Accuracy of Long-Range Probability Forecasts for Iowa Rivers
After the 1993 Upper Mississippi River Flood, the National Weather Service (NWS) began implementing its Advanced Hydrologic Prediction Services (AHPS), choosing the Des Moines River as its first demonstration basin. One component of AHPS uses ensemble streamflow prediction (ESP) techniques to issue forecasts of the probability of flooding in the upcoming season (the next 90 days). This research intends to estimate the accuracy of these long-range flood forecasts, or in other words, how likely is the river to flood once it has been forecasted?
Operational long-range river forecasts are made for the eastern two-thirds of Iowa by the NWS North Central River Forecast Center (NCRFC) in Chanhassen, MN (Figure 1).
Forecasters use recent weather observations to assess current moisture conditions within a river basin, and then run computer models for different possible scenarios. For instance, what would the river flows be over the next 90 days if the weather was the same as observed last year? These “what if” scenarios can be run for any year in which historical weather observations are available (in practice, about 50 years into the past). By analyzing the scenarios with statistical methods, forecasters can predict whether a flood is more (or less) likely given the current moisture conditions in the basin (Figure 2).
To evaluate the accuracy of these long-range forecasts, we can compare the forecast probability of a flood with what actually happens. If a flood is forecasted to be more likely, one would expect that the river would flood more often (at least more often than it does on average). Such an assessment can only be done by comparing the forecast probability with the outcomes issued over many years. Of the 133 NCRFC forecast sites in Iowa, 54 have at least 30 years of flood observations to do such analysis (see Figure 1). Our evaluation of the quality of long-range flood forecasts will focus on these sites.
Using the operational forecast model from the NWS North Central River Forecast Center (NCRFC), we will generate past forecasts retrospectively, for a period from 1950 to 1999 (the historical period available for operations). We will compare what would have been forecasted historically had AHPS been available, with what happened when forecast flood probabilities were unusually high (or low). The comparison will provide insights on the skill of long-range forecasts, how the skill varies throughout the Iowa flood season, and the potential value of long-range forecasting for flood planning and advance preparation.
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