Probability of exceedance
Meat & Livestock Australia October 24, 2023
The probability of exceedance is a graph with lines representing the forecasted and historical data of a certain location for a particular rainfall total as shown in Figure 10. The graph can be accessed by selecting a location on the map or entering a location in the search function.
The red line in Figure 10 shows the likelihood of the chosen location receiving the median rainfall over the specified timeframe. In the example, Murray Bridge typically has a 60% chance of receiving 18.6mm rainfall, 40% chance of receiving 27.6mm and 20% chance of receiving 40.8mm during October. A drier time of year would typically show a steeper red line, whereas a wetter time of year would decrease the slope of the red line.
The blue line in Figure 10 represents the forecast. It shows how often the 99 model runs exceed a range of rainfall values. A blue line below the red line would indicate a drier outlook. A blue line above the red line, as shown in Figure 10, indicates a wetter outlook.
Note the shading of the graph aligns with the defined deciles for rainfall summarised in Table 1 and Figure 3a. Hovering over the graph brings up the percentage chance for the corresponding rainfall.
Figure 10. Rainfall outlook for the month of October 2022 at Murray Bridge, SA. Reproduced from Agriculture Victoria’s eLearning course ‘Using seasonal climate prediction tools’.
Key message
- The four weather extremes of most interest to farmers are heat, cold, wet and dry.
- Meaningful interpretation of forecasting information enables better decision making, which is why it is important to understand the involved terminology.
- The Bureau’s five climate outlook and forecasting tools provide additional context to forecast results by including the decile distribution of forecast model runs and the likelihood of them occurring, which helps you plan operations past the seven-day weather forecast.
- There are two types of tools: maps (the chance of extremes and the chance of 3-day totals) and location-specific graphs (decile bar charts, timeline graph and probability of exceedance).