When we lived in WA state, if the chance of rain was 30%, we got rain. Now in IL, ut seems like no matter the % it doesn't rain.
I checked with the three weather providers and found that they don’t all handle the probability of precipitation, known as POP, the same. “Technically, the POP is the probability that a measurable amount of precipitation (at least 0.01 inch of rain or water equivalent) falls during a given time period at the specific rain gauge that is at the ‘official’ verification site,” said Mike Steinberg, a meteorologist and senior vice president at AccuWeather.com.
The Weather Channel uses a similar definition, only it counts any precipitation, not just water amounting to 0.01 inch or more, and it includes precipitation three hours before or after the forecast time, according to Bruce Rose, a vice president and principal scientist for weather systems for the Weather Channel. So if there’s a certainty of a slight drizzle at 2 p.m., that leads to a forecast of 100% probability of precipitation at noon. “We’ve experimented with more objective methods … but found that, from a consumer-centric viewpoint, the most well received version was something of a worst-case (i.e. highest or most-significant-event) approach,” Rose said.
The National Weather Service, meanwhile, has the same minimum standards for precipitation as AccuWeather, but displays the probability of rain “at any given point in the forecast area,” rather than at the specific point where the gauge is located, according to spokesman Christopher Vaccaro. (So if there’s a 50% chance that it will rain somewhere in the area, and, if it does rain, 80% of the area is likely to be covered, the probability of precipitation is 40%.)(http://blogs.wsj.com/numbersguy/deciphering-a-20-chance-of-rain-470/)
Probability is Just a Guide
Probability does not tell us exactly what will happen, it is just a guide(http://www.mathsisfun.com/data/probability.html)
probability, in mathematics, assignment of a number as a measure of the "chance" that a given event will occur. There are certain important restrictions on such a probability measure. In any experiment there are certain possible outcomes; the set of all possible outcomes is called the sample space of the experiment. To each element of the sample space (i.e., to each possible outcome) is assigned a probability measure between 0 and 1 inclusive (0 is sometimes described as corresponding to impossibility, 1 to certainty). Furthermore, the sum of the probability measures in the sample space must be 1. (http://encyclopedia2.thefreedictionary.com/probability)
I would have thought that the various probabilities of rain would be the same! Silly me!
So if the % of rain is a certain % then it may rain or not!