This is kind of the purpose of the "50% chance of rain" things. The process is called calibration and is usually done with linear regression, and it means that in historical forecasts, the actual outcome was rain 50% of the time. Surface precip is notoriously hard to predict, so this is what we've got right now.
But they should publish that... And then compare that figure to their competitors... To demonstrate to their users that their service is actually better, not just has a shinier UI...
