| Big weather: What to look for While cyclones and other big storms stick in the minds of sailors, gentler events often demand more immediate attention. Take the humble low pressure trough and its associated band of cloud that seems to sit overhead, refusing to budge. On a weather map a trough is usually drawn as a dotted line. On satellite cloud images the trough is invisible but a thick band of cloud to its east gives its location away. When it comes to making storms, all the action happens east of a trough in the band of moist cloud. In Australia during La Nina, troughs tend to stall west of the dividing range and their cloud bands sit over our east coast, causing grief to the sailing public. The tropical cloud band that sat over the country on Christmas and Boxing Day 1999 was a good example. It rained from Cape York to Hobart, ruining cricket in Melbourne while the leading yachts in the Sydney to Hobart relished in fresh north-east winds. "Upper troughs can trigger lows any time of the year," says Brisbane severe storm forecaster Jeff Callaghan. "Rain-bearing trough systems always have the potential to bring cyclones in and develop east coast lows." Moist easterly winds entering a cloud band are like adding fuel to a fire, says Callaghan. Easterlies are a good sign that further trouble could be brewing. Yachts in the Sydney to Hobart relished in easterly winds up to 30 knots before the cloud band moved offshore. Fifty knot south-westerlies followed. The weather was later complicated when a double-low pressure system formed out of the trough and sat stationary in the Tasman Sea for days afterward. Race tail-enders were literally hammered. At the same time up north, cruising sailors in the vicinity of Lady Elliot Island, off Queensland's Capricorn Coast, found themselves running for the cover of Bustard Head when a low developed in the cloud band to their north. It moved quickly offshore, and created 40 knots and more of easterly winds. In June 1994 a huge trough system moving away from the Australian mainland, gave birth to a cyclonic 'bomb' in waters between the North Island of New Zealand and Kiribati. A cruising fleet of sailboats en route to Kiribati from the Bay of Islands was caught in its path and the destruction that followed provided enough material to write a best-selling book ... © weather-wise.com 2000 |