East coast lows

What is an East Coast Low?

East Coast Lows (ECL) are intense low pressure systems which occur, on average, several times each year off the eastern coast of Australia, in particular southern Queensland, NSW and eastern Victoria. Although they can occur at any time of the year, they are more common during autumn and winter with a maximum frequency in June. East Coast Lows will often intensify rapidly over a period of 12-24 hours making them one of the more dangerous weather systems to affect the eastern coast. East coast lows are also observed off the coast of Africa and America and are sometimes known as east coast cyclones.

How do they form?

East Coast Lows may form in a variety of weather situations. In summer they can be ex-tropical cyclones. At other times of the year, they will most often develop rapidly just offshore within a pre-existing trough of low pressure due to favourable conditions in the upper atmosphere in combination with warm sea surface temperatures. ECLs may also develop in the wake of a cold front moving across from Victoria into the Tasman Sea. The sea surface temperature gradients associated with the warm eddies of the East Australian Current are an important contributor to the development of the lows.

The gales and heavy rains often occur south of the low centre, while to the north of the low there can be clear skies. The challenge for forecasters is to accurately predict the location, movement and intensity of the centre of the low.

Why are they dangerous?

ECLs can generate one or more of:

• Gale or storm force winds along the coast and adjacent waters
• Heavy widespread rainfall leading to flash and/or major river flooding
• Very rough seas and prolonged heavy swells over coastal and ocean waters which can cause damage to the coastline.

Falling trees and flash flooding have caused loss of life on the land, many small craft have been lost off the coast and larger vessels have run aground during these events.

How often do they form and is there a trend?

The Bureau has a detailed database of these lows beginning in 1973. Each year there are about ten "significant impact" maritime lows. Generally, only once per year do we see "explosive" development. Looking at all the lows since 1973 there is no evidence of a trend.

What warnings does the Bureau issue for East Coast Lows?

Over land areas Severe Weather Warnings are used to warn of the dangerous winds, damaging surf and heavy rain potentially leading to flash flooding. If needed, Flood Warnings are also issued to warn of river flooding. Over the sea the standard Marine Wind Warnings are used.

What is the difference between an East Coast Low and a Tropical Cyclone?

Tropical cyclones develop over very warm tropical waters where the sea surface temperature is greater than 26°C. They have relatively long life cycles, typically about a week, and severe tropical cyclones can produce significant property damage with wind speeds over 180 km/h near the centre, heavy rainfall and coastal inundation through storm surge.

East Coast Lows can produce gale to storm-force winds, very heavy rainfall and in some cases coastal inundation. While maximum wind speeds recorded are lower than in severe tropical cyclones, a gust of 165 km/h was recorded at Newcastle associated with the East Coast Low that sunk the bulk carrier Sygna in 1974. During the first of the ECLs in June 2007, when the bulk carrier Pasha Bulker ran aground, gusts of 105 km/h at 6:21 am on 8 June and 124 km/h at 1:32 am on 9 June were recorded at Newcastle.

June 2007 East Coast Lows

Five East Coast Lows occurred during June 2007 - a rare, if not unprecedented sequence. The first (8-9 June), known as the Queen's Birthday Storm, had the most impact, resulting in loss of life and widespread damage along the Hunter, Central Coast and Sydney coastlines associated with sustained heavy rain, flooding, strong winds and large ocean waves and swell. The coal carrier Pasha Bulker (76,741 tonnes) ran aground at Nobby's Beach in Newcastle.

Fig 1: Pasha Bulker grounded on a reef just off Nobby's Beach near Newcastle by an East Coast Low in June 2007. Photo courtesy of Brett Delaney, ex-Bureau weather observer.

AskBOM: What is an East Coast Low?