What exactly does 30% chance of a shower mean?

Let’s break it down!

Three weather ideas you have to understand are:
-Forecast area
-Forecast period
-Measurable precipitation

FORECAST AREA is the region that the forecast is for. For example, if I’m producing a Kelowna forecast, then the forecast will be for Kelowna, Lake Country, West Kelowna, and Peachland. This may vary for difference forecast sources - keep in mind that forecasts do differ depending on where you get your forecast! This is very important - different meteorologists and computers may not think the same way. (Even though if we did it would make our lives much easier…)

FORECAST PERIOD is usually a 10 to 24 hour time frame. Once again, this may vary for different forecast sources.

MEASURABLE PRECIPITATION is 0.2mm of rain, or 0.2cm of snow.

The chance of precipitation is the chance that measurable precipitation will fall on anywhere within the forecast area during the forecast period.

Let’s say today’s forecast for Kelowna is a 40% chance of precipitation.

This means from 5am Thursday to 5pm, there is a 4 out of 10 chance that anywhere between Peachland and Lake Country will get at least 0.2mm of rain, or 0.2cm of snow.

Personally, I often prefer using the terms slight, moderate or high when describing precipitation risk instead of using a percentage because forecast confidence can vary depending on the weather pattern!

There are times when it’s easier to predict the weather, and then there are times when I lose sleep at night because the pattern is too complex to know exactly where the rain or snow will fall!