General Information-East Coast Lows

Started by Bus 400, May 30, 2010, 11:13:56 PM

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Bus 400

Since these are our biggest weather events during the year, below is a bit of information off a recent Emergency Services Agency briefing which could could be of some help to others.

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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 overnight making them one of the more dangerous weather
systems to affect the NSW 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?
ECLs 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.
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 also contribute to the development of the lows.
The gales and heavy rain occur on and near the coast 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 and movement 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 fatalities 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 between 1973 - 2004, 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 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 (category 3 or greater) can produce
significant property damage with wind speeds over 180km/h near the centre, heavy
rainfall and coastal inundation through storm surge.
Tropical Cyclone "Justin", which affected the Queensland Region in March 1997,
lasted for 18 days!

ECLs generally have much shorter lifetimes than Tropical Cyclones and last only a few
days. They develop over the Tasman Sea close to the NSW coast and can intensify
rapidly in the overnight period. Unlike Tropical Cyclones, where the warm seas
provide the energy source, East Coast Lows are driven by the temperature gradient
between the Tasman Sea air and cold air in the high levels of the atmosphere over
the continent.
They 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 6:21am on 8 June and 124 km/hr at 1:32am on 9 June
were recorded at Newcastle.


A bit more information can be found at http://www.bom.gov.au/nsw/sevwx/facts/ecl.shtml