A recent TikTok video has been making the rounds recently talking about rain chances, and it’s not entirely accurate about what exactly goes into a chance for rain.
So, our chance of rain takes two things into account: confidence, and area.
Your daily rain chance (the technical term is Probability of Precipitation), is a combination in the forecaster’s confidence rain will occur, and how much of the area being forecasted for will see rain. So, quite often, confidence is at 100%, and this equation is dependent solely on area, but that’s not always the case.
For those of you who pay close attention to the 7 day forecast every day, you’ll often notice rain chances can often trend up quite a bit from days 5-7 to the day of the event. That’s the “confidence” part of the equation. Rain chances go up the closer we get to the event, because forecast confidence increases the closer we get to the event too!
Some of you may be noticing an inherent problem with this rain chance though. It doesn’t take into account when the rain will occur in the day, or how much rain will fall. If a tenth of an inch of rain falls at 11:30 p.m., odds are it’s not as impactful to most of you as two and a half inches of rain at 3:30 in the afternoon is. So, a little dirty secret is, a lot of meteorologists try to take into account how that rain will impact you, and put that into the rain chance too (source). There have absolutely been times I increased or reduced a rain chance based on the impact I thought the rain would have on you, because at the end of the day, that’s what is important.
So, what does that rain chance mean to you? Well, it’s a little complicated, as you can see. I’d advise using the rain chance as a quick way to determine if rain will be possible on a day, and then dig into the forecast from there to see what kind of impacts that rain could have on you.
There’s a lot of confusion surrounding rain chances. You can check those rain chances, or check the forecast discussion for a little context surrounding the rain chances, using Live Alert 19.