SOUTHERN COLORADO — Most of the time when we talk about rain or snow in weather forecasts, we use percentages and chances when telling you what to expect. But these percentages are frequently misunderstood. So, let’s talk about what probability means with weather forecasting.
Let’s start with what it’s not. Let’s imagine a forecast has a thirty percent chance of rain. Chances don't relate to physical area; it doesn't mean 30 percent of the area of the city will be getting rain at any given time. It’s also a myth that you’ll see rain or snow for 30 percent of the time forecasted.
Instead, think of it like playing the odds in Vegas. A 30 percent chance of rain during the afternoon in Colorado Springs means that there is a three in ten probability of any point, such as your house, in the Springs getting measurable rainfall at some time during the afternoon.
Let’s use Colorado Springs and Pueblo as examples. A 30 percent chance of rain during the afternoon in Colorado Springs means that there is a three in ten chance of any point in the Springs getting measurable rainfall at some time during the afternoon. If Pueblo has a 40 percent chance, any house in the city of Pueblo has, equally, a four in ten chance of getting measurable precipitation.
Here’s where things get a bit more complicated. The way meteorologists calculate these chances is by multiplying the amount of area we expect to see rain or snow with our confidence in seeing it. For example, if I expect 50 percent of Colorado Springs to see afternoon showers, and I’m 60 percent confident in the weather system being able to produce the showers in the first place, my forecast would show a 30 percent chance of showers. This is why probability isn’t related to area; it’s related to forecast confidence over a given area.
These chances apply to the entire time frame of the forecast. In other words, an 80 percent chance of rain on a given day doesn’t mean a washout; it simply means there’s a very high chance of seeing rain at some point in your day. Of course, we always try to provide much more detail on what we expect, including how long storms will last and how much snow will fall if we’re expecting snow, but this is the basic idea.
The biggest thing to know about rain and snow chances is that they do not have anything to do with area or duration. Those aspects of the forecast are things we talk about using other tools, like Futurecast, and aren’t things automated forecast tools will be as solid at telling you.
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