Meteorologists predict thunderstorms by looking for the atmospheric ingredients that allow storms to develop and then using weather models, radar, satellites, weather balloons, surface observations, and lightning detection to monitor those conditions.
Three basic ingredients are needed for a thunderstorm to form: moisture, atmospheric instability, and a source of lift. When these ingredients come together, warm, moist air can rise rapidly through the atmosphere and develop into the towering cumulonimbus clouds associated with thunderstorms.
Predicting exactly when and where a thunderstorm will form can still be difficult. Individual storms can develop quickly and affect relatively small areas, meaning one community could experience a powerful thunderstorm while another nearby receives little or no rain.
How Do Meteorologists Know When Thunderstorms Will Form?
Before thunderstorms develop, meteorologists examine the atmosphere to determine whether conditions are favourable for convection.
They generally look for three essential ingredients:
- Moisture: There must be enough moisture in the atmosphere to support cloud and storm development.
- Instability: Warm, moist air near the surface beneath colder air higher in the atmosphere can create an environment where rising air continues moving upward.
- Lift: Something must initially force the air upward. This can come from a cold front, warm front, terrain, surface heating, or another atmospheric boundary.
Having all three ingredients does not guarantee a thunderstorm will form at a particular location. Meteorologists also need to determine how strong these ingredients are, where they overlap, and whether anything in the atmosphere could prevent storms from developing.
What Tools Do Meteorologists Use to Predict Thunderstorms?
Thunderstorm forecasting involves both predicting the environment before storms form and monitoring storms after they develop.
Meteorologists use several tools to accomplish this.
Weather Models
Computer weather models allow meteorologists to look several hours or days into the future and identify areas where the atmosphere may become favourable for thunderstorms.
Models forecast variables such as:
- Temperature
- Humidity
- Atmospheric pressure
- Wind speed
- Wind direction
- Precipitation
- Atmospheric instability
- Cloud development
Meteorologists compare multiple forecast models and model runs to see whether they consistently show favourable thunderstorm conditions developing in the same region.
Higher-resolution models can also provide guidance on where individual clusters or lines of thunderstorms could develop.
However, thunderstorms can occur on scales small enough that models cannot perfectly predict the location and timing of every storm.
Weather Radar
Once thunderstorms begin developing, weather radar becomes one of the most important tools available to meteorologists.
Radar sends pulses of energy into the atmosphere and measures the energy reflected back by precipitation particles.
Meteorologists can use radar to determine where precipitation is occurring, how intense it is, and how storms are moving.
Modern Doppler weather radar can also provide information about movement within storms.
Forecasters use radar to monitor:
- Thunderstorm location
- Storm movement
- Rainfall intensity
- Hail
- Strong wind signatures
- Rotation within thunderstorms
- Developing lines and clusters of storms
Radar becomes especially important when meteorologists are determining whether an existing thunderstorm is becoming severe.
Weather Satellites
Satellites allow meteorologists to watch thunderstorms develop from above.
Before storms form, satellite imagery can reveal areas of moisture, cloud development, and atmospheric boundaries.
Once thunderstorms begin developing, meteorologists can watch clouds rapidly grow vertically.
A thunderstorm can eventually develop into a towering cumulonimbus cloud extending high into the atmosphere.
Satellite observations can help meteorologists identify rapidly developing storms, monitor cloud-top temperatures, and follow large thunderstorm complexes as they move across a region.
Satellites are particularly valuable in areas where radar coverage is limited.
Weather Balloons
Weather balloons provide meteorologists with a vertical snapshot of the atmosphere.
Instruments carried by weather balloons measure temperature, humidity, pressure, wind speed, and wind direction at different altitudes.
This information allows meteorologists to determine how unstable the atmosphere is and whether conditions support thunderstorm development.
A weather balloon sounding might reveal warm, humid air near the surface with much colder air above it.
That type of temperature structure can create an unstable atmosphere capable of supporting strong rising air.
Weather balloon observations also allow meteorologists to examine how winds change with height, which can become particularly important when forecasting severe thunderstorms.
What Is Atmospheric Instability?
Atmospheric instability describes the tendency for air to continue rising after it has been lifted.
Imagine a parcel of warm, humid air near the ground.
If that air is pushed upward and remains warmer and less dense than the surrounding atmosphere, it can continue rising. As it rises, it cools and water vapour condenses, helping clouds develop.
In a highly unstable atmosphere, air can rise rapidly and produce powerful thunderstorm updrafts.
Meteorologists use several measurements and indices to evaluate instability.
One of the most commonly discussed is CAPE.
What Is CAPE?
CAPE stands for Convective Available Potential Energy.
It is a measurement meteorologists use to estimate the amount of energy available to rising air in the atmosphere.
Generally, greater CAPE indicates that stronger thunderstorm updrafts may be possible if storms are able to develop.
However, CAPE should not be treated as a simple thunderstorm score.
High CAPE does not automatically mean severe thunderstorms will occur. The atmosphere still requires sufficient moisture, lift, and other favourable conditions.
Meteorologists therefore examine CAPE alongside many other atmospheric variables rather than using it by itself.
What Causes the Air to Rise?
Even when the atmosphere is unstable, something often needs to provide the initial push that gets air moving upward.
Meteorologists call this lift.
Several weather features can provide lift.
Weather Fronts
Cold fronts are common triggers for thunderstorms.
As denser cold air advances, it can force warmer, moist air upward. If the atmosphere is sufficiently unstable, thunderstorms may develop along or ahead of the front.
Surface Heating
Strong sunshine can heat the Earth’s surface.
The surface then warms the air immediately above it. If that air becomes sufficiently warm and buoyant, it can begin rising through the atmosphere.
This is one reason thunderstorms frequently develop during warm summer afternoons.
Terrain
Mountains and other elevated terrain can physically force air upward.
If the air contains enough moisture and the atmosphere is unstable, this lifting can help initiate thunderstorms.
Outflow Boundaries
Existing thunderstorms can also help create new thunderstorms.
Cool air flowing outward from a thunderstorm can act somewhat like a small cold front. When this outflow encounters warm, humid air, it can force that air upward and potentially trigger additional storm development.
How Do Meteorologists Predict Severe Thunderstorms?
Predicting that thunderstorms could occur is different from determining whether those thunderstorms could become severe.
Meteorologists need to evaluate whether the atmosphere could support hazards such as damaging winds, large hail, intense rainfall, or tornadoes.
One particularly important factor is wind shear.
Wind shear refers to changes in wind speed or direction with height.
When winds change significantly through the atmosphere, a thunderstorm’s updraft can become tilted.
This can help separate the storm’s rising air from its rain-cooled downdraft, allowing the thunderstorm to remain organized for longer.
Strong wind shear combined with substantial instability can support more organized and potentially severe thunderstorms.
Meteorologists therefore examine instability and wind shear together when assessing severe-weather potential.
How Do Meteorologists Predict Where Thunderstorms Will Develop?
Thunderstorm forecasting often involves identifying boundaries where air is most likely to rise.
Meteorologists examine weather maps for features such as:
- Cold fronts
- Warm fronts
- Low-pressure systems
- Lake breezes
- Outflow boundaries
- Areas of strong surface heating
- Terrain
- Moisture boundaries
They then compare these features with areas containing sufficient moisture and atmospheric instability.
Where these ingredients overlap, thunderstorm development becomes more likely.
The difficult part is determining exactly where the first storm will form.
A small atmospheric boundary that is difficult for computer models to resolve could become the trigger for thunderstorm development.
This is one reason meteorologists continually monitor observations throughout days when thunderstorms are expected.
How Do Meteorologists Predict When Thunderstorms Will Start?
Computer models can provide an approximate window for thunderstorm development, but predicting the exact time a storm will form can be challenging.
Meteorologists watch how atmospheric conditions change throughout the day.
For example, increasing temperatures during the afternoon may gradually increase instability.
A cold front approaching from the west could then provide the lift needed for thunderstorms to develop.
Satellite imagery can become particularly useful at this stage.
Meteorologists can watch small cumulus clouds begin developing and determine whether they are growing vertically.
Rapidly growing clouds can indicate that the atmosphere is becoming increasingly supportive of thunderstorms.
Once precipitation develops, radar allows meteorologists to track the storms directly.
How Is Lightning Used to Track Thunderstorms?
Lightning detection networks provide another important source of information.
Lightning is produced as electrical charges separate within a developing thunderstorm cloud.
Detection networks can locate lightning strikes and allow meteorologists to monitor where thunderstorm activity is occurring.
An increase in lightning activity can sometimes accompany a rapidly strengthening storm.
Lightning information can therefore be combined with radar and satellite observations to provide meteorologists with a more complete picture of how a thunderstorm is evolving.
Lightning detection is especially valuable because thunderstorms are defined by the presence of lightning and thunder.
How Do Meteorologists Know Which Direction a Thunderstorm Will Move?
Meteorologists use radar observations and atmospheric wind data to estimate how thunderstorms will move.
Once a storm has developed, consecutive radar images reveal its direction and speed.
Forecasters can use this movement to estimate which communities may be affected next.
However, thunderstorms do not always move in a perfectly straight line.
Storms can strengthen, weaken, merge, split, or develop along boundaries.
New thunderstorms may also repeatedly form on one side of an existing storm system.
Meteorologists therefore continuously update short-term forecasts as storms evolve.
Why Are Thunderstorms Difficult to Predict?
Thunderstorms can develop quickly and on relatively small geographic scales.
The atmosphere might be favourable for thunderstorms across a large region, but only certain locations may actually experience a storm.
Small differences in cloud cover, temperature, humidity, or the location of an atmospheric boundary can determine where thunderstorms develop.
Computer models may correctly predict that a region will experience thunderstorms without correctly predicting the exact neighbourhood where the first storm develops.
This becomes particularly important with isolated thunderstorms.
One location could experience heavy rain, lightning, hail, and damaging winds while another location only a short distance away remains dry.
Why Can Thunderstorm Forecasts Change So Quickly?
Thunderstorm forecasts are continually updated because the conditions responsible for storm development can change throughout the day.
For example, unexpected morning cloud cover could limit surface heating and reduce instability.
Alternatively, temperatures may rise more quickly than expected, increasing instability.
An earlier thunderstorm could also produce an outflow boundary that triggers additional storms somewhere forecasters had not initially expected.
Meteorologists continually compare actual observations with what computer models predicted.
When the atmosphere develops differently than expected, the forecast may change.
How Far in Advance Can Meteorologists Predict Thunderstorms?
Meteorologists can sometimes identify favourable conditions for thunderstorms several days in advance.
At longer ranges, the forecast is generally about potential rather than the exact location of individual storms.
As the event approaches, meteorologists gain access to more detailed observations and updated model guidance.
In Canada, Thunderstorm Outlooks can provide an early indication of areas where thunderstorms are likely to develop up to 36 hours in advance.
As thunderstorms actually begin developing, radar, satellite imagery, lightning detection, and surface observations become increasingly important.
Short-term forecasts can then focus on the movement and intensity of individual storms.
What Is the Difference Between a Thunderstorm Forecast and a Warning?
A thunderstorm forecast indicates that atmospheric conditions may support thunderstorm development.
That does not necessarily mean every location within the forecast area will experience a storm.
A severe thunderstorm warning is much more immediate.
Warnings are issued when a severe thunderstorm is occurring or expected to pose a significant threat to an affected area.
Meteorologists use radar, lightning information, surface observations, weather models, and reports from the ground to assess developing storms and determine whether warnings are necessary.
Because thunderstorms can develop and intensify rapidly, people should continue monitoring official weather information when severe weather is possible.
How Are Thunderstorms Forecast in Canada?
Environment and Climate Change Canada uses a combination of computer models, weather satellites, Doppler radar, weather stations, weather balloons, and lightning detection data to produce forecasts and monitor severe weather.
Meteorologists can issue Thunderstorm Outlooks when conditions appear favourable for thunderstorm development.
Once storms develop, radar and lightning observations allow forecasters to monitor their location, intensity, and movement.
If a storm becomes severe enough to pose a significant hazard, weather alerts can be issued or updated as conditions evolve.
During active thunderstorms or severe weather, Canadians should follow current forecasts and official alerts from Environment and Climate Change Canada.
The Bottom Line
Meteorologists predict thunderstorms by looking for three basic atmospheric ingredients: moisture, instability, and lift.
Weather models help forecasters identify where these ingredients could come together before storms develop. Weather balloons provide information about instability and winds throughout the atmosphere, while satellites allow meteorologists to watch clouds begin growing.
Once thunderstorms form, Doppler radar and lightning detection become particularly important for tracking their development, movement, and intensity.
Meteorologists also examine factors such as CAPE and wind shear to determine whether ordinary thunderstorms could become organized or severe.
