Explaining precipitation chances

Posted at 9:44 AM, Dec 13, 2021
and last updated 2021-12-13 11:44:04-05

From your television screen to your StormTracker weather app, I am sure you're all quite familiar with rain or snow percentages and what they mean. However, the true meaning behind these values may come as a shock to you. I would like to preface that this is still controversial for meteorologists, and many meteorologists use different methods when calculating precipitation chances.

The National Weather Service utilized this formula to calculate your area's chance for rain or what we call a PoP: P = C x A . In simple terms, the probability of precipitation (rain/snow chance) is the meteorologist's confidence times the percentage of the forecast area expected to see precipitation.

Let's walk through how we come to these percentages. If I am very confident parts of central Montana will see rain, I would give my forecast confidence a 90%. However, the rain showers will be scattered in nature so I forecast about 40% of the forecast area will see rain. We change that number to a decimal, 0.4. This leaves us with 0.36 or 36% chance of precipitation, which we would round to 40%. Therefore, the values you often see us using are not the percent chance you will see rain in Great Falls.

We cover 15 counties in the Great Falls forecast area, from Glacier County to Daniels County and 3 counties in Helena's forecast area. On a given day, Great Falls may see a few inches of snow while Havre is dry. Our forecast of say a 60% chance of snow would still verify for the forecast area.

Some meteorologists have scrapped using percentages altogether and designated words to describe precipitation chances. The StormTracker weather team will often use these terms as well. An isolated chance, 10-30%, will likely only impact a few towns, if that. A scattered chance, 40-50%, will impact several towns while some still stay dry. Numerous rain or snow showers, 60-70%, will impact most towns in the viewing area. A widespread chance, 80-100%, would likely be a line of thunderstorms or a snowstorm impacting the entire viewing area.

If this is news to you, you are not alone. Meteorologists have still not come to a firm agreement as to whether or not this is the proper method.