2.0 Ensemble Forecasts in Hydrology » 2.2 Flood Risk Example
Probabilistic guidance can be generated from an ensemble forecast to assist forecasters and end users with visualizing the range of possible outcomes and their associated probabilities.
Question
In this single-valued forecast, what is the probability of reaching flood stage? (Choose the best response.)
The correct answer is a.
There are no absolutely correct answers since your experience and familiarity with the situation play a role in developing the forecast. Given the situation, you may say there’s a small, non-zero chance because the forecast peaks close to flood stage and you know that that there’s uncertainty in the forecast. However, if you believe that the single-valued forecast is perfectly accurate, you would give a 0% chance.
This HEFS graphic provides probability guidance for the same time period and initialization time as the single-valued forecast above.
Question
Based on this product, what should you forecast for reaching flood stage? (Choose all that apply.)
The correct answers are d, e.
The probabilistic forecast provides quantitative guidance about the forecast uncertainties. But like the single-valued forecast, it doesn’t provide an absolutely correct answer. It does, however, suggest a much greater chance of reaching flood stage (75%) and a small chance (10-25%) of reaching major flood stage.