SHORT-RANGE ENSEMBLE FORECASTING OF EXPLOSIVE AUSTRALIAN EAST-COAST CYCLOGENESIS

Explosive cyclogenesis occurs on average once a year over the coast of New South Wales (NSW), Australia. Known locally as east coast lows, t hese storms are characterized by very strong winds and heavy rain. Int ensity, size, proximity to the coast, and speed of movement of the cyc lone are important in their impact on coastal NSW, especially Sydney. Predicting the location of the system, the maximum sustained wind spee ds, and the rainfall totals all are operational forecasting challenges . Warnings are issued when predictions exceed threshold values. For ex ample, land gale forecasts are issued if sustained wind speeds are exp ected to reach or exceed 34 kt (about 17 m s(-1)). The east coast low of 30-31 August 1996 featured land gales over the greater Sydney area. No warnings were issued as the forecasters estimated that the wind st rength would fall below gale force. In this study, uncertainty in the predictions is estimated and reduced by providing, in addition to the routine single operational numerical weather prediction, a Monte Carlo -based short-range ensemble (SREF) approach. The intention is to impro ve the forecasts and also to provide valuable statistical information such as sea level pressure probability ellipses and estimates of the v ariances in the wind and rainfall predictions. For this event, both th e unperturbed and ensemble forecasts predicted sustained maximum wind speeds in excess of 40 kt (20 m s(-1)) at the official Sydney observat ion station. However, the SREF provided vital additional information, namely, that over 70% of the forecasts were within one standard deviat ion (plus or minus 5 kt) of the mean. The SREF guidance therefore stro ngly supported the prediction of land gales. Moreover, although the en semble forecast mean slightly underpredicted the rainfall total at Syd ney, the forecast spread encompassed the observed 24-h total of 127 mm .