why does it always rain when there is a 30% chance of precipitation?

Answer 1

I wish it would rain. I've seen days when there was a 50% chance of rain and no rain. All it means is that some where near you you have a 3 in 10 chance of seeing some rain.

Answer 2

Seems like another case of selective memory. For every 10 times on the average an unbiased forecaster (including a computer model's numerical forecast) forecasts a 30% chance of precipitation, a measurable amount will occur 3 times. Same thing as rolling a 10-sided dice numbered 1-10 - 3 times of 10 (on the average) you'll get a 1, 2, or 3. Do the experiment with the dice and you'll see how you can go on streaks - maybe 11 rolls without a 1, 2, or 3 - or 4 straight of them. The same thing can happen with weather forecasts even if they are unbiased. Some forecasters have biases such that when they say a 30% chance it verifies 50% or 20% of the time.

There also is some truth to the idea that forecasts are made for an area - whether it be several counties which comprise an NWS zone or a metropolitan area which a broadcaster forecasts for. Yet it is not correct to say that some places in that area always get the rain and some don't. I have been taking daily precipitation readings 3 miles from an airport (KMPO) for 7 years and can vouche that sometimes they get more rain and sometimes we do, but over the course of a season that tends to average and we get about the same amount. Similarly, scattered showers miss some places and hit others - but they tend to be different places each time. Long-term precipitation totals will indicate there can be locations in a region which are more favorable spots and average more precipitation than nearby areas, but the difference is rarely large. If you do happen to be in one of those spots, you may have to assume a forecast is a slight underestimate or overestimate for your particular spot.

Nor is it correct to say a probability represents the amount of area in a region which will get rain. The probability is a combination of that and the fact that the entire region may be hit or missed any particular time. E.g., a storm tracks differently than expected, a front passes sooner than expected, etc. - so no place in the region gets rain despite being given a 60% chance.

Answer 3

30% chance of rain means that 30% of the viewing area will have rain. You probably live in that 30% that always get it.

Answer 4

The last poster was correct.

That 30% is COVERAGE.

In other words..... It`s not, "There is a 30% chance of rain here."
Its, "30% of the forecast area will see rain."


Some other stations may do it differently, but that's how we do it here.

Weather_Wise911

Answer 5

It doesn't, unless you're in the 30% coverage area that's gonna get some rain.

Answer 6

It DOES, but that means that there is only a 30% chance in that specific spot and place.

Answer 7

well i hear that its a posible chance of 30 percent always rains now dont get mad at me its just what i hear

Answer 8

Well it doesnt, but it certainly does seem so sometimes doesnt it? Please keep in mind that meteorologists dont really forecast the weather anymore. they rely on computer models.... accurate records are kept, and the computer tells them that on average, the last time these conditions existed this is what happened next. Welcome to computer model forecasting..... sigh.... all this technology, and we cant rely on the person interpreting it.

Answer 9

i would like to know where you live we had a 100% chance last night and it didn't rain...80% today still nothing....60% tomorrow ....::Sigh:: i doubt it will happen

Answer 10

Because the weather person you're watching doesnt like to be wrong.