Image Credit: ADOBE STOCK

If you’re not a meteorologist you might reasonably assume a 40 per cent chance of precipitation in, say, Kelowna means there’s a 40 per cent chance it’s going to rain throughout the city. You’d be wrong though.

It doesn’t mean there’s a certain chance of rain in the entire city, it’s the probability that there’s going to be rain somewhere within the area.

The probability of precipitation is calculated by Environment Canada with a complex computer model that uses the conditions in the atmosphere to predict a likelihood of rain for a representative point, or a typical spot, within a region with oversight by a meteorologist.

Environment Canada meteorologist Brian Proctor gave iNFOnews.ca this technical definition for a 40 per cent chance of precipitation.

“Statistically what it means is four times out of 10, out of the identical synoptic situation, a representative point within Kelowna would see measurable precipitation,” he said.

Proctor said the way forecasts are presented is a balancing act between the math and jargon meteorologists work with and giving people information that’s going to make sense without a bachelor’s degree in atmospheric science.

That’s why Environment Canada won’t say there is a 50 per cent chance of precipitation.

“We've done some public outreach, it came out to the fact that the public really didn't understand what 50 per cent meant,” he said. “They wanted a better sort of idea if it's going to be a better chance of seeing precipitation or a lower chance of seeing precipitation, which is why we opted for 30, 40 or sort of 60 and above values to a large degree.”

Calculating the probability of precipitation in the Okanagan is naturally trickier than in flatter regions like the prairies since the terrain plays a huge role in whether a particular spot is going to see rain.

“If you think about all the terrain we see and oftentimes our communities in the valley bottoms in the Interior. So we could say, 'yeah, there's a chance of seeing maybe a 40 per cent chance of showers or something.' But at the higher elevations on the ridge lines is probably a much greater chance of seeing measurable precipitation over the Coquihalla or coming up across going over towards the cusp from Vernon,” he said.

Proctor said that Environment Canada uses several computer models to try to come up with the most accurate forecast.

The models aren’t perfect and can’t predict how the weather is going to be a bit different in every pocket of the valley, so the chance of precipitation calculation is the best way to tell people how likely it is that they’ll get rained on, even though it isn’t saying how likely it is for the whole city to get some rain.

He said something to look out for is the fact that some weather apps just use a computer model to give a chance of precipitation and don’t have a meteorologist to interpret and analyze the data.

Proctor said when some computer programs look further ahead into the future they have to fill in some gaps in the calculations that predict rain and often the program will skew the prediction to say that it’s more likely to rain.

“Oftentimes if people are using a model, just driven by one model, you'll see a bias towards heavier precipitation the further you go out in time. And we know that's incorrect,” he said.

In case you're now wondering, there's a 40 per cent chance of showers in Kamloops, Kelowna and Vernon tomorrow afternoon, Aug. 7, while there is a 60 per cent chance in Penticton.

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