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Ohio River Forecast Center
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The Models and the Final Product

 

The Model

The basic jobs of a river forecast model are to estimate the amount of runoff a rain event will generate, to compute the routing, how the water will move downstream from one forecast point to the next, and to predict the flow of water at a given forecast point throughout the forecast period.

There are six different rainfall-runoff models made available to River Forecast Centers. The OHRFC uses the "API Event Based" model, an older model with a long history of usage. API stands for Antecedent Precipitation Index, meaning that the model uses an index number based on previous rainfall amounts and the length of time since the rainfall. This index value is then used on a graph that relates rainfall to runoff. This process used to be performed entirely on a series of charts and tables. Now it has been entirely computerized.

There are also a variety of routing methods available to be used in our forecast models. At the OHRFC, we primarily use Recession Coefficient and Muskingum routing. These methods attempt to numerically simulate attenuation, or how a wave of water is lowered and spread out as it flows downstream. No matter how much the peak of the water flow is lessened, the same amount of water must pass through the downstream point, barring human intervention. A graphical example of attenuation is shown below.

The Output

The last thing a river forecasting model does is to plot the flow and stage for a given forecast point. The OHRFC currently uses the Interactive Forecast Model (IFM) to generate the river forecasts. This is an alpha-numeric (tabular) interactive model that allows the hydrologist to modify the amount of runoff that occurs due to rain and snowmelt, the base flow (or groundwater), and the recession coefficient (rate of recession of the river). With each change, the hydrologist instantly sees the effect on the river. The Interactive Forecast Program (IFP) is a graphical replacement for IFM. IFP is built around a nationally standardized forecast program that is far more flexible than the older IFM. The IFP plots show observed flow, local flow, which is runoff and baseflow combined, upstream routed flow, and the model output flow. Here is an example of this type of plot (graph and table).

Then, after examining and manipulating the model output, the computer prepares the written forecast. The forecasts that we produce are not released to the public, only within the NWS. The office whose area of responsibility includes one of our forecast points is responsible for issuing public statements regarding that particular section of river.


National Weather Service
Ohio River Forecast Center
1901 South State Route 134
Wilmington, OH 45177-9708
Page last modified: Tuesday, September 24, 2013 6:47 AM
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