Storms of Southern Alberta
Understand the thunderstorms, hail, tornadoes, blizzards and Arctic blasts that shape life here, and exactly what to do when one is coming.
In this lesson
- How a thunderstorm is built, from updraft to anvil to gust front
- Why Alberta is known for hail, and how a hailstone grows
- Tornadoes on the prairie, and where to shelter
- Upslope snowstorms, blizzards, ground blizzards and Arctic outbreaks
- The difference between an Environment Canada watch and a warning
Thunderstorms: the summer engine
Back in Lesson 10 you met the three ingredients a thunderstorm needs: moisture, unstable air and something to lift it. On a summer day in southern Alberta, all three often come together by mid-afternoon. Strong sun heats the ground, moisture builds up near the surface (growing crops add a surprising amount by breathing out water vapour), and the foothills give the air a push upward.
That is why storms often pop up over the foothills in the early afternoon, then drift east over the prairie toward Lethbridge in the late afternoon and evening. If you have ever watched towering clouds build over the mountains on a hot July day, you have watched the engine starting.
The updraft is the storm’s fuel line, sucking warm, moist air upward. When it reaches the top of the weather layer it can rise no further, so it spreads out sideways into a flat anvil. A bump poking above the anvil, called an overshooting top, means the updraft is especially strong.
Rain and hail falling through the cloud drag cold air down with them. This downdraft hits the ground and spreads out like water poured on a table. Its leading edge, the gust front, is that sudden cool, dusty blast you feel a few minutes before the rain arrives. The most powerful storms, called supercells, have a rotating updraft and can last for hours.
If you can hear thunder, you are close enough to be struck by lightning, even under blue sky. Get inside a house or other enclosed building, or a hard-topped vehicle with the windows up. Picnic shelters, tents, golf carts and trees do not protect you. Stay inside until 30 minutes after the last rumble. Avoid open fields, hilltops, water and lone trees.
Hail alley
Alberta has a reputation as one of the most hail-prone places in Canada. The most famous stretch, often called “hailstorm alley,” runs along and east of the foothills from roughly Calgary to Red Deer, but southern Alberta, Lethbridge included, gets its share too, mostly from June to August.
How a hailstone grows
A hailstone starts as a tiny ice pellet or frozen drop high in the storm. The updraft carries it through a region full of supercooled water, droplets that stay liquid even below freezing. They freeze onto the pellet on contact, adding layer after layer like an onion. When the stone grows too heavy for the updraft to hold up, it falls. A stronger updraft can hold up a bigger stone, so the biggest hail comes from the strongest storms.
Our high ground helps hail survive. Lethbridge sits around 910 m above sea level, so the freezing level is closer to the ground than it is in lower places, and falling hail has less warm air to melt in. Add strong summer updrafts and the strong winds aloft that help organize storms, and the prairie near the Rockies is ideal hail country. Insurers even fund a long-running cloud-seeding program in central Alberta aimed at producing more, smaller hailstones.
Tornadoes on the prairie
Alberta sees a number of tornadoes each summer. Most are weak, short-lived and over open country, and many are only discovered later from photos or damage in a field. Some form beneath powerful rotating supercells. Others, called landspouts, form under growing cumulus clouds when spinning air near the ground gets stretched upward; they are usually weaker but can still do damage. Alberta also knows how serious tornadoes can be: Edmonton’s 1987 tornado was one of the deadliest in Canadian history.
Go to a basement, or to a small interior room on the lowest floor (a bathroom, closet or hallway) away from windows, and protect your head. Mobile homes, RVs, tents and vehicles give poor protection: leave them for a sturdy building if you have time. Caught outside with no shelter? Lie flat in a ditch or low spot away from vehicles and trees, and cover your head. Never shelter under a highway overpass.
Winter storms: upslope snow, blizzards and ground blizzards
Here is a southern Alberta surprise: our biggest snowstorms usually arrive on an east wind, not a west one. The prairie rises gently from Saskatchewan toward the Rockies. When cold air from the north and east is pushed westward, it is forced uphill, cooling as it rises, and squeezes out snow. This is called upslope snow, and it is often heaviest closest to the foothills. Spring upslope storms in March, April and May can dump heavy, wet snow that snaps branches and closes roads.
A blizzard is a long stretch of strong wind with snow and blowing snow, cutting visibility to almost nothing. A ground blizzard needs no new snow at all: strong wind simply lifts what is already on the ground. On the open prairie you can have blue sky overhead and a total whiteout at highway level.
Check 511 Alberta and the forecast before you go, and tell someone your route. If you hit a whiteout, slow down and turn on your full headlights (daytime running lights may leave your tail lights dark). Never stop in a driving lane; if you must stop, pull well off the road and turn on your hazard lights. If you are stuck, stay with your vehicle, run the engine only now and then, keep the exhaust pipe clear of snow and open a window slightly. Carry a winter kit: blankets, warm clothes, food, water, a shovel, booster cables and a charged phone.
Arctic outbreaks
The same mountains that bring us chinooks also act as a wall for Arctic air. Bitterly cold air from the far north is dense and shallow, so when it heads south it slides along the eastern side of the Rockies like water poured down a trough. An Arctic front can turn a mild afternoon into a frigid evening, with north winds and blowing snow. Once the cold settles in, wind chill can freeze exposed skin quickly. Watch for Environment Canada extreme cold warnings, cover up and check on neighbours.
Watches, warnings and statements
Environment Canada uses a ladder of alerts:
- Special weather statement: unusual or changing weather worth knowing about.
- Advisory: a specific weather condition that is a concern but less serious than a warning.
- Watch: conditions are favourable for severe weather to develop. Keep an eye on the sky and be ready to act.
- Warning: severe weather is happening or about to happen. Take action now.
A watch means “get ready.” A warning means “take action.” The most urgent warnings, such as tornado warnings, can also be pushed straight to your cellphone through Canada’s Alert Ready system.
Try it yourself
Next time a storm builds, count the seconds between a lightning flash and its thunder. Every three seconds is roughly one kilometre. If you can hear thunder at all, head inside. In winter, when the forecast calls for northeast winds and snow, compare totals in Lethbridge with places closer to the foothills.
West Lethbridge now (3:14 am): 0°C, wind S 2 km/h, humidity 95%. Live station ›
Key terms
- Updraft
- The rising current of warm, moist air that feeds a thunderstorm.
- Anvil
- The flat, spreading top of a thunderstorm where the updraft can rise no higher.
- Gust front
- The leading edge of cool air rushing out from a storm’s downdraft.
- Supercell
- A long-lived, powerful thunderstorm with a rotating updraft; the main source of large hail and strong tornadoes.
- Landspout
- A usually weaker tornado that forms under a growing cumulus cloud rather than a rotating supercell.
- Upslope snow
- Snow produced when easterly winds push moist, cold air up the rising prairie toward the foothills.
- Ground blizzard
- A whiteout caused by strong wind lifting snow already on the ground, with no new snow falling.
- Watch and warning
- A watch means severe weather could develop; a warning means it is happening or about to happen.
Check your understanding
1. How does Lethbridge’s high elevation help hail reach the ground?
With less warm air between the freezing level and the ground, falling hailstones survive with less melting.
2. You are out golfing and hear thunder, but it is not raining yet. What should you do?
Lightning can strike well away from the rain. When thunder roars, go indoors, and wait 30 minutes after the last rumble.
3. A tornado warning is issued while you are in an RV at a campground. What is the best action?
RVs and mobile homes offer poor protection. A basement or small interior room in a sturdy building is far safer.
4. Big upslope snowstorms in southern Alberta usually come with winds from which direction?
Easterly winds push cold air up the rising prairie toward the foothills, cooling it and squeezing out snow.
5. Environment Canada issues a severe thunderstorm watch. What does it mean?
A watch means “get ready”: severe storms could develop. A warning means one is happening or about to happen.
Weather School is for learning and enjoyment. It is not official guidance and may contain errors. For watches and warnings, always use Environment Canada (WeatherCAN app or weather.gc.ca).