Subseasonal Forecasts of the Northern Queensland Floods of February 2019: Causes and Forecast Evaluation

Abstract

During the austral summer 2018/19, devastating floods occurred over northeast Australia that killed approximately 625,000 head of cattle and inundated over 3000 homes in Townsville. In this paper, the disastrous event was identified as a record-breaking subseasonal peak rainfall event (SPRE). The SPRE was mainly induced by an anomalously strong monsoon depression that was modulated by the convective phases of an MJO and an equatorial Rossby (ER) wave. The ER wave originated from an active equatorial deep convection associated with the El Niño warm sea surface temperatures near the dateline over the central Pacific. Based on the S2S Project Database, we analyzed the extended-range forecast skill of the SPRE from two different perspectives, the monsoon depression represented by an 850-hPa wind shear index and the 15-day accumulated precipitation characterized by the percentile rank (PR) and the ratio to the three-month seasonal (DJF) totals. The results of four S2S models of this study suggest that the monsoon depression can maintain the same level of skill as the short-range (3 days) forecast up to 8–10 days. For precipitation parameters, the conclusions are similar to the monsoon depression. For the 2019 northern Queensland SPRE, the model forecast was, in general, worse than the expectation derived from the hindcast analysis. The clear modulation of the ER wave that enhanced the SPRE monsoon depression circulation and precipitation is suspected as the main cause for the lower forecast skill. The analysis procedure proposed in this study can be applied to analyze the SPREs and their associated large-scale drivers in other regions.

Multi-week prediction of livestock chill conditions associated with the northwest Queensland floods of February 2019

“…It is possible that the different hindcast periods across the S2S models present a challenge in comparing the extreme probabilities between models; however, we repeated the analysis using a common hindcast period (1999–2010) and the probability patterns remain largely unchanged (figure not shown). As detailed in recent studies, the week-long weather event that led to the extreme chill was associated with a stalling of the convective phase of the MJO over the western Pacific for ~ 10 days 1 , 15 . We next investigate whether the stalling of the MJO was predictable in the S2S models based on lead week 1 forecasts of the MJO.…”

“…Limitations on the prediction skill of the event's extreme rainfall beyond 1 week was also seen in a suite of ensemble numerical model forecasts from the UK Met Office 14 , with prediction errors generated locally in a lead week 1 forecast with convective rainfall overestimated along the western Cape York coast. This was further supported in a study focusing on a subset of four S2S models 15 , which suggested that three of the models could only predict an extreme rainfall event (off the coast of Townsville in far northeast Queensland) at a higher percentile ranking than their hindcast median, at a lead time of just over 1 week. The fourth model, on the other hand, showed good rainfall prediction skill beyond 2 weeks, which the authors argued to result from the small ensemble size ( N = 4) limiting the robustness of the statistical significance.…”

“…We also used available MJO indices from nine S2S models (Table 1 ), to verify if the stalled MJO was predictable, extending the analysis of recent studies on the extreme event 1 , 15 . One hypothesis is that the MJO stalling in phases 6 and 7 (i.e., western Pacific) was associated with the near-stationary tropical low and hence extreme weather conditions 1 .…”

“…The low responsible for the flooding formed a core on 30 January, and then deepened to 992 hPa over the next five days whilst remaining somewhat stationary inland from the Gulf 13 , 14 . At the same time, an active pulse of the Madden Julian Oscillation (MJO) stalled in the western Pacific 15 for about 18 days, in contrast to the five days the MJO remained in Phases 6 and 7 in early January 1 . The stalling of the MJO is thought to have been a key driver of the event 1 , 15 .…”

“…At the same time, an active pulse of the Madden Julian Oscillation (MJO) stalled in the western Pacific 15 for about 18 days, in contrast to the five days the MJO remained in Phases 6 and 7 in early January 1 . The stalling of the MJO is thought to have been a key driver of the event 1 , 15 . The near-stationary tropical low drove day-time temperature anomalies down to −10 °C (absolute temperatures of ~ 25 °C) at particular locations across the Cloncurry and McKinlay Shires, and wind gusts between 70 and 80 kph in the towns of Cloncurry, Julia Creek, and Winton 1 .…”