WA, NT, QLD, NSW: Extreme cold and wet "dry" season in the north. See story 19 June.
NSW, VIC, QLD, TAS: Storm in teacup as third ECL brings widespread snow, rain and big seas. The third June East Coast Low in two weeks threatened coastal NSW overnight, causing chaos on the Central Tablelands as heavy snow covered the region, lashing the Tasman with storm force winds or greater, and bringing moderate rainfall totals from the Hunter to the South Coast. While it was a significant meteorological event, most Sydneysiders will remember it as a storm in a teacup when media-hyped forecasts of cyclonic winds failed to eventuate.
The event unfolds
The low formed in a trough in the southern Coral Sea on Monday and drifted slowly towards the NSW coast yesterday, slowly deepening at the same time as a very cold airmass moved north across the state. Usually reliable model guidance indicated that the low would "bomb" as a classical East Coast Low, with central pressures forecast around 980hPa near the Sydney region overnight into today. The Bureau of Meteorology issued severe weather warnings early Tuesday morning for damaging winds along the entire NSW coast south of the Mid North Coast with local gusts in excess of 125km/h on exposed parts of the coast, heavy rain and flash flooding on the South Coast, and blizzard conditions in Alpine areas. Around mid-morning, damaging surf warnings were added for the coast south of Forster.
With the Hunter Valley floods two weekends before still strong in public memory, media picked up the threat to Sydney as a major news event and Bureau forecasters were thrust into the spotlight during Tuesday. ABC Radio, for example, carried the following news item at 5pm Tuesday:
"The head of forecasting operations for NSW, Robb Webb, says that kind of wind is known by a different name elsewhere. 'The kind of wind speeds that we'll see in this particular event are the equivalent of a category two tropical cyclone - wind gusts in excess of 100 kilometres an hour, so quite a dangerous situation,' he said."
However, at 3.35pm the Bureau significantly downgraded its severe weather warnings, removing the reference to gusts in excess of 125km/h that would have been equivalent to a category two tropical cyclone. While earlier forecasting opinion and guidance had been that the low would approach very close to Sydney, it was now believed that it would stay farther out to sea, though still maintaining the forecast strength. This change did not register with media, and evening news bulletins went to air warning Sydney residents that cyclone-strength winds were likely overnight.
By 9pm Tuesday, the low was analysed by the Bureau to be "well east of Ulladulla [and] expected to intensify
further and move north and then northeast overnight with gale force southerly
winds extending northwards along the New South Wales coast." By this time the central pressure of the low was estimated at 986hPa and still deepening while satellite wave reports were indicating areas of high winds in the strong pressure gradient to its south. Radio and television stations maintained special coverage, while many state authorities had deployed emergency staff in preparation for the expected damage.
But as the night wore on, strong winds did not eventuate. Updated severe weather warnings and interviews with Bureau forecasters pointed to relatively feeble gusts of 90km/h at Green Cape, Montague Island and Ulladulla, all fully exposed coastal locations, as an indication that there were strong winds about, and the time of arrival of the strong winds in Sydney was pushed back hour by hour as the early hours of the morning ticked by. In the meantime, heavy snow falling in the Central Tablelands was almost ignored.
The low behaved much as forecast and brought conditions to match. It peaked around 6 this morning about 400km east of Sydney with a central pressure of 980hPa and winds around its centre estimated in excess of 100km/h. Despite the very strong pressure gradient across Sydney and the Illawarra, however, strong winds remained offshore with ships around 20km out to sea from Nelson Bay and Ulladulla reporting sustained wind speeds of over 90 km/h at 4am and 7am. Seas, as can be seen in the plots at right, rose to the forecast levels, with average waves exceeding 5m and maximum wave heights around 11m. The effects of the system were felt from Eastern VIC, where flood rain fell yesterday, to SE QLD where damaging winds were experienced and progressively east to New Zealand where snow fell to sea level on the South Island during the weekend.
While there was relief that the city had been spared, there was also a feeling that the Bureau had cried "wolf", and NSW Regional Director Barry Hanstrum was on the media carpet to explain why the forecasts had gone wrong. The answer was connected to the Central Tablelands snow and is clear from the 6am upper air diagram at right, which would not have been available to forecasters until about 7.30am.
Mr Hanstrum told the Sydney Morning Herald that "the winds were forced 'offshore, and also overhead'. At 5am, most Sydney residents would have been sound asleep, unaware that winds of 90 to 100 kilometres an hour were howling just 1000 metres up. The coast was saved from potentially the damaging gale by cold air that dropped snow overnight across parts of the Blue Mountains and towns to the west... Forced over Sydney by westerly winds, the unusually cold air hugged the ground, sending overnight temperatures in the city tumbling to just 8 degrees. Acting as a wedge, the cold air shielded the city, pushing yesterday's much warmer gale force winds, roaring in from the sea, over the top... The westerly winds that brought the cold air also acted as a barrier, keeping the storm further to the east. 'We were very lucky,' said Mr Hanstrum, adding that the inversion happened too quickly to be taken into account by the bureau's predictions."
While the situation could have been deduced from the continued presence of westerly winds at the surface around the Blue Mountains and Sydney basin, it was not until the forecasters got hold of the 6am balloon data from Sydney Airport that they could confirm what was happening. This trace is reproduced at right. It indicates a 20 knot (37km/h) WSW wind blowing at the surface with a temperature of 11C shown by the right-hand red trace. At 700m, the wind is SW at 45 knots (83km/h) and the temperature has only dropped to 10C, marking the top of a temperature inversion. Above this the temperature drops off in the normal way with height, with 50kt (93km/h) winds to above 3km (700hPa). The temperature and dew point (left-hand red) traces are close all the way up to 4km, indicating very moist air.
The blue traces are from the ascent made at 3pm yesterday; again the right-hand trace is temperature and the left-hand one is dew point. The winds at that time (not shown) were west or WSW up to 800hPa, turning sharply S or SE above. These traces show drier air in the westerlies becoming moister in the S to SE winds above, then a fairly normal drop-off in temperature with the increase in height and certainly no indication of a temperature inversion. The forecasters' expectations would have been that the low-level westerlies would turn gradually south then SE as the winds around the low asserted themselves.
The cold air that moved into NSW on Monday was intensified yesterday by a deep SSE stream across SE AUS. The coldest air moved up through western NSW, giving Cobar its coldest day in over 100 years yesterday. This air then arced eastward bringing the coldest air across northern NSW. However, an absence of moisture across the Northern Tablelands resulted in little snow there.
However, in the Central Tablelands, the southern edge of this cold air overlapped moist air feeding around the southern side of the East Coast Low. Snow began falling in earnest during the early evening hours yesterday as temperatures at around 1,000m reached 1 or 2C. It began to fall in Oberon about 8pm and was settling by 9pm. By about 10pm, snow had begun to fall in the upper Blue Mountains, the Great Western Highway was closed by snow and ice between Bathurst Airport and Mt Lambie, and heavy snow was reported falling at Taralga and on the Oberon Plateau with snow settling above about 900m. All roads out of Oberon were reported closed soon after 11pm. Taralga reported 2cm on the ground in the town and 5cm on neighbouring hills.
In the Blue Mountains, snow began accumulating in Blackheath around 11pm and continued to fall steadily until around 2.30am today by which time there was a level cover of 5 to 10cm across the town. Lighter falls were reported about the same times at Mount Victoria and Medlow Bath. As the snow level gradually dropped to around 800m, rain turned to snow in Katoomba, Leura and the higher parts of Wentworth Falls, as well along Chifley Road and Bells Line of Road from Lithgow east to around Mt Tomah. The Great Western Highway was closed to all traffic between Katoomba and Mt Victoria, and all but local traffic between Wentworth Falls and Katoomba shortly before 1am today. Chifley Road from Lithgow to Bell was closed about the same time.
By 2am today, snow was falling farther afield, with the Mid Western Highway closed between Fitzgeralds Mountain and Blayney and some motorists reported stranded. Bells Line of Road closed some time before 4am between Bell and Mt Tomah. Snow in the Oberon area ceased around 5am. By daybreak, the Great Western Highway was closed all the way from Katoomba through to Bathurst due either to snow or ice. Most roads had been reopened by mid to late morning today.
In the Blue Mountains, and probably other areas, rain gradually turned to snow with the result that trees and surfaces were wet when clinging, wet snow began to fall. Trees and shrubs therefore accumulated snow rapidly, and in areas where there wasn't enough wind to dislodge it, the weight of ice and water proved too much for many branches. From about midnight, those awake in an otherwise silent snowscape reported hearing occasional loud cracks followed by a muffled slump as another tree shed a limb or an entire tree crashed to the ground. Some of these brought down powerlines, one blocking the Great Western Highway at Mount Victoria. About 1,500 properties were blacked out at Blackheath, Lithgow, and Mounts Victoria, Wilson and Irvine and there were remote area blackouts, such as around Oberon and Burraga, where field staff had to use a snowmobile to gain access in the snow.
Snow depths reported by press, contributors to weather forums, and the Bureau included:
- Katoomba 12cm (reported by the Bureau, but most observations and melt rates later in the day suggest 3 to 5cm)
- Blackheath 5 to 10cm (observations when roads reopened suggest the heaviest snow in the Blue Mountains fell between Mt Boyce and just north of Medlow Bath)
- Mt Victoria 5cm
- Lithgow area 2 to 10cm (10cm reported by the Bureau from a source unknown to AWN)
- Portland 7cm
- Sunny Corner 10 to 15cm
- Meadow Flat 6cm
- Orange 2cm (not known whether this was in town or at the Airport about 100m higher. Conditions on Mt Canobolas unknown.)
- Oberon 5 to 10cm
- Mt Trickett (6km west of Jenolan Caves and 1340m asl) 30cm with drifts said to be more than a metre deep where strong winds had piled snow against cuttings or in gullies. Lowest temperature on the mountain during the night was -1.
- Taralga 2 to 3cm in town, 5 to 6cm surrounding hills
Snow continued to fall in Alpine areas. The Canberra Office of the Bureau of Meteorology reported that higher peaks of the Brindabella Range west of Canberra had a 10cm cover, while Snowy Mountains resorts reported an additional 15cm had fallen overnight.
Despite low moisture levels, some snow was reported on the Northern Tablelands. Falling snow was reported at Deepwater (1040m) about 11pm, while Guyra reported 1cm of settled snow. Snow was also reported in the press at Uralla, Walcha and Glen Innes. In the upper Hunter, snow fell to as low as 400m around Stewarts Brook, about 50km NE of Muswellbrook with depths of 1 to 2cm on the ground in the settlement itself. It is highly likely that significant snow fell on the nearby Barrington Tops.
Winds and seas
As indicated above, expected strong surface winds on the Central and Illawarra Coasts were suppressed when colder, denser and lighter westerly winds undercut the stronger southeasterlies, keeping them above the surface. A number of exposed coastal locations, however, experienced gusts in the damaging range above 90km/h. These were mostly on the South Coast, where Montague Island reported 104km/h around midnight. Other reports are in highest AWS wind reports and highest gusts. There was a report of winds reaching 90km/h at Wottamolla in the Royal National Park south of Sydney. SES reported about 80 callouts across the state, mostly from the Central Coast and Sydney's North Shore, mostly for fallen trees or branches or water inundation in houses, but a spokesman added "That's about what we would expect from an average wet and windy night in Sydney". About 30 coal ships waiting off the Newcastle coast headed out to deeper water in readiness for the predicted storm.
The winds were felt over a wide area. In QLD, strong winds blew over a 4x8m wall in Fortitude Valley Brisbane onto the roof of an adjacent building. There were also reports of fallen trees and powerlines across the city, and morning peak hour rail services were disrupted when debris fell on the Cleveland line. Gale to storm force winds were also experienced in Bass Straight VIC, with gusts to between 90 and 100km/h at coastal stations, and a gust of 102km/h at the Kingfish A oil rig off the coast near Sale yesterday morning.
Much of the NSW Coast from the Hunter south to the VIC border received falls of 25 to 50mm in the 24 hours to 9 this morning, by which time rain had mostly cleared. Heavier falls, up to 81.6mm at Bruthen, were recorded in Gippsland across the border in VIC, causing some flooding (see story yesterday). Stormwater flooding cut power to about 1400 properties in Mosman on Sydney's lower North Shore, while river flooding cut roads at Audley and the Wakehurst Parkway around Sydney.
This and earlier rain has greatly benefited Sydney's water supplies, filling the system to exactly 50.0% of capacity by Thursday 21 June. Warragamba was 43.5% full, Cataract, Cordeaux and Avon between 60 and 75% full, and Nepean, the Blue Mountains dams and the Tallowa dam on the Shoalhaven River were overflowing. Rainfall estimates across the catchment between 7 and 21 June were Warragamba 180 mm,
Upper Nepean 300 mm,
Woronora 320 mm,
Shoalhaven 275 mm and
Blue Mountains 280 mm. Storage increase for the week to 21 June was a massive 276,750 megalitres, the biggest weekly rise since 1998, bringing the dams to their highest level since May 2004.