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Sydney

Sydney coasted without southerly buster for 14 months

Sydney is in the midst of a long period of missing one of the city’s weather staples, the infamous Southernley Buster.

Strong wind shifts can turn a scorching hot sunny day into a blistering cold mess in minutes.

Sydney experiences an average of five southern busters per year, mostly in the spring and summer, but the last buster dates back 14 months to December 2021.

What is Southernly Buster?

A true baster isn’t just a southerly change, it’s a violent reversal phenomenon that can cause a sharp drop in temperature and damage winds, often forewarned in advance by ominous horizontal roll clouds.

The following criteria are commonly used to classify busters:

  • Wind direction change to the south wind
  • Wind gusts of at least 54 kilometers per hour after the change
  • 3 hours temperature drop of at least 5 degrees
  • Absence of Tasman Low

The most powerful Buster can generate gusts of wind exceeding the gale force and drop temperatures to nearly 15 degrees Celsius in an hour.

New South Wales’ record gust of wind was 130 km/h over Port Kembla on 20 November 1973, equivalent to the winds of a Category 2 tropical storm and the strongest ever in Sydney’s history in December 1948. A gust of wind hit 113 km/h at Sydney Airport.

What is the cause of Southern Lee Buster?

A simplified process leading to the formation of Southern Buster:

1) A relatively weak, shallow cold front is moving eastward across southeastern Australia, preceded by strong northwesterly winds.
2) The northern arm of the front is blocked by the Great Dividing Range.
3) The southern frontal belt spanning southern Victoria and Bass Strait continues to move eastward, causing frontal deformation.
4) Cold air reaches the NSW border and begins to rise over the south coast. This will bend the front into an “S” shape.
5) Cold air along the south coast becomes a density current. The cold air, which is heavier than the warm air, accelerates along the coast as a strong southerly surge to a hot northwesterly wind.

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A weak front is deformed by the Great Dividing Range, setting conditions for a southerly buster along the New South Wales coast

Winter cold fronts are deeper and extend well above the height of the Great Divide, preventing the blocking process, but cold season northwesterly winds are not warm enough, so the components of the above process are generally It exists only in the warm season.It creates the required density current.

Buster strength is primarily governed by the density contrast between the northwest and south winds, which is the temperature contrast.

As long as well-placed weak cold front overview patterns occur simultaneously, the greater the difference, the greater the wind speed, and the strongest busters usually occur after very hot days.

where did the busters go?

The most obvious explanation is that there are no hot days in Sydney.

The last time temperatures were above 32°C was in December 2021, the longest since the early 1950s.

The last time Sydney hit 35 degrees was over two years ago, and it was the longest absence of high temperatures in 115 years.

Another factor could be a decrease in cold fronts. This is evident by the extension of the Southern Hemisphere Annular Mode (SAM) positive phase over the last three years. If the SAM is positive, the cold front will contract from southern Australia to the Antarctic Ocean.

Timing also plays an important role in Southernly Buster.

If southerly winds move up the coast overnight, density currents are not strong enough to generate gusts of over 54 km/h, or moving to cooler night air will not result in a 5 degree temperature drop in 3 hours. .

South Wind Buster in the weather forecast?

Examining the weather charts over the next week shows little chance of a buster from the south.

Saturday will see warm northerly winds and a cold front passing over Tasmania, but the front may go too far south to generate the required density of flow.

https://www.abc.net.au/news/2023-02-16/sydney-weather-no-southerly-buster/101979014 Sydney coasted without southerly buster for 14 months

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