To me, weather prognostication is like stacking pillows: The first two or three may find themselves accurately above the ones below, but once you’re ten pillows high the tower will come crumbling down in a heap of inaccuracy.
That is to say, tomorrow or the day after’s weather prediction may be accurate, but I’ve always operated under the assumption that 10-day weather forecasts are bullshit (and judging by conversations I’ve had with my friends this is not an unpopular opinion.)
The problem is, who is going to call out weather forecasters on their faulty 10-day predictions? Nobody remembers what tomorrow’s weather forecast was nine days ago. People only look at tomorrow’s forecast, because frankly anything beyond that is fairly inaccurate.
But just how inaccurate? How wrong is that 10th day prediction? And does it get more accurate the closer that day gets to today? That’s what I wanted to find out.
First I poked around the Internet looking for old 10-day weather forecasts. When I couldn’t find any, I started recording my own data by taking screencaps of weather.com’s 10-day weather forecast for my old Manhattan zipcode.
I assembled my first ten days worth of data into the info-graphic below:

Superficially this looks like a regular 10-day forecast, though take a closer look and you’ll notice one large exception: These are all predictions for the same day: Saturday, May 01. (GROUNDHOGS DAY!!! No, just joking.)
It’s organized in chronologically descending order with May 1st forecast at the top, and April 22nd prediction at the bottom — I should note now that 9 days away is the last day in a 10-day weather forecast, with the first day being todays prediction. You fucking idiot, did you really not know that?
It hardly takes a statistician to notice accuracy and time are inversely proportional. Or perhaps more simply put: The farther you are away from the day you are predicting, the less accurate your prediction becomes. Again, this is common sense, lets look at the farts. Just kidding, the facts.
Nine days away, on April 22nd, Weather.com had predicted a rainy day, with a high of 66 degrees. Well I hope people checked the weather for May 1st since that day, because they would have looked awfully dumb in a rain jacket and umbrella when May 1st approached and it was in fact 86 degrees and sunny! “At least take off that jacket you dumb fuck!”
Some other interesting things: you’ll notice a great leap in accuracy between 4 and 5 days away from “today’s date.” Meaning by April 27th, four days away from May 1st, weather.com had finally zeroed in on an accurate prediction, getting the high wrong by only 3 degrees Fahrenheit and the lows completely accurate!
Certainly one set of data isn’t very telling, which is why I kept recording forecasts until we landed on a rainy day. I was eager to find out if weather.com, which wasn’t very accurate about temperature, was perhaps a little better at predicting precipitation. I’m crying right now. It won’t stop.
Below is a similar info-graphic compiled from 10-day forecasts from May 3rd thru May 12th.

So for this set of data, these are all predictions for May 12. Based on temperature alone, weather.com was even less accurate then our first info-graphic. Nine days away, on May 3, they were not only off by 22 degrees (predicting a high of 73, vs the actual 51) but they were incorrect about rain (predicting mostly cloudy, but no precipitation). And they say banks need to be bailed out? …What?
You also may notice that 4 days away, May 8th, weather.com is still very off with their prediction, estimating highs and lows of 69/54 against the real 51/44 — a very chilly day for May! Yo quiero taco HELL!
However, on a purely precipitation level, by May 4th, weather.com pretty much nailed it, and never relented: it was going to rain on May 12th, and you know what? They were right!
Obviously this is just twenty dates worth of information (out of a possible two-million-eighty-thousand-five-hundred — Creationists do it from behind!) but it should hopefully help shed a little bit of light on just how much these so-called “professional” weather predictors get away with when it comes to 10-day weather forecasts.
;)