Deciphering the 7-day forecast: what it means in real life

Here is a little secret in the weather world — not all days in the seven-day forecast are created equal.

There! The cat’s out of the bag.

It is as simple as this: as we go further into the future, confidence drops in the forecast by those putting the forecast together.

So let’s break down how I approach and look at the seven-day and how you should too.

Since I love charts, here is one that can give you a high level look at confidence and likelihood for the forecast to change. This may seem obvious, but if it is, why do we still stick with the seven-day?

So at what point do we start to take the forecast as fairly set in stone? When do we hit a point when the variability in the numbers, the type of precipitation and how much, start to get dialed in?

I say on fairly straight-forward storm systems — not those lovely rain-snow line around Steeles-type of systems — it is about Day 3.

How I view the seven-day forecast:

Day 1: high confidence

The Gospel! This is when hour-by-hour forecasts can be made with confidence and often success. Timing precipitation duration, amounts and temperatures are going to be pretty darn close.

Confidence level: 95-100 per cent.

*Disclaimer: Sometimes things just don’t go according to plan, and that is why our friends at 680 NEWS have the weather guarantee (enter to win here). It wouldn’t be fun for listeners if they got it right every single day. Although meteorologist Jill Taylor would have a big ol’ grin on her face!

Day 2: very good idea what will happen

From here we can start forming a plan of action. Timing may still be adjusted a few hours earlier or later for precipitation, but we should already have a general sense of the severity and amounts. For temperatures it will hold within reason barring a dramatic shift.

Confidence level: 92-97 per cent.

Day 3: pretty good idea what will happen

Confidence will build for a forecast if a weather-maker has appeared in it in the previous days. If a new player arrives it will be taken more seriously that it will likely happen in some capacity.

Confidence level: 90-95 per cent.

Day 4: trend and precipitation type

At Day 4 we are less likely to see wild temperature swings in the forecast over the next couple of days. We also get a pretty clear picture of what type, if any, of precipitation we may face. It is treated sort of as a swing day in the forecast, but it is still subject to change.

Confidence level: 80-90 per cent.

Day 5: likely to change, but helps us gauge impact

Storm’s on the way? Well maybe, but we start to get an idea of big storm potential or just a run of the mill type of storm system. Temperatures could still fluctuate — even by more than a few degrees.

Confidence level: 70-75 per cent.

Day 6: I see you

So the weather maker showed up yesterday and now it is something that we need to take seriously, so it is not just a one off. Confidence is still low, but it could happen. We pay more attention to the temperature trend than the actual number.

Confidence level: 65-70 per cent

Day 7: Oh hey, nice to meet you

It is our introduction to the potential weather that day. We take a look at it and the temperature trend but don’t adjust our plans just yet. That storm system is likely still out in the middle of the Pacific and the forecast can swing wildly. It could go from storm system to sunny the next day. The forecast pattern is still pretty uncertain.

Confidence level: 60-65 per cent.

Here is hoping that this can help serve as a guide to decipher the seven-day forecast and help you plan for those days when you have little Johnny’s outdoor birthday party or that bicycle wine tour booked in Niagara, and when you need to pull the chute on Plan B.