What does it mean when "X percent chance of rain" is predicted? An Update??

Cecil's Columns/Staff Reports
1

I was just listening to my local radio station here in Houston, TX, where they do “Dumb question amnesty” at the end of each month. A caller asked the question “What does 30% chance rain really mean? Is it a probability or is it that 30% of the area will get some amount of rain?”

The radio station called our local channel 11 meteorologist, and he said that, in no unclear terms, 30% chance means that meteorologists believe 30% of the area will recieve some measurable amount of rain, and that it did NOT refer to an overall probability…

sigh So what’s the SD? Have they updated what X chance of rain means since the original column was written, or did our local guy screw it up?

I saw this column was discussed in '04, but have things changed since then? Or has there been some massive misunderstanding?

2

http://www.straightdope.com/columns/read/854/what-does-it-mean-when-x-percent-chance-of-rain-is-predicted <— link to the original column.

3

It means that under similar meteorological conditions in the past, it rained 30% of the time.

Another thing is that forecasts are only really useful for about 3 days out; after that, there’s too much variation to effectively forecast.

So if it says you have a 50% chance of rain tomorrow, it’s as likely to rain as not. If it says you have a 50% chance 5 days from now, I wouldn’t make plans one way or the other based on that.

4

But the time factor is still not covered. Does a 50% chance of rain from 2pm to 3pm mean that there is a 50% chance that rain will occur at some moment during that hour, or does it mean that at every moment of the hour there will be a 50% chance of rain, i.e., that an approximate total of 30 minutes of rain can be anticipated?

5

FAQ - NOAA says: