calls in the space of just five hours, and the hospitalisation of 1400 people.

See also

• Black nor'easter
• Fremantle Doctor
• Australian east coast low