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Image © Coastal Views An unusual east coast low created the worst flooding since 1954 in the Clarence Valley, on the north coast of New South Wales, Australia. Here the river has burst its banks and spilled beyond into the surrounding low country in the vicinity of Lawrence. Sugar cane is shown as green vegetation along the edge of the river. Hybrid low packs a punch North-east New South Wales witnesses storm-force
winds and the worst floods in more than 50 years ... March 14, 2001 What began with awesome surf on Queensland's Gold Coast ended in disaster for dairy farmers along the banks of the Macleay River further south. East Coat lows are a product of the central Tasman Sea but rarely do they migrate to the north-west, intensify to storm force then track across the Australian coast to the south-west, bringing devastating floods. While the storm was every bit a category one cyclone when it crossed the beaches south of the NSW- Queensland border, it didn't start out that way. As a result, its impact bewildered residents not used to this sort of weather. When it developed near Lord Howe Island on March 3 - five days before it crossed the coast - the mechanics behind its formation did not belong to the cyclone definition. The low formed in classic 'ex-tropical' fashion with a little help from the jet stream, creating spinning winds at mid levels which then spiraled downwards to the surface. Winds at the surface were powerful indeed and the fetch extensive for the Tasman. A peak wave of 9m was recorded by a monitoring buoy off Cape Byron, Australia's easterly-most point early on March 5. Throughout the week, the swell remained at a significant average of between 4-5m, but for anybody other than surfers and property owners by the sea, the northern rivers region remained unaffected.
Quicksilver Pro revels in ideal conditions Image © Jonathon Wood. Ryan Turner of the US floats across the lip of a perfect barrel at Burleigh Heads, Queensland, during the first round of the Quicksilver Pro. The 1.5m (eight foot) waves were the best this coast had seen for more than 12 months. Deepening system Things began to happen along the coast on March 6, as the low pressure cell intensified out to sea, bringing rain squalls from the south. The next day was similar, but by the morning of March 8 the familiar roar of wind in the tree tops signaled the storm was deepening and moving towards the coast. The Australian Bureau of Meteorology issued a storm warning between Wooli and Cape Byron with winds 45-50 knots at times gusting higher. While it is unusual for a storm to travel to the west, it is not unheard of. After all, the region's great flood-bearing cyclones of 1954 and 1974 traveled this way and left a legacy of destruction in their wake. What made this storm different, was the fact that it first drew its energy from the jet stream, and later sucked moisture and warmth off the ocean surface - thanks to sea surface temperatures above 26 degrees C - just like a tropical cyclone. Before it closed with the coast the storm formed a cyclonic 'eye' around which winds whirled. Squalls bringing gusts as high as 75 knots and sustained winds of 56 knots were recorded at Evans Head. Yamba, 30 km south copped a hammering with buildings facing the open ocean taking the full brunt of the winds. However, other than downed trees and limbs, little structural damage was reported. According to Brisbane severe weather forecaster Jeff Callaghan, the storm should be called a 'hybrid'. "It's debatable where you pigeon-hole this kind of storm," he said.
Weather report Infra red cloud over Australian
and New Zealand Image courtesy of the Australian Weather Bureau. This satellite image shows the Tasman Sea storm as it developed. The jet stream - highlighted with moisture from ex-cyclone Abigail in Western Australia - is helping to get the storm spinning in a clockwise direction at upper levels. This spinning migrates down to the surface and is typical of storm development outside the tropics. For this reason the Tasman Sea storm cannot be called a cyclone. Note the 'fetch' of winds at the surface from the south-east towards the north-west: This long run of winds in the 30-40 knot range created huge swell on North-eastern NSW and South-eastern Queensland beaches. For a view of the NSW wave buoy data for the week of the storm click here. For a view of the Cape Byron wave buoy showing a maximum swell of 9m (30 feet!) click here. Images courtesy of the Manly Hydraulics Laboratory. From here the storm migrated to the north-west - unusual for this region. However, when you consider that this westward movement of weather has been common all summer, this storm is less of a surprise. In fact, the ex-cyclone pictured over Western Australia was created well to the east, over the Solomon Islands! Satellite derived winds (quickscat) These two images, run together as an animation, show the north-west movement of the storm 48 hours after formation to 24 hours before it crossed the coast. Not the fetch of winds over 30 knots from the south-east. Fraser Island can be seen jutting out from the Australian coast north-west of the storm, with Moreton and Stradbroke Islands further south. Burleigh Heads, where early trials of the Quicksilver Pro were held, is south of Stradbroke. Cape Byron is the eastern-most point on the mainland. Visible spectrum satellite image This image shows the east coast low approaching the NSW coast on Thursday at 5pm, local time. The storm is at its most intense and has developed an eye. About this time Evans Head - located in the south-west quadrant of the storm - recorded a gust of 75 knots, with sustained winds of 56 knots. Radar loop, centred on Grafton
NSW, Image courtesy of the Australian Bureau of Meteorology This animated loop shows the storm's eye - top left corner - crossing the coast between Ballina and Byron Bay starting 5.30pm local time. The yellow spots circulating around the eye are squalls, with rain rates up to 40mm per hour. They also packed significant winds, with one squall crossing the coast at Evans Head producing a gust up to 75 knots. For a larger and longer animated loop (692kb), click here. Animated picture of the
storm's Image courtesy of the Australian Weather Bureau. This animation shows the storm from go to whoa! Note how it takes days to evolve into a tightly-packed system, all the while deepening and easing. The wave buoy charts reflect the storm's rises and fall. Extreme flooding (click here for flood figures) After the eye crossed the coast between Ballina and Byron Bay, the storm moved south-west over the tablelands near Inverell. Moist winds from the north-east followed in its wake bringing heavy downpours from north of Brisbane all the way south to Taree. Severe forecaster Jeff Callaghan noted that the dewpoint the next day in south-east Queensland was about 27 degrees C, and thunderstorms were easily created out of such high moisture. One storm blew out of all proportions over Brisbane on Friday night, dumping 203mm (8 inches) in two hours over Logan City. In fact, rainfall figures for Logan recorded 134mm (5 1/4 inches) in the first hour and 232mm (9 1/8 inches) after three hours. Damage from flash flooding in this heavily populated region is expected to top more than AU$10 million. Further south, these same moist winds blew onto the dividing range and condensed as torrential rain. Dorrigo, west of Coffs Harbour lies atop the plateau at 1500m and within sight of the sea. More than 642mm (25 inches) fell within three days here sending the Orara River into extreme flooding. The tiny township of Bellingen east of Dorrigo found a converted butter factory - now arts centre - filled with a metre of mud and debris as the tiny Bellinger River broke its banks. Bridges and roads suffered accordingly. Water from the southern arm of the Clarence River, the largest in New South Wales, created the worst flooding since 1954 and came within centimetres of overtopping the levee at Grafton and Maclean. In fact, the weather bureau predicted the peak to top the levee at 8m built to withstand the worst flood recorded - in 1890 - which peaked at 7.9m. Panic ensued as shop owners and residents prepared for the first evacuation of Grafton in 43 years. Trains and buses were readied for refugees and the main highway running north and south was closed to all other traffic. As it turned out, the flood peaked 250mm below the bureau's predictions and the levee held. Further south on the Macleay River, Kempsey and the flat flood plains around Smithtown and Gladston copped the worst inundation since 1963. The floodplains are prime dairy country, but with recent deregulation and other financial hardship, this natural disaster may put many farmers out of business. Considering the flat nature of the lower Macleay, it will be days before water recedes from this country.
Clarence Valley rainfall: Three days from March 8: 726mm
(29") at Lowanna - located between Dorrigo and Glenreagh, affecting the
Nymboida and Orara rivers. (total over 5 days 880mm.) Peak heights at Grafton in the Clarence Valley: 1890:
7.9m NOTES: In 1954 there were no levees at north and south Grafton. Chief flood forecaster for NSW with the Australian Bureau of Meteorology, Gordon McKay estimates the effect of the two levees on peak flood heights at Grafton can raise the river up to .5m. While this year's flood peak at Grafton was higher than 1954 the flood, itself, was not. However, both the north and south arms of the Clarence were affected by rainfall and both tributaries contributed to this flood. However, the Orara and Nymboida fed by waters draining from the Dorrigo plateau, provided the greatest runoff. The bureau predicted a peak of 8m - the height of the levee - for 2001. Models for the Orara River pointed to slightly higher runoff than what actually occurred contributing to the lower-than-predicted peak, according to Gordon McKay, chief flood forecaster with the bureau of Meteorology in Sydney. Maclean, influenced by tide, lower Clarence Valley: 1890:
3.54m Macleay River Valley: 1949: 7.98m In this year's flood, most of the rain fell in the lower 60 percent of the catchment, whereas previous major floods recorded widespread rainfall, particularly in the headwaters. This year, as little as 46mm fell over seven days at Walcha, at the extreme headwaters. Rainfall
for the Macleay catchment Wollomombi:
140mm over 7 days. © Jamie Brown/weather-wise.com 2000-2001 |
