How we choose weather alert colours
Get your weather forecasts and alerts: Use our mobile app WeatherCAN or visit our Weather Information map. Stay safe during severe weather with guidance from Public Safety Canada.
Weather forecast confidence and impact
We have recently added colours to our weather alerts (Warnings, Advisories and Watches). These colours (yellow, orange, red) reflect the risk of a weather hazard. To choose the colour we look at two types of information. We consider:
- how confident we are the weather will happen
- the possible impact that weather may have on you
Alert Colour Matrix
We use an Alert Colour Matrix to help us choose the colours for our alerts. It has a vertical and horizontal axis, to show the different levels of forecast confidence and weather impact. The confidence we have that the weather will happen goes vertically up the side of the matrix from low to very high:
- low forecast confidence
- moderate forecast confidence
- high forecast confidence
- very high forecast confidence
The possible weather impacts are horizontal along the bottom from low to extreme:
- low weather impact
- moderate weather impact
- high weather impact
- extreme weather impact
Where our level of confidence meets our level of impact on the matrix, we find the colour to describe that alert.
Figure 1: Alert Colour Matrix
Long description
The Alert Colour Matrix used by forecasters to guide the selection of an alert colour. The vertical y-axis shows confidence levels: Low, Moderate, High, and Very High. The horizontal x-axis shows impact levels: Low, Moderate, High, and Extreme. Various combinations of impact/confidence result in yellow, orange, and red.
Threshold to choose an alert colour
For each alert, there can be different combinations of weather impact and forecast confidence.
No alert:
- Impact is expected to be low
- Impact is expected to be moderate and confidence is low or moderate
Yellow alert:
- Impact is moderate and confidence is high or very high
- Impact is high and confidence is low or moderate
- Impact is extreme and confidence is low
Orange alert:
- Impact is high and confidence is high or very high
- Impact is extreme and confidence is moderate or high
Red alert:
- Impact is extreme and confidence is very high
How the colours show weather impact
Let’s say a thunderstorm is on its way. Assume the first part of the information ‘forecast confidence’, is very high that it will happen (we are certain it’s coming). The alert colour will change depending on the possible impact of the storm. If we expect the possible impacts will be:
- moderate, we would issue a yellow alert
- high, we would issue an orange alert
- extreme, we would issue a red alert
Levels of weather impact
We only issue weather alerts for three levels of weather impact: moderate, high, and extreme.
Our Impact Guides help us decide what impact level we use to describe a weather event. This in turn helps us choose the alert colour. Not all the potential impacts listed in the guides need to occur during a particular weather event for that impact level to apply.
Weather impact levels describe how the weather will affect:
- travel delays
- utilities disruption
- damage to property
- danger to people's health
- time for society to recover
For low-impact weather, when day-to-day activities are not affected, we do not send out a weather alert. Instead, we may create a MetNote or a Special Weather Statement.
Important safety information about our Impact Guides
Our Impact Guides are examples of the range of primary impacts the average person in the forecast area may expect. They do not include secondary impacts like business or school closures.
Factors that influence impact
The same weather event can have different impact levels depending on various considerations and situations. For example, some factors that affect weather impacts are (this list is not exhaustive):
- location
- existing conditions (for example, significant snow on the ground before a snowfall)
- timing
- the vulnerability of the area and/or people
Impact carries through all levels
Any impacts described at lower levels in the guides (for example, moderate) also apply to higher levels (for example, extreme), even if they are not mentioned directly.
Levels of forecast confidence
How do we decide our level of forecast confidence (how sure we are) that a weather event will happen? We use a variety of information sources:
- Current observations
- Weather prediction models
- Forecaster expertise
Ultimately, it is our forecasters who decide when there is low, moderate, high, or very high confidence that a weather event will happen. We use this confidence in our Alert Colour Matrix to help choose a weather alert’s colour.