Weather Banter - View Single Post

Dennis M. Rodgers · April 26th 05, 12:00 AM posted to sci.geo.meteorology

wrote:
If the weather forcast says we have a 50% chance of rain today, and a
40% chance of rain tomorrow, what is the chance that we will have rain
during that 48 hour period? And what is the probability that we will
have rain each day during that 48 hour period?

I've tried:

P(A||B) = A + B - A * B; # Is this approach right or wrong?

P(A&&B) = (A + B) / 2;

Please, help so that I may better understand what our weather man is
really saying!

If the forecast is 50% today and 40% tomorrow, then that's
what the forecast is. To then subject a forecast to some
statistical post-processing to obtain some less useful
homogenized number seems like a silly waste of time. This
exercise simply dilutes the original forecast with 24 hour
resolution to a less precise forecast with 48 hour
resolution. What's the purpose?

If you want to "better understand what our weather man is
really saying!" than don't attempt to combine consecutive
probabilistic forecasts into something the forecaster didn't
forecast.

Disclaimer: Opinions expressed are my own.