What Is a Rainfall Warning?
A Rainfall Warning is issued by Environment and Climate Change Canada when significant rainfall is occurring or expected and could create hazardous conditions.
Periods of heavy rainfall can overwhelm drainage systems, cause water to accumulate on roads and increase water levels in rivers, creeks and other low-lying areas. Particularly intense rainfall can also contribute to flash flooding, especially in urban areas and locations with poor drainage.
The impacts of a rainfall event depend on more than the total amount of rain. Rainfall intensity, duration, existing soil moisture, snowmelt and local drainage conditions can all influence the risk of flooding.
Related Warnings
| Warning or Alert | Description |
|---|---|
| Yellow Rainfall Warning | Indicates significant rainfall is expected to cause moderate impacts, such as localized flooding, water accumulation and difficult travel. |
| Orange Rainfall Warning | Indicates an unusually significant rainfall event is expected with potentially major impacts, including widespread flooding and travel disruptions. |
When Is a Rainfall Warning Issued?
A Rainfall Warning is issued when significant amounts of rain are expected to fall within a specified period and create potentially hazardous conditions.
Environment and Climate Change Canada’s rainfall warning criteria vary across Canada. Thresholds account for regional differences in climate and the amount of rainfall that can typically produce significant impacts.
Warnings may be triggered by large rainfall totals over many hours or very heavy rainfall over a shorter period.
The impacts can also depend on conditions before the rain begins. Saturated or frozen ground may absorb less water, while melting snow can combine with heavy rain to increase runoff and the potential for river flooding.
Under Canada’s colour-coded weather alert system, a Yellow Rainfall Warning indicates moderate impacts are expected, while an Orange Rainfall Warning indicates a higher-impact and less common event.
What Weather Conditions Should You Expect?
During a Rainfall Warning, expect persistent heavy rain, periods of torrential rain, or both, depending on the weather system.
Rainfall may cause water to collect quickly on roads, parking lots and other poorly drained surfaces. Visibility can also decrease significantly during the heaviest rainfall, making driving more difficult.
Possible conditions and impacts include:
- Heavy or prolonged rainfall
- Torrential downpours
- Reduced visibility
- Ponding or pooling water on roads
- Rapidly rising creeks and streams
- Localized or widespread flooding
- Increased river and lake levels
- Saturated or waterlogged ground
If rain falls very rapidly, flash flooding can develop with relatively little warning. Longer-duration rainfall can instead cause a gradual rise in rivers and other waterways.
What Should You Do During a Rainfall Warning?
Monitor local forecasts and rainfall warnings closely, particularly if you live or travel through an area that is susceptible to flooding.
During a Rainfall Warning:
- Avoid unnecessary travel during periods of extremely heavy rain.
- Reduce your speed and increase following distance when driving.
- Watch for water accumulating on roads and in low-lying areas.
- Never drive through a flooded roadway.
- Stay away from fast-moving rivers, creeks and drainage channels.
- Avoid flooded underpasses and other low-lying areas.
- Move valuables away from flood-prone areas when significant flooding is possible.
- Keep drains and catch basins clear when it is safe to do so.
- Monitor local flood information and instructions from emergency officials.
- Be prepared to change your plans if flooding makes roads unsafe.
Heavy rainfall conditions can vary significantly over short distances, particularly during thunderstorms, so continue monitoring updated warnings throughout the event.
Frequently Asked Questions About Rainfall Warnings
A Rainfall Warning means significant rainfall is occurring or expected and could cause hazardous conditions such as flooding, water accumulation and difficult travel.
There is no single rainfall threshold for all of Canada. Environment and Climate Change Canada uses regional criteria because rainfall amounts that create significant impacts vary by location and season.
A Yellow Rainfall Warning indicates that moderate impacts are expected. An Orange Rainfall Warning indicates a more significant and less common event with the potential for major impacts and disruption.
Yes. Flash flooding can occur when intense rainfall causes water levels to rise rapidly, particularly in urban areas, low-lying locations and areas with poor drainage.
There is no standard duration. A warning may remain in effect for several hours or longer depending on the weather system, expected rainfall, impacts and forecast confidence.
