The Rain Dilemma? Vancouver and TO

Both Toronto and Vancouver will have a brush with showers tomorrow, but this blog will highlight why the risk is fairly minimal and not significant (As of writing TO's risk has increased).  

But first I want to discuss a little bit about what is P.O.P really means

Probability of precipitation is one of the most misunderstood really terms in existence.

What does 40% P.O.P really mean? 

  • 40 percent chance of seeing precipitation during the day?
  • 40 percent chance of the whole forecast area seeing precipitation? 
  • 40 percent chance of any one point in the forecast area seeing precipitation?

I think the general public as a whole misinterprets the P.O.P in slightly different ways so I'm going to attempt to set the record straight. 


At it's core, the P.O.P is a mathematical equation. Don't fret, it's quite simple really.

The National Weather Service uses the following formula:


  • Area of Coverage = forecast area or area of concern
  • Confidence = forecaster's confidence in the given forecast or situation 
 Graphic by Brad Panovitch 

Graphic by Brad Panovitch 

P.O.P's are typically given with the intention of the person staying in one position, so if you drive all through the forecast area to run a view chores you will increase the probability of precipitation -- it's as simple as that.

Toronto for tomorrow?

Looking at the SREF's the POP's have increased from 20 to 60 percent for late afternoon showers, but confidence right now is low unfortunately until tonight's model runs come in. Just a fact of the business, folks.

So a 40% chance of showers for the GTA is reasonable right now, 60% in St. Catherines.