What Is Heavy Rainfall?
Heavy rainfall occurs when a large amount of rain falls within a relatively short period or when persistent rain produces substantial accumulations over a longer period. In Canada, heavy rainfall can occur during thunderstorms, slow-moving weather systems, tropical systems and prolonged periods of moisture moving into a region.
The impacts of heavy rainfall depend on more than the total amount of precipitation. Rainfall intensity, duration, soil conditions, terrain, drainage and existing water levels all influence whether heavy rain results in flooding.
Heavy rainfall can occur during any season when temperatures are warm enough for precipitation to fall as rain, although the atmospheric processes responsible for it vary throughout the year.
Quick Facts About Heavy Rainfall
Main Characteristic: Large amounts of rain falling rapidly or accumulating over an extended period
Common Seasons: Spring, summer and fall, but possible year-round in parts of Canada
Measured As: Millimetres (mm) of rainfall
Major Hazards: Flooding, poor visibility, erosion and hazardous travel
Common Causes: Thunderstorms, low-pressure systems and moisture-rich air
Related Conditions: Torrential rain, flash flooding and river flooding
Related Alert: Rainfall Warning
What Causes Heavy Rainfall?
Heavy rainfall develops when the atmosphere contains substantial moisture and conditions allow that moisture to condense into clouds and precipitation.
For significant rainfall to occur, moist air generally needs to rise. As air rises, atmospheric pressure decreases and the air cools. Eventually, water vapour condenses into cloud droplets.
When cloud droplets or ice particles grow sufficiently large, they fall toward the surface as precipitation.
Several atmospheric processes can repeatedly lift moist air, including:
- Low-pressure systems
- Weather fronts
- Thunderstorms
- Terrain and mountains
- Tropical systems
- Converging winds
The heaviest rainfall events often occur when a weather system can continuously draw moisture into the same region.
How Is Heavy Rainfall Measured?
Rainfall in Canada is generally measured in millimetres.
One millimetre of rainfall represents approximately one litre of water falling over each square metre of horizontal surface.
Meteorologists examine both rainfall accumulation and rainfall intensity.
Accumulation describes the total amount of rain received during a particular period. Intensity describes how quickly that rain falls.
For example, the impacts of a given rainfall total can be very different if it falls gradually over an entire day rather than during a short, intense downpour.
What Is Considered Heavy Rainfall?
There is no single amount that makes rainfall “heavy” in every situation.
The significance of rainfall depends on how much falls, how quickly it falls and the conditions in the affected area.
A rainfall amount that causes few problems when spread over several days could overwhelm drainage systems if it falls within a few hours.
Local geography also matters. Urban areas, steep terrain, saturated soil and locations near rivers or streams can respond differently to the same rainfall event.
Official rainfall alerts therefore use defined criteria appropriate to the region and season rather than relying on one universal Canadian threshold.
Heavy Rain vs. Torrential Rain
Heavy rain describes rainfall occurring at a high intensity or producing substantial accumulations.
Torrential rain generally describes exceptionally intense rain falling rapidly.
A prolonged weather system may therefore produce heavy rainfall over many hours without rainfall being torrential throughout the event.
A strong thunderstorm, by comparison, may produce torrential rain for a relatively short period.
Both can cause flooding, but short bursts of extremely intense rainfall are particularly important for flash flooding.
Heavy Rainfall vs. Showers
Rainfall can be relatively continuous when produced by widespread cloud systems associated with fronts or low-pressure systems.
Showers are generally more intermittent. They can begin and end relatively quickly and may vary considerably in intensity from one location to another.
Showers can still become heavy.
During unstable weather, individual showers or thunderstorms can produce substantial rainfall while nearby locations receive considerably less.
Why Do Thunderstorms Produce Heavy Rain?
Thunderstorms contain powerful rising currents of warm, moist air known as updrafts.
These updrafts transport moisture high into the atmosphere, where it condenses and contributes to the development of large clouds and precipitation.
Strong thunderstorms can contain enormous amounts of water.
When precipitation becomes concentrated beneath the storm, extremely heavy rainfall can occur.
The flooding risk becomes particularly significant when thunderstorms move slowly or repeatedly develop over the same location.
What Is Training?
Training occurs when multiple areas of rain or thunderstorms repeatedly move across approximately the same location.
The process resembles railway cars travelling along the same track.
An individual thunderstorm may only affect a location briefly, but several storms following the same path can produce very large cumulative rainfall totals.
Training thunderstorms are an important setup for excessive rainfall and flash flooding.
Why Do Slow-Moving Storms Cause More Rain?
The speed of a weather system can strongly influence rainfall accumulation.
A fast-moving storm may produce intense rain but leave the area relatively quickly.
A slow-moving or nearly stationary system can continue producing rain over the same region for many hours.
If moisture continues flowing into the system, rainfall accumulations can become substantial.
This is why forecasters consider both rainfall intensity and storm movement when evaluating flooding potential.
How Do Weather Fronts Produce Heavy Rain?
Weather fronts form along boundaries between different air masses.
When warm, moisture-rich air is forced upward along a front, it cools and produces clouds and precipitation.
A rapidly moving front may produce a relatively short period of rain.
A slow-moving or stationary front can produce much larger rainfall totals because moisture and precipitation remain focused over the same region.
If thunderstorms repeatedly develop along the boundary, rainfall can become particularly intense.
Can Tropical Systems Bring Heavy Rain to Canada?
Yes.
Tropical cyclones and their remnants can transport enormous quantities of atmospheric moisture northward.
Even after a tropical cyclone loses its tropical characteristics, its moisture can interact with fronts and other weather systems over Canada.
This can produce widespread and locally intense rainfall.
The rainfall impacts of a tropical system can also extend well beyond the area experiencing the strongest winds.
How Do Mountains Affect Heavy Rainfall?
Terrain can enhance precipitation through a process called orographic lift.
When moisture-rich air encounters elevated terrain, it is forced upward.
As the air rises, it cools, encouraging condensation, cloud development and precipitation.
This process is particularly important in mountainous regions of Canada, including parts of British Columbia.
Some locations exposed to moisture-bearing winds can consequently receive much greater rainfall than areas located farther downwind.
Can Heavy Rainfall Occur During Winter?
Yes.
Heavy rainfall can occur during winter when temperatures are sufficiently warm for precipitation to reach the ground as liquid rain.
Warm, moisture-rich air can sometimes move into areas containing existing snowpack.
This creates the possibility of a rain-on-snow event, where rainfall combines with melting snow.
When the ground is frozen or already saturated, water may be unable to infiltrate the soil efficiently, increasing runoff into streams, rivers and low-lying areas.
How Does Heavy Rain Cause Flooding?
Flooding develops when water accumulates faster than it can drain, infiltrate the ground or be carried away by waterways.
During heavy rainfall, soil may initially absorb some of the water.
As the ground becomes saturated, its ability to absorb additional rainfall decreases.
More water then travels across the surface as runoff.
This runoff can enter streams and rivers, cause water to collect in low-lying areas or overwhelm urban drainage systems.
The type of flooding that develops depends on the rainfall event and surrounding environment.
Heavy Rainfall and Flash Flooding
Flash flooding can develop rapidly during exceptionally intense rainfall.
If rain falls faster than the ground and drainage systems can handle it, water can accumulate within a short period.
Small streams and normally dry channels can also rise quickly.
Urban areas can be particularly susceptible because pavement, rooftops and other impermeable surfaces prevent much of the rainfall from soaking directly into the ground.
Heavy Rainfall and River Flooding
Heavy rainfall can also contribute to river flooding.
Water falling throughout a river’s drainage basin eventually flows toward streams and rivers.
Prolonged rainfall can cause river levels to rise gradually, particularly when soils are already saturated.
The response may continue even after rain has stopped because water from across the watershed is still travelling toward the river.
Heavy rainfall combined with spring snowmelt can further increase water levels in some parts of Canada.
Why Does Saturated Ground Increase Flood Risk?
Soil can only hold a limited amount of water.
When soil is relatively dry, some rainfall infiltrates into the ground.
After prolonged rainfall or snowmelt, the soil may become saturated.
Additional rainfall is then more likely to remain at the surface or become runoff.
Consequently, a moderate rainfall event occurring after several wet days can sometimes cause more flooding than a heavier rainfall event occurring after an extended dry period.
Why Are Cities Vulnerable to Heavy Rain?
Urban areas contain large amounts of pavement, concrete and rooftops.
These surfaces are largely impermeable, meaning rainfall cannot readily soak into the ground.
Instead, water flows toward storm drains, streets, ditches and other drainage infrastructure.
During extremely intense rainfall, water can arrive faster than drainage systems can remove it.
This can result in urban flooding, including flooded roads, underpasses and low-lying areas.
How Does Heavy Rain Affect Driving?
Heavy rainfall can make driving hazardous even when flooding does not occur.
Rain reduces visibility and creates water on road surfaces.
As rainfall intensity increases, windshield wipers may have difficulty maintaining clear visibility.
Standing water can also increase the risk of hydroplaning, which occurs when a vehicle’s tires lose effective contact with the road surface because of a layer of water.
Flooded roads present an additional hazard because water depth and road conditions can be difficult to judge.
Can Heavy Rain Cause Landslides?
Heavy or prolonged rainfall can increase the risk of landslides in susceptible terrain.
Water entering soil and rock can increase their weight while reducing the stability of slopes.
The risk depends on factors including slope steepness, geology, vegetation, previous rainfall and soil moisture.
Mountainous and steeply sloped areas can therefore face hazards beyond flooding during prolonged periods of intense precipitation.
How Is Heavy Rainfall Forecast?
Meteorologists use several tools to forecast and monitor heavy rainfall.
Numerical weather prediction models estimate where moisture, rising air and precipitation are likely to develop.
Weather radar allows meteorologists to observe the location, movement and intensity of precipitation as it occurs.
Weather satellites provide information about cloud systems, atmospheric moisture and large-scale storm development.
Weather stations and rain gauges measure how much precipitation actually reaches the ground.
Forecasters combine these observations with information about storm movement, soil conditions and expected rainfall duration to assess potential impacts.
How Does Doppler Radar Track Heavy Rain?
Doppler weather radar sends pulses of radio energy into the atmosphere and measures energy reflected by precipitation particles.
This allows meteorologists to identify areas of precipitation and estimate their intensity.
Radar imagery is particularly valuable during thunderstorms because conditions can change rapidly.
Repeated radar observations also show whether storms are moving quickly, remaining nearly stationary or repeatedly crossing the same area.
Can Heavy Rainfall Be Predicted Days in Advance?
Large weather systems capable of producing heavy rain can often be identified several days before they arrive.
However, predicting exactly where the heaviest rainfall will occur can remain difficult.
Thunderstorms are particularly challenging because rainfall totals can vary dramatically over short distances.
As an event approaches, forecasters use newer model runs, radar, satellite data and surface observations to refine rainfall forecasts.
Heavy Rainfall in Canada
Canada experiences several different meteorological setups capable of producing heavy rainfall.
Summer thunderstorms can generate short periods of intense precipitation.
Large low-pressure systems can produce widespread rainfall over many hours.
Atmospheric moisture arriving from the Pacific can produce substantial precipitation in western Canada, particularly where terrain enhances uplift.
Eastern Canada can also receive heavy rainfall from tropical systems or their remnants.
Spring events may become especially important when rain combines with melting snow, frozen ground or already elevated rivers.
Because Canada’s climate and geography vary considerably, the causes and impacts of heavy rainfall differ between regions.
Heavy Rainfall Warnings
Heavy rainfall as a weather condition is different from an official Rainfall Warning.
A rainfall warning is an alert issued when forecast or observed rainfall is expected to meet established criteria and create potentially hazardous conditions.
Warning criteria can vary by location and season.
The condition page explains the meteorology of heavy rainfall, while the alert page should explain when a Rainfall Warning is issued and what actions people should consider when one is in effect.
How to Stay Safe During Heavy Rainfall
Monitor weather forecasts and official alerts when significant rainfall is expected.
Avoid driving through flooded roads because water depth, current strength and the condition of the road underneath may not be visible.
Use additional caution while driving in intense rain because visibility and tire traction can deteriorate rapidly.
People near rivers, streams and low-lying areas should pay particular attention to changing water levels during prolonged rainfall.
During thunderstorms, remember that heavy rain may occur alongside other hazards such as lightning, strong winds and hail.