The Bureau of Meteorology (BOM) has released its climate update for May 2026, and the forecast presents a complex "double-edged sword" for stormwater managers across Australia. While the headline suggests a transition toward drier conditions, the underlying oceanic signals warn that when rain does fall, it may carry more energy and intensity than usual.
As we move into the June to August period, here is what the long-term forecast means for the industry and how to prepare.
The Forecast: A Shift Toward El Niño
The BOM indicates that the El Niño-Southern Oscillation (ENSO) is currently neutral, but the tropical Pacific is warming rapidly. An El Niño development is likely during winter. Simultaneously, a positive Indian Ocean Dipole (IOD) is possible for winter-spring.
What this means for rainfall:
- The Dry Signal: There is a 60% to 80% chance of below-average rainfall for much of eastern Australia, Victoria, southern SA, and south-west WA.
- The Temperature Spike: Maximum and minimum temperatures are likely to be above average across nearly the entire continent. In many southern regions, there is a 70% chance of "unusually high" temperatures.
The Stormwater Paradox: Dry Catchments vs. High-Intensity Events
For stormwater professionals, a "dry" forecast can often lead to a false sense of security. However, the May update contains a critical detail regarding Sea Surface Temperatures (SSTs).
Waters along the NSW and eastern Tasmanian coasts are currently up to 3°C above average. Forecasts for winter indicate these warmer-than-average waters will persist. The BOM warns that "warmer waters can increase atmospheric moisture and energy, raising the potential for more intense rainfall and weather systems."
For stormwater management, this creates three distinct challenges:
1. The "First Flush" Phenomenon
With a forecast of below-average rainfall, catchments are likely to experience longer dry spells. During these periods, pollutants, including hydrocarbons, heavy metals, and organic debris - accumulate on impervious surfaces. When the "likely" high-intensity bursts occur due to the warm coastal waters, the "first flush" will be highly concentrated, potentially overwhelming filtration systems and leading to higher pollutant loads entering waterways.
2. Hydrophobic Soils and Flash Flooding
Extended periods of above-average temperatures and low rainfall can lead to soil hydrophobicity (water repellency). When an intense coastal low or storm cell hits, the parched ground may struggle to absorb initial moisture, leading to higher-than-expected runoff coefficients. Existing infrastructure must be clear and operational to handle these sudden peaks in volume.
3. Asset Maintenance and Fire Risk
Warmer and drier conditions increase the accumulation of dry organic matter in open channels and basins. This not only creates a fire risk within urban corridors but also means that when rain does arrive, large volumes of organic debris can quickly block gully pits and trash racks.
Strategic Recommendations for Winter 2026
In light of the May 2026 update, stormwater managers and local authorities should consider the following actions:
- Accelerate Pit and Pipe Cleaning: Take advantage of the drier windows in June to ensure networks are at 100% capacity. This mitigates the risk of flash flooding from high-energy "warmer water" storm events.
- Monitor Water Quality Systems: Ensure that high-efficiency GPTs (Gross Pollutant Traps) and biofiltration segments are maintained to handle the heavy pollutant loads associated with the first flush after a dry spell.
- Vegetation Management: Prune and manage vegetation around basins and swales to prevent excessive organic loading and reduce fuel loads for the warmer-than-average winter.
- Review Design Assumptions: This forecast reinforces a growing trend: the traditional "average" is becoming less frequent, replaced by extremes. If you are in a coastal zone (particularly NSW or TAS), ensure your emergency overflow paths are unobstructed to handle the "unusually intense" events fueled by record SSTs.
Summary
The Winter 2026 outlook suggests we are entering a period of lower total volume but potentially higher individual event intensity. Success for stormwater management and maintenance over the next few months will depend on not being lulled into complacency by the "dry" forecast, but rather preparing for the volatility that warmer oceans bring to our doorstep.
