What the heck do these Environment Canada alert terms mean?

'Rainfall warning' vs 'weather alert': Demystifying Environment Canada's advisories

Answers from warning preparedness meteorologist with Environment and Climate Change Canada.

A rainfall warning was issued on Jan. 31 for Squamish, for example, because 50 millimetres of rain is expected over the next 24 hours.Alex Ratson/Moment/Getty Images

We arguably have more access to weather information than ever, with all the apps and media updates, but it can be confusing to parse out what the terms meteorologists use in their releases and why.

For example, when is a rainfall warning issued versus a special weather statement?

Lisa Ervin, a warning preparedness meteorologist with Environment and Climate Change Canada, explained that weather warnings are issued when a specific criteria is expected to be met.

A rainfall warning was issued on Jan. 31 for Squamish, for example, because 50 millimetres of rain is expected over the next 24 hours.

A special weather statement can be issued for a variety of reasons.

"[They] are used to explain weather situations that are not adequately described in the public forecast, or within watches and warnings.

"We can use a special weather statement where, for instance, the expected rainfall isn't necessarily forecast to reach the warning level criteria, but still may be impactful due to other reasons. We can use a special weather statement to provide an earlier heads-up for weather for a weather event."

The weather statement may later get upgraded to a warning.

"We can also use the weather statement to express uncertainty. We know something's coming … but the weather models aren't in full agreement. But we do want to give the public a heads-up that impactful weather is on its way and to do that, we use a special weather statement because we have the option of freeform text—we can use our words to highlight what we think is the most likely scenario."

Ervin cited the example of when a special weather statement would be used is for the first heat wave of the season where the daytime high temperatures are not yet reaching a warning threshold, but it is early in the season.

"Especially in the early months of May or early June when people have not acclimatized to hotter temperatures, it can be impactful, but not yet reaching warning level criteria."

Therefore, a special weather statement would be issued.

In the current situation, Squamish is under both a rainfall warning and a special weather statement.

The statement applies to the south coast of B.C. and the weather expected through the whole region.

The rainfall warning is more specific to Squamish.

Some of the language in both can sometimes overlap between warnings and statements.

"Sometimes, they're very similar between the two statements and it's because the weather that's impacting Squamish is also impacting the surrounding regions, but the rainfall just happens to be a little bit lower in those surrounding regions,” she explained. “Those 'call to action' statements are relevant for the broader geographic area."

Ervin said Environment Canada also stays in contact with other agencies, such as the River Forecast Centre and BC Wildfire Service.

"So we are aware of impactful weather in more ways than just simply our weather warnings," she said.