Weather Forecasting ... On-Line
Introduction to Verification
If you routinely prepare weather forecasts, you are likely interested in how well your forecast does compared to the actual weather that occurs. Although this comparison seems relatively easy to do, it can be more complex than it appears. The purpose of this web page is to explore the meaning of verification and examine reasons why verification is important. The companion web page,
Verification Measures, looks at a variety of ways to quantitatively assess the "goodness" of a forecast.
What is Verification?
The Compendium of Meteorology (American Meteorological Society) defines verification as follows:
"the entire process of comparing the predicted weather with the actual weather,
utilizing the data so obtained to produce one or more indices or scores and
then interpreting these scores by comparing them with some standard depending
upon the purpose to be served by the verification."
This definition points out several aspects of verification that need to be highlighted:
The terms evaluation, performance measure, and metric are frequently used in lieu of the term verification. Similarly, verification is usually thought of as a numerical comparison or score, but often verification can take the form of a more qualitative comparison.
Why is Verification Important?
In the early 1980s the National Weather Service examined the verification process and developed a list of reasons why a verification system is needed. These reasons include:
A verification program should have several goals:
What Should You Verify?
You need to verify what you forecast! The question is: what did you forecast? Forecasts can be divided into four basic types:
To verify any of the above types of forecast, you need two things:
- definitions for the terminology used in the forecasts
- time frames over which the forecasts are valid
Similarly, you need to decide how to treat ranges of forecast values, and whether you are forecasting for a point or for an area.
Once your verification system answers all questions, you need to collect the forecasts and their accompanying verifying data. This gives you a set of forecast-observation pairs that are the basis for the statistical comparison described in the companion web page, Verification Measures.
Verification is often overlooked in a forecast office. It should not be. A forecaster needs to know how well he/she does. It aids the learning process and enhances the forecast experience of the forecaster. It helps a forecaster learn what forecast method works in a particular situation and where improvement is needed.
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last updated on 2/27/10