Environment Canada deploys new tornado and thunderstorm alert system Tuesday

Environment Canada's new targeted alert system for tornado and severe thunderstorm warnings launches on its website Tuesday morning, replacing zone-based alerts that sometimes blanket areas up to several hundred kilometres across.

The updated system uses freeform polygons that follow the storm as it moves, narrowing the warning footprint to focus on the area of greatest concern. "What this means that when we're issuing one of those two warnings, the alert goes out to a larger number of people than perhaps what the immediate danger really represents," Jonathan Bau, a senior ECCC meteorologist, said Monday. "The change to freeform polygons is to reduce the amount of over-alerting."

Warning polygons will show the storm location, direction and speed, list locations likely to be impacted based on projected arrival time, and describe associated hazards like wind, hail, and heavy rain. The polygons widen downstream to indicate forecast uncertainty.

When targeted warnings are issued, the National Public Alerting System (Alert Ready) will send emergency text notifications to cellphones in affected polygon areas — narrower than the current broad-zone approach. However, the source does not specify when exactly the system will activate or say whether mobile weather apps like WeatherCAN will display the polygon visualization.

Television and radio emergency interruptions will see "marginal" improvements under the new system. The upgraded alerts apply to tornado and severe thunderstorm warnings only; watches will continue under the existing process.