Does a 20% chance for rain leave you 100% confused?

One of the most confusing parts of our weather forecast deals with the use of rain and snow chances. We express those as percentages… 20% chance for rain, 50% chance for snow, etc. People are often misled by these numbers as they think these are the actual chances for rain or snow. This is incorrect.

In meteorology, the percentage for precipitation refers to how much area that precipitation will cover… not whether it will occur. In other words, if we say there is a 50% chance for rain, what we are really saying is that there is a 100% chance for rain, but the COVERAGE of that rainfall will be 50% of a given area.

In the case of our weather forecasts, the NewsChannel 8 viewing area covers 21 counties and extends from near Rochester in the west, to Chippewa Falls in the north, to Mauston in the east, and Prairie du Chien in the south. Imagine the viewing area broken up into 100 equal squares. A forecast that calls for a 50% chance of rain means that it will rain, but the rain would only cover 50 of those squares. The other 50 would remain dry.

Of course… actually forecasting which of those 100 squares actually see the rain and at what time are the million dollar questions. For short term forecasts of one to six hours, we do pretty good. Even out to 24 hours, we still bat better than average. However… beyond 48 hours, the accuracy slips quite a bit. Remember… compared to chemistry, physics, and mathematics, meteorology is a very young science. We’ve only had satellites for 40 years, and radar technology since World War II. Don’t worry, research meteorologists are hard on the case working to improve those numbers. In addition, new computer technology is taking the science forward by leaps and bounds. While it will be years down the road before we can tell you that it will rain one-quarter inch in your backyard at 3:06pm Friday afternoon, we are getting closer!

Talk to you all next week!