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John Wheeler, WDAY, Published March 27 2013

Weather Talk: Forecast not meant to be a spot-on crest prediction

Until the spring thaw has progressed to the point at which rivers are rising, the National Weather Service predictions are limited to what is called the “probabilistic outlook.” This is a computer model that takes into account past floods and the conditions that led to those floods in order to come up with an array of probabilities that rivers will reach or exceed certain levels.

The model starts with known factors, such as the shape of the land, the water content of the snowpack and the current levels on the rivers and then uses probabilities to factor in the unknowns, such as future weather and how much water will be absorbed by the soil.

The result is a complicated list of probabilities. Most people tend to fix on the 50 percent probability as the forecast, but really the entire range of probabilities is the forecast at this point. The chance of rain a week or three weeks away is really and truly unknown.

The probabilistic forecast is designed to help people make planning decisions. It is absolutely not intended to be a spot-on crest prediction.


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