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September 24, 2006

Accuracy of Temperature Forecasts

ForecastAdvisor provides an accuracy measurement for one- to three-day-out forecasts combined. But ForecastWatch keeps data on forecasts out to nine days, and has data on forecasts for each day out. The percentage of temperatures within three degrees is a basic measure of accuracy. It isn't the only measure, but it is the accuracy measure most commonly known to non-meteorologists. Every city, it seems, has a television meteorologist who proclaims a "three degree guarantee".

Another interesting measure is a forecast "miss". If a temperature forecast is off by ten degrees or more, it is called a miss. That means that if the actual temperature was 80 degrees, a forecast is considered a miss (or a blown forecast) if the forecast temperature was 70 degrees or below, or 90 degrees or above.

The chart below shows the "within three degrees" and "missed forecasts" for both high and low temperature forecasts from all providers for one to nine days out in 2005. Not all of the providers tracked provide forecasts out nine days (and some offer even more). Each bar at one day out represents about one and a half million forecasts. Each bar at nine days out represents about 800,000 forecasts.

At one day out, for the entire country, high temperature forecasts are within three degrees of the observed afternoon high about 68%% of the time. High temperature forecasts are blown one day out about 3%% of the time. Many of these blown forecasts one day out are because of climate extremes that the models don't handle well, or timing errors with cold or warm fronts.

You might notice that the low temperature accuracy is lower than the high temperature accuracy. There are a couple of reasons for this. One, forecasts are taken at 6 pm, and the high is usually around 3-6 pm, whereas the low is around 3-6 am the next morning. Most forecasters, when they forecast a low temperature, forecast the overnight low. For "tomorrow's" (one-day out) forecast, the high will occur in around 24 hours from the forecast, the low, 12 hours after that. That 12 hour difference is important one day out, but becomes less important the farther out the forecast is. This is apparent in the graph. At nine days out, the difference between high and low temperature accuracy is only 1.5%%, whereas at one day out its 7%%.

Notice also that the "within three degrees" accuracy seems to taper off, and if you draw an imaginary line, it looks almost like if you continued the accuracy forward to 10, 11, 15, etc. days out, that it would converge on an accuracy around 30%% or 35%%. You might need to click on the graph to view the larger version to notice this. This is significant because the average accuracy of a climate forecast is about 33%%. A climate forecast is taking the normal, average high and low for the day and making that your forecast. So at nine days out, forecasters still show some skill. They are better than just using the normal temperature for the day. But not by much.

 

 
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