07/13/2025
Ever wonder why meteorologists always use words like "chance" and "scattered"? To understand why we often speak in this "probabilistic" space, you first need to understand a little bit on how weather modeling and forecasting works, and how we forecast the weather in the modern era.
Long story short, the forecast is quite complicated, and if we were to just say the most likely temperature, precipitation, wind, etc... each day, we'd be hiding a lot of the details. If we wanted to have a perfect forecast, we'd need perfect observations of every square inch of earth's surface and every layer of the atmosphere out to outer space. We have a lot of tools that help us get close, but unfortunately, modeling every air molecule is computationally impossible at this day in age. So what do we have? We have hundreds of different models that try to get close to modeling most of the atmosphere, running daily. All of these come together to make an "ensemble". This inherently gives us a range of "possible outcomes", and very rarely are all outcomes the same (see the image below for outcomes for the next 7 days). This becomes important when a degree Fahrenheit of difference could make the difference between storms or no storms, and a few knots of wind could make the difference of a tornado or not. These ensembles tell us stories, and it us up to us as meteorologists to go through the data and use these our expertise to tell these stories for you.
Maybe one day, all models show the high temperature in Pittsburgh above 90F. In this case, we'd be "very confident" of a high in Pittsburgh >90F. But lets take the more realistic scenario where some models are below 90F and some are above 90F. Well what does this mean? Today, for example, if we stay less cloudy temperatures may push into the low 90s in Pittsburgh, but if we have more cloud cover and rain, highs may be as low as the mid-80s. But lets say 70% of the models have us in the 80s, while 30% have us in the 90s. We could always say that "it will be in the upper 80s today" but in reality, there's a chance it could be in the 90s. If that's important to YOU, it might be important to include that detail in the forecast.
Maybe temperature is not your thing, so lets heighten the stakes a bit with another example. Most days of the year if we were forecasting whether there was going to be a tornado or not, we would almost always forecast that there there would not be a tornado. By doing this, we can hide a lot of the forecast detail; wouldn't you want to know if there was even a 5% chance of a tornado near your house? It's also important to communicate what that 5% chance of a tornado "means". It could be everything from "there will be storms today, but any given storm has a 5% chance of a tornado" to "there only a 5% chance of storms today, but if one forms, it will likely produce a tornado". This is important information to convey to not just you, but also to emergency managers and first responders working to keep us safe.
Everyday, it is out job to work through the data using our expertise, so you don't have to. Because our mission is saving lives, we will always strive to communicate high-stakes scenarios, even if there is a low probability of them occurring. If you ever have any questions on the forecast, fell free to chime in in the comments and we can try out best to help you out. The marginal chance of severe wind still lingers near and east of Pittsburgh today (10% chance of severe winds within 25miles of you), and a marginal-slight risk of flash flooding (10%-20% chance highest east of Pittsburgh).
