By Jon Nese | December 26, 2013 at 12:15 pm
With AccuWeather having recently introduced its 45-day forecasts in August, I thought it would be interesting and informative for this year’s students to be some of the first to quantitatively assess the skill of this new product.
With that in mind, from September 2 to December 3 (a 93-day period), my students retrieved the maximum (high) temperature forecasts from AccuWeather.com for 15 cities: Denver, Glasgow, Bismarck, Minneapolis, Marquette, Buffalo, Caribou, Boston, Philadelphia, Cleveland, Chicago, Kansas City, Memphis, Louisville and Cape Hatteras. The students then computed the absolute value of the difference between the forecasts and the observed high temperature, and averaged those “absolute errors” for each forecast day to arrive at mean absolute errors for the Day-1, Day-2, Day-3, … , Day-45 forecasts.
All forecasts were retrieved at 7:30 a.m. ET using zip codes appropriate to the observing site at which the forecasts were verified (typically, the airports). For quality assurance, I also independently retrieved the forecasts and the observed maximum temperatures.
To assess the skill of the AccuWeather forecasts, the historical National Weather Service averages (“normals”) derived from the period 1981-2010 were also used as “forecasts” – this “climatology forecast” is commonly used as a basis for comparison.
The results for Minneapolis, Boston and Memphis (below) are representative of the rest of the cities. Depending on the location, AccuWeather becomes less accurate (on average) than climatology at anywhere between 9 and 11 days lead time. The average of the results for the 15 cities demonstrates that beyond this time frame, the AccuWeather forecasts actually have “negative skill” – that is, climatology, on average, is more accurate than the deterministic AccuWeather forecasts.