Atomic Shrimp - Clouds Of Suspicion - An Audit Of The 5 Day Weather Forecast

Clouds Of Suspicion - An Audit Of The 5 Day Weather Forecast

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5-day weather forecasts are available from a number of sources now - but how accurate are they, really?

My reason for asking is this:

If I make any kind of prognostication about something supposed to happen 5 days hence, and I have unlimited opportunity to change my prediction, right up until the morning of the day, doesn't that grant me licence to say whatever I like now, knowing I can bother with the trifling business of being accurate when we get closer to zero hour and proper prediction is that much easier

An Audit

So let's test the value of the 5 day forecast. What I intend to do is record the weather forecast for each day, as it progresses toward the present, through the 5-day forecast on the Met Office website.

I'll also be recording the actual weather outcome for the same local area.

As the data builds, we'll hopefully be able to see two things:

How often, and by how much, the 5 day weather forecast changes as it progresses through to the present
How accurately and how reliably can the weather be predicted, from 5 days out

Metrics

These are the various different weather types as defined by the Met Office - I've arranged them here into a sort of periodic table of weather - and it's this grid I'll use to score the forecasts on reliablility and accuracy.

Each day's forecast will be given a consistency penalty as follows:

One point for each time it moves a column from the previously forecast weather for the day
Two points for each time it moves a row from the previously forecast weather for the day
Four points if the final forecast for the day is different from the initial forecast (this penalty avoided if the forecast reverts to the original one)
Eight points if the forecast on the day is different from all of the previous forecasts

Each day's forecast will be given an accuracy penalty as follows:

One point for each column away from the actual observed local weather on the day
Two points for each row away from the actual observed local weather on the day
Four points if the actual observed local weather on the day is different from the initial forecast
Eight points if the Actual observed weather on the day is different from all of the forecasts

Location

I'll be monitoring two geographical points - Portsmouth for weekday forecasts and Southampton for weekend ones (because that's work/home - so I can make the observations on the day).

So, without further ado, let's start measuring...

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