Never believe year-long forecasts

On my to-do list, this post is titled “Chad Evans, you son of a bitch.” Though the specifics are about the failings of a specific local TV meteorologist, the broader lesson is that weather forecasts longer than about a week aren’t worth the time it takes to make or read them. AccuWeather’s 45-day forecasts have caught some flack for being awful, as everyone expected they would be. Less attention has been paid to verifying the long-range forecasts from WLFI meteorologist Chad Evans.

I decided to take a look at the September 2011 forecast¬†to see how it fared (there’s probably a forecast from September 2012, but I’m too lazy to search for it). As the graphs below show, it’s hard to beat climatology for long-range forecasts. Interestingly, there’s not a noticeable drop in skill over time with temperatures. The precipitation forecast does seem to get worse over the life of the forecast, with the exception of a lucky break in the summer.

Forecast and climatology monthly average temperatures.

Forecast and climatology monthly average temperature errors.

 

Forecast and climatology precipitation total errors.

Forecast and climatology precipitation total errors.

Mr. Evans was smart enough not to include day-by-day specifics, except for Christmas. This year, he claimed ¬†claimed to be 4-0 on his white Christmas forecasts. The forecast called for 1″ or more of snow on Christmas morning. Unfortunately, there was none. Several inches fell the week before, but warm and rainy weather the weekend prior took care of that. Speaking of snowfall, 10″ was forecast for January 2014. In six days, we’ve already passed that, and the snow continues to fall as I write.

In the first two months of the most recent annual forecast, the temperature errors aren’t awful, but the precip forecasts miss the mark pretty hard (though the direction of the error was right in both cases). As the year progresses, you’d expect to see the skill diminish.

Nov Dec
Tmax 9 1
Tmin 5 8
Tavg 3.9 2.1
Precip .73″ (25%) .95″ (38%)
Forecast absolute error

And that’s really the point here: seasonal (or longer) outlooks are really bad at giving specific information. You can sometimes make use of them for trends, but even then they’re not very reliable. I can’t fault a forecaster for busting a forecast, I’ve had plenty of busts. But presenting skill-less forecasts to the public is a disservice to the public and to the reputation of the meteorology profession.

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