MONTGOMERY, Ala. (WSFA) - It’s one of the largest sources of confusion surrounding any weather forecast: when we give you a forecast and tell you there is a chance of rain, what does that actually mean? The problem with explaining chance of rain is that it can be very subjective...
There are a lot of theories out there, but here is how both WSFA 12 News and the National Weather Service define it. A rain chance - also known as a PoP (probability of precipitation) - is often expressed as the coverage of showers and storms in a give day and describes the chance of precipitation occurring at any point in a selected area.
So, on a day where our forecast calls for 50% chance of rain, it means we expect that 50% (or half of our viewing area, which is basically all of central and south Alabama) will see some kind of wet weather.
Some confuse the percentage with the amount of time it could potentially rain in a given day or with the odds it will rain at one particular location on the map... while both of those theories could potentially make sense, that’s not what exactly what forecast is calling for.
At no time does a percentage given in a forecast tell you how long it will rain, how much rain is expected, what time the rain will arrive or what the potential impacts are to you. We as meteorologist will dive deeper into that when we give a forecast on air, online or through our First Alert weather app, so it’s important to pay attention to more than just a percentage given on the 7 day forecast.
Bottom line... the correct way to interpret the forecast is as follows: if there is a 50 percent chance of rain in the forecast, then we are calling for half of our viewing area to see some kind of wet weather at any given point throughout the day/night we are forecasting for. As that number goes up or does, so does the coverage area we expected to see rain.