How accurate are daily weather forecasts?

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How can meteorologists predict weather forecasts with accuracy? After how many days out do forecasts lose accuracy? What factors make weather forecasts hard to predict?

In: Earth Science
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It’s partially done by looking at historical norms for the region (e.g. you know it’s more likely to rain when the wind is from a certain direction etc), and partially done by using big computers to analyse weather patterns and predict how they’ll change. Accuracy for short-term forecasts isn’t really guaranteed after three or four days, though, and that’s because the weather is a chaotic system–that doesn’t mean it’s entirely impossible to predict, but that the slightest error in your starting conditions will rapidly diverge from the actual weather.

To give an example: let’s say you had a magic device that could measure the wind speed, temperature and air pressure at every point one metre apart in the atmosphere. You feed those values into a perfect supercomputer that can exactly simulate every interaction there is and let it run. The resulting prediction will *still* be totally wrong after a month, because you didn’t measure the bits in between those 1m points and have no idea what was happening there!

Pretty accurate.

Meteorological predictions are made on data collected from decades even centuries of research and with the help of extremely intelligent super computers.

Inspite of this there are some errors which can happen & do happen all the time.

By rough estimate, you’ll get right predictions 22-25 days a month.

3 days in general the time at which forecasts break down, due to the inherent chaotic nature of weather by the time you get to 3 days the random nature of the system tends to reduce the accuracy to a level too low to be significantly useful.

>How

As mentioned, using lots of data points (and an increased understanding of their cause and effect) fed to computers. But also note that answers aren’t given explicitly but as percentages. So imagine feeding data 100 times through your computer, which also accounts for chaos/randomness. This means you’ll get 100 different forecasts. If 60 of them predict rain in a certain area and 40 don’t, you’ll have a 60% chance of rain. (How they handle 50/50 may change as viewers want answers, not a coin flip.)

>After how many days

Also as mentioned, about 3 days

>What factors

“Weather also has two additional properties that make forecasting even more difficult. First, weather is nonlinear, meaning that it abides by exponential rather than by arithmetic relationships. Second, it’s dynamic — its behavior at one point in time influences its behavior in the future.” -*The Signal and the Noise*

When dealing with exponential relationships, time is only compounding this problem, hence why 3 days has been mentioned.

If the question is whether it will rain tomorrow, weather forecasts are going to be pretty reliable. But beyond 48 hours predictions are significantly less accurate.