Seasonal Climate Outlook Message for March to June 2009

The bottom line

Severe TC Hamish batters Queensland coast. Dave McRae, Qld Climate Change Centre of Excellence, 10/03/09

Category 4 Tropical Cyclone Hamish is the first cyclone for some time to travel south along the Queensland coast. It will be interesting to see if it delivers high rainfall totals to southern Queensland especially in dam catchment areas (but preferably without the wind damage to buildings and agricultural crops that occurred with TC Larry in March 2006).

For a cyclone to develop it needs a pre-existing low pressure system or disturbance (trough or surface level low), warm ocean temperatures (greater than 26.5 degrees C), a moist unstable atmosphere (such as suitable for strong thunderstorm development), be at least 500 km away from the equator (to allow for coriolis deflection or spin) and have little change in wind with height (low/weak vertical wind shear needed to allow for development of the cyclone centre).

For a cyclone to be rated as a category 4 it should have wind gusts between 225 to 280km/hr, an average maximum wind speed between 160 to 200km/hr and would usually have an approximate central pressure of 955 to 930hPa. Tropical cyclone Tracey was a category 4 (Darwin 1974).

For a cyclone to be rated as a category 5 it should have wind gusts in excess of 280km/hr, an average maximum wind speed in excess of 200km/hr and would usually have an approximate central pressure of less than 930hPa. A category 5 cyclone would be extremely dangerous and cause widespread destruction.

Tropical Cyclone Larry was the last significant cyclone to impact on Queensland. It crossed the north Queensland coast near Innisfail in north Queensland and caused much devastation in that region. A maximum wind gust of 294km/hr was recorded at the Bellenden Ker Tower (CSIRO weather observation site, elevation of 1450m) during cyclone Larry.

For more information on cyclones and warnings, go to the Bureau of Meteorology tropical cyclone warning centre at www.bom.gov.au/weather/cyclone/

As autumn is a key time for the establishment of climate phenomena such as El Nino and La Nina QCCCE climate staff will continue to closely monitor what happens over the next few months. The 30 day average of the SOI as of 10 March is plus 7.4. You can receive a text message with the latest SOI values sent to your mobile phone. To subscribe to this free service, call me on (07) 4688 1459.