|Local weather forecast by
and the Final Product
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 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
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.
Monday, September 16, 2002