Following on from Robin Hewitt’s article in July 2017 ‘It’s All About The Weather’ this article focuses on the East Coast Low storm events that occur on the eastern seaboard of Australia. East Coast Lows are intense low-pressure systems that typically occur between latitudes 25 to 40 degrees south in the western Tasman Sea, approximately between Fraser Island Queensland to Tasman Island, Tasmania. As such, they can occur on the Melbourne-Osaka racetrack and delivery route to or from Melbourne or Osaka.
East Coast Lows bring storm conditions that can be particularly vicious and can occur any time of the year, although are most common in the winter months. Many destructive storms that have been recorded on the eastern seaboard can be attributed to these weather systems. This includes:
- The Sygna Storm of May 1974, where the coal ship ‘Sygna’ was driven ashore at Stockton Bight. Wind gusts up to 90 knots and maximum wave heights of 17m were recorded at Newcastle during the storm.
- The 1993 Sydney Hobart Race, where an East Coast Low formed off the southern NSW soon after the Boxing day start, with gale force southerlies and heavy sea conditions contributing to a number of incidents and retirements amongst the fleet.
- The Pasha Bulker Storm of June 2007, where an East Coast Low formed off the central NSW coast, again with gale force southerlies and a heavy seaway that drove the bulk carrier ‘Pasha Bulker’ ashore at Nobby’s Beach, Newcastle.
Characteristics: A common weather pattern that often precedes an East Coast Low is an inland trough over eastern Australia, which intensifies as it drifts eastwards to the coast. Coastal weather from the trough is often humid and muggy, with calm to light winds, overcast skies and areas of heavy rainfall and thunderstorms. As the trough approaches or crosses the coast, a low-pressure centre can form which can deepen rapidly to form an East Coast Low if suitable conditions all line up, with factors including:
- Warm sea temperatures of the East Australian Current;
- Lee effects from the Great Dividing Range;
- The position and amplitude of the Jetstream, particularly where the jet has a high amplitude ‘hook’ shape that occurs above the low which can enhance its clockwise rotation; and
- Presence of an upper cold pool of air, often pushed up from associated cold fronts from the southwest.
When formed, East Coast Lows can be characterised by rapid deepening of the low pressure centre, with winds quickly rising to gale/storm force along the coast. Once formed, East Coast Lows generally follow an easterly track into the Tasman Sea, usually weakening as they head further offshore. Large and confused seas accompany the winds that can be exacerbated by current versus wind effects with the East Australian Current. In addition to the strong winds and confused seas, East Coast Lows also bring heavy rainfall and poor visibility.
Case Study – E Coast Low of July 2001: A case study of the July 2001 East Coast Low is presented to illustrate typical conditions that can occur in the lead up, formation, intensification and weakening of an East Coast Low. The East Coast Low of Friday July 27th to Sunday July 29th 2001 brought gale and storm force winds to high seas and NSW coastal waters. Some of the incidents included:
- Mayday from a yacht rolled by huge seas in Stockton Bight. The three crew were recovered from a life-raft by helicopter on the Saturday night and the yacht abandoned. The yacht was later wrecked on a beach near Seal Rocks. The yacht had been on passage to Newcastle from New Zealand.
- Securite message from a merchant vessel near Seal Rocks where a number of timber logs had washed off the vessel. A securite message was also broadcast from a merchant vessel near Eden, which lost a number of sea containers over the side. At the time, Montague Island was reporting average winds of 50 knots plus.
- The sinking of a 30m vessel off Malabar with the crew of four rescued by Water Police. Weather observations from the Launch reported average winds of 50 knots with gusts of 65 knots and 10m seas.
- The closure of the Port of Sydney with a 7 to 10m swell reported outside Sydney Heads.
- Postponement of the Sydney- Gold Coast Yacht Race.
Lead Up and East Coast Low Formation: The weather leading up to the event featured the formation of a trough in inland NSW and Queensland, bringing humid conditions to coastal areas with calm to light winds and overcast conditions. Periods of heavy rainfall in NSW and Queensland were also recorded, including 60mm on the Illawarra coast. The synoptic pattern (Figure 1) shows the location of the trough on the afternoon of Thursday 26th July, appearing as a broad trough extending into NSW. Two relatively strong and slow moving high-pressure systems were present over the Tasman Sea and Australian Mainland. A low-pressure system was located south of the Bight with an associated cold front. Mean Sea Level (MSL) prognosis charts forecast a Low to develop off the NSW Central coast by late Friday. The humid, wet weather and synoptic pattern of the preceding days were indicators that the weather conditions could be favourable for the formation of an East Coast Low.
Figure 1. Mean Sea Level (MSL) Analysis 4pm Thursday 26th July (from the BoM)
By dawn on Friday morning, a low pressure centre had formed within the trough and crossed the Hunter coast with a pressure of 1012 mbar, as shown on Figure 2. The cold front passing through Bass Strait brought cold air (including an upper cold pool) towards the low centre and against the warm humid air feeding into the low from its north-eastern side. Favourable conditions for intensification of the low exist with the relatively dense cold air of the front mixing with the warm humid air and pushing it upwards in a convective process, with release of latent heat as the humid air rises and condenses. This convective process is further aided by the upper cold pool, increasing the buoyancy of the humid air and its rate of ascension.
If available at the time, analysis of Convective Available Potential Energy (CAPE) charts would have indicated elevated CAPE levels and a further sign of severe weather development.
The deepening of the low was reflected in coastal waters forecasts issued by the Bureau of Meteorology (BoM), with strong and gale wind warnings issued for a weather pattern described as ‘Low off NSW central coast expected to deepen today and move slowly east southeast’.
Figure 2: MSL Analysis 10am Friday 27th July 2001 (from the BoM)
East Coast Low Intensification: Following formation during the Friday morning, the Low rapidly deepened throughout the day. The initial gale wind warning that had been issued for waters south of Seal Rocks was upgraded to a storm force wind warning with average winds of 45-55 knots and 6 to 8m seas over Hunter, Sydney and Illawarra coastal waters.
Figure 3 shows the Low formed up on the Friday evening with a barometric pressure of 990 mbar. Barometric pressure had dropped 22 mbar between morning and evening, which included a 15 mbar drop over 6 hours between 4pm to 10pm.
The inflow of cold air into the Low from the south is evident (associated with the cold front), along with the inflow of warm moist air from the western side of the Tasman High. Winds along the coast quickly rose to gale force from the southerly quadrant, with heavy rainfall and poor visibility along the coast.
Figure 3: MSL Analysis, 10pm Friday 27th July 2001 (from the BoM)
By Saturday morning on 28th July, the Low was located 120 nautical miles east of Sydney, had deepened further to 988 hPa and appeared to be fully developed (Figure 4). Isobars were squeezed close together between the Low and adjacent high-pressure systems, particularly on the western flank of the Low, as gale to storm force south quadrant winds swept across coastal waters. The Low dominated the weather along the entire NSW coast, parts of the southern Queensland Coast, and Eastern Bass Strait. Throughout Saturday, storm force wind warnings remained in effect for coastal waters south of Port Macquarie, with an increase in the forecast average winds to 50-60 knots from the S-SW, with forecast significant wave heights of 9 to 14 metres.
Figure 4. MSL Analysis, Saturday 28th July 10am (from the BoM)
Weather Observations – Winds: The strongest recorded winds during the event were during the Saturday at Montague Island on the southern NSW coast. Wind speeds (average and gusts) and barometric pressure at Montague Island during the storm are graphed on Figure 5. Montague Island recorded a steady drop in barometric pressure during Thursday 26th and Friday 27th with calm light winds from the southerly quadrant during Thursday night and early Friday morning. With the formation of the low on Friday morning, wind speeds quickly increased to gale force. Winds continued to build for the next 12 hours and by midday Saturday had reached storm force, with the western flank of the Low over Montague Island. Storm force winds were maintained until 7pm Saturday night, peaking at 5pm with average winds of 50 knots and gusts of 60+ knots. The winds continued unabated at gale force for another 14 hours until 9am on Sunday morning. Overall, the coastal waters around Montague Island had been swept with gale or storm force southerly quadrant winds for around 36 hours.
Figure 5: Weather Observations at Montague Island July 26-30, 2001 (data from the BoM)
Weather Observations – Waves: Some 150 nautical miles north of Montague Island, the Sydney Waverider Buoy had begun to feel the effects of the storm by Friday afternoon, with steadily increasing wave heights throughout the day, as shown on Figure 6. At midnight the buoy recorded significant wave height of 4m and maximum wave heights of 6m. During the remainder of the night, the seas continued to build and by Saturday morning, wave heights of 6m (sig) and 10m (max) were recorded. The highest recorded waves at the buoy occurred around midnight Saturday, with significant wave height of 7m and maximum wave height of 12m. No further data was recorded for the event when the buoy broke its mooring sometime on Sunday 29th July and later washed ashore some distance north on the NSW Central Coast.
Figure 6: Wave Data from Sydney Waverider Buoy 26-29th July 2001 (data courtesy of NSW Department of Natural Resources and NSW Department of Commerce Manly Hydraulics Laboratory)
East Coast Low Weakening: By Sunday morning, approximately 48 hours after its formation, the Low had weakened to 996 mbar as it slowly tracked east across the Tasman Sea (Figure 7). Weather observations along the coast indicated a large area of High Seas and NSW coastal waters had been subject to gale or storm force winds for much of this 48 hour period.
Figure 7. MSL Analysis Sunday 29th July 10am (from the BoM)
Figure8: MSL Analysis, Monday 30th July 10am (from the BoM)
By Monday morning, the Low continued to weaken at 1008 mbar and move eastwards, with widely spaced isobars indicating moderate winds (Figure 8).
East Coast Lows are a hazardous storm event on the eastern seaboard of Australia. Once developed, their influence can dominate weather for hundreds of miles, bringing storm force or gale wind conditions that can persist over multiple days. Recognising and understanding the weather that often precedes and occurs during an East Coast Low can be used for strategies to deal with these storms. This includes utilizing available weather sources, such as marine weather forecasts and prognosis charts, routing software grib files, onboard weather and barometer observations, and other recently available information such as CAPE charts.
By Andrew Roberts, November 2017
Andrew sails aboard the Joubert 42 sloop ‘Tilting at Windmills’ and is a member of the ORCV and SYC. He has been a competitor in a number of Hobart and Lord Howe Island races, and other races and deliveries along the Australian eastern seaboard and to/from New Zealand.