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Old 02-19-2009, 08:05 AM
NineToTheSky is offline
Join Date: Mar 2006
Location: UK
Posts: 2,774

How accurate are weather forecasts?

I'm going away in ten days (to celebrate my 25th wedding anniversary ), and I looked up the forecast for the weekend after next. On this site, it tells me temperatures, cloud cover, whether it will rain or not, and wind speed. Can they really tell to that degree of accuracy (or any degree) what the weather will be like? Don't forget that this is for the UK where the weather changes from day to day.

I've always meant to make a note of the forecast and then compare it to the actual weather, but I've never got round to it.

How far ahead can forecasts be relied upon? I know the very basic principles of weather forecasting, but what information do they need to compile a forecast for weeks or months away?
Old 02-19-2009, 03:20 PM
cainxinth is offline
Join Date: Dec 2000
Location: Philadelphia
Posts: 2,509
You could do a study and see how often a five day forecast is accurate but I don think it would do you much good. The chance of the weather pattern changing is based on so many variables that for all intents and purposes its unknowable. So it would be like counting how many times a coin flip results in heads or tails to predict which will come up next. The odds are always going to be 50 50 even if it came up heads a hundred times. Just like the chance that the weather will unexpectedly change on any given day is still unknowable even if the forecast was right 100 times before.
Old 02-19-2009, 03:50 PM is offline
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Join Date: Mar 2003
Location: Minneapolis, MN
Posts: 15,201
In general, weather forecasts are:
  • more reliable closer to the date (tomorrow's is more accurate than the 10-day forecast, etc.).
  • Certain times of the year have more stable weather patterns, so forecasts then are more reliable.
  • Locations near changes in terrain have more weather changes, so forecasts are less reliable. (Here in Minnesota, with neither oceans nor mountains for a thousand miles in any direction, our weather is fairly predictable. Britain, with oceans fairly close on all sides, is less so.)
These are only general guidelines, but may help somewhat.
Old 02-19-2009, 06:00 PM
Fubaya is offline
Join Date: Apr 2008
Posts: 3,898
I wouldn't put any faith in a 10 day forecast. I usually consider forecasts for the next 24-48 hours to be pretty accurate, say 75% or better, 3-4 days about 50/50, and anything beyond that to be speculation.

Predicting what a fluid will be doing next week is just too complex of a problem. The study of weather prediction led to chaos theory, so that should tell you something.
Old 02-19-2009, 06:37 PM
Markxxx is offline
Join Date: Apr 1999
Location: Chicago,IL
Posts: 14,962
Modern weather forecasts don't tell you what WILL happen, they tell you what is LIKELY to happen.

For instance, if Joe Weather says "there is a 90% chance of rain tomorrow," what he's saying is when conditions in the past were similar to the conditions that exist today, 90% of the time it's rained before.

So you see weather forecasters take models and feed them data and the computers quickly analyze the results. Then the weather person uses that to forecast.

The further out your trying to forecast the more variables come into play. The more variables, the greater the chance is you'll be wrong.

I recall WGN-TV's cheif meterologist Tom Skilling saying that forecasts were pretty accurate 3 days out and then each day after they get less and less accurate.

Indeed when I was younger I recall the weathermen to be wrong a lot, even the next day out. But now they are pretty much on the mark


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