Statistical Accuracy of Internet Weather Forecasts

markmcb writes "Brandon Hansen considers the statistical accuracy of popular on-line weather forecast sources and shows who's on target, and on who you probably shouldn't rely. Motivated by a trip to a water park that was spoiled with hail despite a 'clear sky' forecast, he does a nice job of depicting deviations, averages, and overall accuracy in a manner that stats junkies are sure to love."

Comments

The more the merrier?

[Reverse Gear]

What a nice piece of work.

I can't help but smile a bit that MSN weather in this test turns out to be the absolutely worst when it comes to accuracy in almost all categories.

I would think there is a lot of difference on how the forecasts are made in the different channels, some of them probably do get a lot of their information from meteorologist working on their own stations. I wouldn't wonder if MSN doesn't have a meteorologist (or maybe only one) working to provide their forecasts.
Computers and simulations play a big role in predicting the weather today, but human eyes are worth a lot still.

I don't myself live in the USA, so my primary use of these are to check on when there is severe weather in areas where I know someone.
I have gotten used to check on weather underground for this information, I haven't checked on many other weather channel, but I feel quite well capable of following what is going on in the USA with tornadoes and such here from Denmark.

For a long time we only had one weather forecast service here in Denmark, a national institute. Since a primarily private TV station (TV2) a few years ago started their own weather forecast service, I really feel the national institute have been pulling themselves together and have provided many services that they didn't provide until now. So even though some of the services provides terrible accuracy they might still serve a good purpose in giving the other services competition and thereby forcing them to improve also.

When I am really dependent on the forecast I tend to study the information behind the simple prediction of the given weather, that way I am also much better prepared for possible scenarios, knowing which front move where and can better "read the skies".

Re:The more the merrier?

[Gabrill]

It seams odd to me that he started the project because of rain, and then completely ignored rain in his observations. Otherwise, the study was very cool.

NOAA/NWS

[zoward]

I use the NWS website, mostly because I hate all the annoying flash ads on most of the other sites. I was also under the impression that most of these sites get their information from the NWS and pass it along to you (along with a bucket of ads). There was a lot of complaining amongst the popular weather sites when the NWS opened its own web site.

What is your source?

[Lord Satri]

(I work at the Canadian Meteorological Centre, but I am not a meteorologist myself)

One thing that struck me is the 'abnormal diversity' of weather information sources. In Canada, weather models are computed in one place, a ~1000 processors computer in a basement which does only one thing: forecasting weather (the constant real-world observations that are ingested are used to adjust the models). Only one 'real' source (of course, there's the american, british, french, etc. official forecasting models to which we compare 'scores' on a daily basis). However, there's plenty of other canadian websites which will give you weather forecasts (one example). From what I know, these "other websites" have a significantly smaller workforce of meteorologists to interpret the models results than the Meteorological Service of Canada (the CMC is part of the MSC). That's why I would favor the 'original' source instead of a 'second-hand' source. I must however admit, commercial online sources of weather forecasting sometimes offer value-added products, such as the number of ski trails opened, offer general weather information capsules, etc.

And by the way, the official Environment Canada weather website is the most visited website in Canada (or at least, that's what they tell us, the employees! :-).

Reliable forcasting method...

[AmIAnAi]

I remember some years ago a radio presenter saying that you could achieve greater accuracy than supposed weather forcasters simply by using the assertion: today's weather will be the same as yesterday. Have we moved on from this position?

Re:Reliable forcasting method...

[Bob(TM)]

In the forecasting vernacular, that method describes forecasting based on "persistence".

Persistence is the yardstick all forecasters use to determine if they should find another line of work (or be asked to do so by others). If you can't demonstrate an understanding of the processes and data such that extend beyond the data source everyone else has (ie., the weather their experiencing), it's just snake oil.

(IAAM)

Re:Reliable forcasting method...

[Anonymous Coward]

persistence is 75% reliable.

On any significant sample, weather reports were never worse than this.

Currently, models are able to make 85% or a little more accuracy.

This may sound paltry, but where this really works out is in longer term forecasts. At 75% you are probably wrong at 3 days forecast. Even if you take the assumption that forecasts are independent from day to day, 85% means you are probablt wrong after 5 days.

The extra two days you can predict for is what the money is going towards.

Statistics don't lie Statisticians do!

[wesborgmandvm]

Kudos to this guy for the work he put into the effort but it is really comparing apples and oranges. A forecast is a time sensitive product. You can't look at the forecast provided on day x from two different sources and compare them unless the forecast was provided at the same time of day.

The National Weather Service collects all the weather data used by forecasters, they also provide the 1st forecast. AccuWeather and others take the National Weather Service forecast then watch the new data (using National Weather Service provided data) to offer a refined forecast a few hours latter. Who do you think is going to be the most accurate the guy who provides the first forecast or the guy who waits for more data and then refines the for cast? AccuWeather's has statistics that show they are more accurate then the National Weather Service but if you used the AccuWeather forecast then waited for the next National Weather Service update I bet National Weather Service would be more accurate.

I am surprised that this guy used the weather.com and not the National Weather Service for the actually temp for all his calculations. (It doesn't matter b/c I am sure weather.com is right from National Weather Service data). He did point out that AccuWeather is the only one who provides forecasts > 10 days in advance.

My preference for weather forecasts is:

National Weather Service
AccuWeather (easy to understand graphics and 2 week forecasts)
The Weather Underground (Years ago they were the1st to provided free access to hurricane computer models)

Re:Statistics don't lie Statisticians do!

[Overzeetop]

Actually, you're only considering the accuracy of the scientific forecast. That is essentially useless to the end user - what they (I) want is accuracy of the reported forecast. The difference? I don't care when the actual prediction was made, I want to know that when I look at the forecast, it is likely to be correct. A very accurate forecast that is only updated once every three days is not nearly as useful as a farily accurate forcast updated every ten minutes. The former would be the best by your yardstick, but wouldn't necessarily help determine if the likilihood of a hail storm was high for this afternoon as much as the latter.

Can we believe the forecasts?

[gavink42]

Personally, I take weather forecasts with a couple of grains of salt.

However, the last cold blast that came through Memphis was forcast almost a week ahead of time. Weather radar of the middle part of the country showed about 90% clear of storms. So, I had a hard time with that one.

To my surprise (and right on time), down came the blast of cold air. Soon after was the promised snow/ice.

It still seems like an inexact science... with a touch of art and a pinch of luck thrown in for good measure.

Re:Can we believe the forecasts?

[Daniel Dvorkin]

What I find funny is that the entire practice of weather prediction is based on a logically fallacy. They take the data from previous years and say, ok, last time conditions looked like this x happened, so we predict x will happen again. Anybody who's taken an introductory logic class knows that you can't correlation does not equal causation.

And anyone whose understanding of correlation goes beyond "an introductory logic class" knows that in fact, as long as you're very careful about what you're doing, you can in fact very often use observed correlations to make valid predictions.

There's this whole field of study called "statistics," see. Not the "X% of people surveyed believe Y" type of thing you hear on the news, but an actual science, grounded in rigorous mathematical theory and growing more sophisticated all the time at producing useful knowledge from mountains of data. People get PhD's in it and stuff. Really. Maybe you ought to read about it some time. Maybe even take a class.

Or perhaps you'd rather remain secure in your prejudices, repeating "correlation does not equal causation" like a mantra, snickering at people whose knowledge you choose not to understand.

Whom

[mdsolar]

A preposition is awkward to end a sentence with. But, "whom" is the word "on" is followed by.
--
Solar follows the rules for grammer. http://mdsolar.blogspot.com/2007/01/slashdot-users -selling-solar.html

Interpretation of the models is everything

[Aliks]

I am not a meteorologist, but I have worked with them a few times.

Generally the competing weather models will show a range of possible outcomes with various probabilities. You can average across all scenarios and come up with a 60% probability of rain, but the more days out you go more the scenarios diverge, so the less useful a single average will be.

Most people would not find it useful to hear that "there will be probably be thunder on Wednesday if it remains hot enough, but if it cools down on Tuesday then the thunderstorm will be off to the north somewhere"

Additionally, a lot of weather conditions are influenced by thin layers of cloud high up, so thin that precise measurements are critical so precise forecasts in one location more than 3 days out are difficult.

You can't fool mother nature

[TheHornedOne]

I work for Mother Nature; So I am really getting a kick out of most of these replies. Some of you guys are very good at making it sound like you know what you are talking about. But trust me.... You don't. I think you just want to make yourself sound smart, when in reality you don't know what you are talking about. This is how bad info gets passed around. If you dont know about the topic....Dont make yourself sound like you do. Cos some slashdotters believe anything they hear."

/wrong metasite
//slashies
//dont' kill me

Large metro areas

[Anonymous Coward]

Geographically large metro areas -- especially those with hills or large bodies of water -- make a weather forecaster's job all the more difficult. The chance of rain may be higher on one end of town, but it's difficult for a TV or radio announcer (or a newspaper spread, for that matter) to present the distinctions clearly and quickly.

Too long ago, when I was an undergraduate taking Meteorology, we visited the weather department in a Twin Cities (MN) television station. The anchor on duty was pretty blunt: if there's a 100% chance of rain on one end of town and a 10% chance on the other end, the broadcast would distill that as a 55% chance of rain. He argued that it was the best his department could offer given the commercial realities of limited airtime and the mandate to serve the entire metro area.