100% Chance of Blog

As I was driving to work this morning, the weather forecast was that there was "a 60% chance of showers" today. This was followed by, "Right now, it's 49 degrees and raining." I can't verify the temperature, but I could certainly agree that it was raining.

That would mean that there was 100% chance of showers. Because, when you come right down to it, either it is raining -- 100% -- or not raining -- 0%.

But it got me wondering. How do they decide that there is a 30% or 60% or 95% chance of rain in a forecast?  Do they look at the radar, see rain somewhere else and then calculate the odds that those rain clouds will arrive in our area?
This works pretty well in the extreme short term: It's raining in Massapequa and the wind is blowing from the south. Since Farmingdale is just north of Massapequa, the odds are good the rain clouds will get there.
But when they are looking at rain in Tennessee, do they use some super-duper calculation to determine that there is a 30% or 70% chance it will make it to Long Island?

Does it have to do with what day of the week it is? Maybe someone has accessed the past 130 years of weather records. Perhaps they show that it has rained on more Tuesdays than Fridays, which would lead someone to forecast a higher chance of rain on a Tuesday, say 60%, than a Friday, say 35%.
Still, as they say in the investment business, past performance does not guarantee future results. The bottom line is that either it is raining or it is not raining. So a forecast, ultimately, comes down to a 50/50 chance: Either it will rain or it won't.

Kind of like this blog. Either I write an entry... or I don't.