Compound weather conditions that caused the north Queensland floods in February 2019: observations and predictions

Compound weather conditions that caused the north Queensland floods in February 2019: observations and predictions

In early February 2019, the compounding effect of record-breaking rainfall, strong winds and relatively cold temperatures over the northern Queensland led to the loss of an estimated 670,000 livestock over an area of 13 million hectares. This was caused by a near-stationary monsoon depression that saw weekly rainfall totals above 1000 mm in some locations and day-time temperatures more than 10°C below average for almost 10 days. This led to large numbers of cattle and native fauna dying from hypothermia (Hall et al. 2020). The insurance cost to Townsville, where flooding caused inundation and damage to more than 3000 homes, was over $1.2 billion AUD.
Here, we present the findings from a first look at the rain-temperature compound event from an observational and multi-week prediction standpoint (Cowan et al. 2019). Observations and reanalysis confirm that an active Madden-Julian Oscillation pulse stalled over the western Pacific during the period of the flooding. To the south, a blocking anticyclone over the northern Tasman Sea promoted onshore easterly flow together with relatively low apparent temperatures. Given El Niño-Southern Oscillation was neutral, it did not play a significant role in the event.
Forecasts from the Bureau of Meteorology's subseasonal-to-seasonal (S2S) prediction system, Australian Community Climate and Earth-System Simulator, Seasonal version 1 (ACCESS-S1) – predicted a more than doubling of the probability of extreme weekly rainfall totals a week prior to the floods, along with increased probabilities of unusually low maximum temperatures and high winds. Ensemble-mean weekly rainfall amounts, however, were considerably underestimated by ACCESS-S1, even in forecasts initialised at the start of the peak flooding week. This is consistent with other state-of-the-art dynamical S2S prediction systems, including ECMWF’s SEAS5. Among the eleven different S2S systems analysed, the ensemble medians of only two models predicted above the highest observed decile rainfall for northwest Queensland in a week 2 forecast. On closer inspection, at least one individual ensemble member of ACCESS-S1 managed to forecast ~85% of the event's rainfall magnitude for lead week 2. Prediction systems like ACCESS-S1 are therefore capable at getting close to predicting record-breaking compound events with at least one week's lead-time, but accurate prediction beyond two weeks of an event like the northern Queensland floods remains somewhat elusive.

References:
Cowan et al. (2019): Forecasting the extreme rainfall, low temperatures, and strong winds associated with the northern Queensland floods of February 2019, Weather and Climate Extremes, 26, 100232, https://doi.org/10.1016/j.wace.2019.100232.

Hall. T. J (2020): Pasture recovery, land condition and some other observations after the monsoon flooding, chill event, Department of Agriculture and Fisheries Report.

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