Originally Posted by Erick L
40% means that there's a 40% chance of showers at any point in the area covered.
I usually check the weather channel, sometimes Environment Canada and even NavCanada for METAR and TAFs (for aviation, made by Environment Canada). It's pretty accurate for the commute since I work at the weather station.
Or to say it differently, 40% chance of showers means a 100% chance of showers in 40% of the coverage area.