What a forecast really says
Before you make your own forecast, learn how the professionals build one, and what every word and percentage in an Environment Canada forecast actually means.
In this lesson
- The “forecast funnel”: how forecasters work from the whole planet down to one town
- Two simple forecasts you can always beat, or lose to
- What “40 per cent chance of showers” really means
- Forecast words like “risk of”, “windy” and “high” and “low”
The forecast funnel
Forecasters don’t start with your town. They start big and work down, like pouring the weather through a funnel:
- The whole hemisphere: where are the big ridges and troughs in the jet stream, and how are they moving?
- The continent: where are the highs, lows and fronts on the surface map, and where are they heading?
- The region: what will the mountains do? Will the air come down the slopes as a chinook, or be pushed up them as upslope snow?
- Your town: local effects like the river valley, the coulees, and a cold, calm night under clear skies.
You’ll learn each step of the funnel in this track. Lesson F6 puts them together into one routine.
Two forecasts anyone can make
Forecasters measure their skill against two simple “no-skill” forecasts:
- Persistence: “Tomorrow will be like today.” It works surprisingly well on quiet days, and fails badly when a front or chinook arrives.
- Climatology: “Tomorrow will be average for the date.” It’s a safe guess a week or more ahead, but useless for a storm.
A good forecaster beats both. When you start making your own forecasts, score these two as well. If you can’t beat persistence, keep practising!
Reading Environment Canada’s forecast
Environment Canada writes its public forecasts using strict rules, so every word means something specific.
Chance of precipitation (the percentage)
- It’s the chance that measurable precipitation (at least 0.2 mm of rain or 0.2 cm of snow) falls at any one spot in the forecast area during that period.
- Precipitation is mentioned in the forecast when the chance is 30 per cent or more.
- The percentage is written out when it’s 30 to 70 per cent, in steps of 10. You’ll never see “50 per cent”. At 80 per cent or more, the forecast simply says the rain or snow will happen.
- So “40 per cent chance of showers” means: at your house, about 4 times in 10 a forecast like this one, you’d get at least a few drops that can be measured.
“Risk of”
Used with thunderstorms, hail, freezing rain and freezing drizzle when the chance is 30 or 40 per cent. The number isn’t written out, but that’s what it means.
Wind
- For today and tomorrow, wind is only mentioned when it’s expected to average 20 km/h or more.
- For days 3 to 7, “windy” means an average of 40 km/h or more, or gusts of 50 km/h or more, for at least six hours. In Lethbridge, you’ll see that word a lot.
High and low
For days 3 to 7, the high and low are the warmest and coldest temperatures expected, usually early afternoon and early morning. For today and tomorrow, when the temperature won’t follow the normal daily pattern (for example, rising overnight as a chinook arrives), the forecast describes that trend instead.
Our forecasts often say “temperature rising to plus 8 by morning” in winter. That’s a chinook arriving overnight. You’ll learn to spot one coming in Lessons F3 and F6.
Try it yourself
Open today’s Lethbridge forecast from Environment Canada. For each period, write down:
- Is there a percentage? What does it mean, in your own words?
- Is wind mentioned? What does that tell you about its speed?
- Is it a normal day (cold morning, warm afternoon), or is a trend described?
Then write your own persistence forecast for tomorrow from today’s readings on the live station. Tomorrow, see which one was closer.
🏫 For teachers and parents
- Time: 30 to 40 minutes, plus the next-day check.
- Big idea: scientists measure how good a prediction is by comparing it with simple baselines. Good for a discussion of probability and fair tests.
- Math link: collect a week of forecast percentages and what actually happened. Did it rain about 4 times in 10 on “40 per cent” days?
- Quick check: students explain “60 per cent chance of flurries” to a younger student in one sentence.
Key terms
- Forecast funnel
- Working from the large-scale pattern down to local details.
- Persistence forecast
- “Tomorrow will be like today.”
- Climatology forecast
- “Tomorrow will be average for the date.”
- Chance of precipitation
- The chance of at least 0.2 mm of rain or 0.2 cm of snow at any one spot in the area.
Check your understanding
1. Where does a forecaster start the “forecast funnel”?
Forecasters start with the large-scale pattern, because it steers everything smaller.
2. The forecast says “Chance of showers, 40 per cent.” What does that mean?
It’s the chance of at least 0.2 mm falling at any given point in the area during that period.
3. Which percentage will you never see in an Environment Canada forecast?
Environment Canada’s rules don’t allow 50 per cent.
4. When is a “persistence” forecast most likely to be wrong?
Persistence assumes nothing changes, so it fails when the weather changes.
Source: Environment and Climate Change Canada, Guide to public weather forecasts: weather elements.
Weather School is for learning and enjoyment. It is not official guidance and may contain errors. For watches and warnings, always use Environment Canada (WeatherCAN app or weather.gc.ca).