8 Warning Signs of a Tornado

Severe weather can move in quickly, and tornadoes — those powerful storms with winds reaching up to 300 mph — are some of the most unpredictable.

I grew up in Oklahoma, which averages about 57 tornadoes a year, so school tornado drills were as common as fire drills. At home, we were taught to get in the bathtub or an interior closet if we couldn’t make it to a shelter.

But what if you’re outside, or don’t have access to weather information? An idyllic early summer evening on the lake or golf course can quickly turn dangerous. Are there ominous signs you can look for in the skies?

Absolutely. But by the time you see them, it might be too late.

“Most of the visual clues of an approaching severe thunderstorm or tornado require you to be close enough to the storm,” says Bill Bunting, deputy director of the National Weather Service (NWS)/National Oceanographic and Atmospheric Administration (NOAA) Storm Prediction Center (SPC). Because of that, he says, “It would be unlikely you could take protective action in time.”

That’s scary. Still, it’s important to know the warning signs so you can react immediately. I talked to Bunting about how tornadoes form, and what to look for.

Thunderstorm

Before you can have a tornado, you need a thunderstorm. The atmosphere “must be unstable, and with sufficient moisture to allow a developing thunderstorm updraft to rise vertically,” Bunting says.

Warm spring or summer afternoons are perfect. Then you need a source of lift, like a cold front, which makes the unstable, warm air rise.

When the rising warm air meets cooler air above, it condenses into precipitation. That precipitation continues to rise and fall in the turbulent cloud environment until eventually falling to earth as rain, or sometimes hail. (Thunderstorms are complex, so these are just the basics.)

When does a thunderstorm become a potential tornado producer? Lots of things have to happen. Only a small percentage of thunderstorms actually produce tornadoes, Bunting says, but here are some signs that signal a tornado is possible:

Towering Vertical Clouds

As a thunderstorm strengthens, you may see clouds growing into a huge, towering column as moisture condenses when it hits cooler air. What starts out as a fluffy, cottony cumulus cloud grows rapidly into a massive cumulonimbus that can shoot miles into the air.

This vertical cloud growth is caused by the updraft, which feeds the storm and gives it power. Sometimes, the updraft twists or tilts as it develops into a strong storm. A tilted updraft can signal a powerful tornado-maker called a supercell.

“Updrafts tilt in the presence of vertical wind shear, which is the change in wind speed and direction with increasing height from the ground to the upper levels of a storm,” Bunting says. “Vertical wind shear is one of the primary ingredients needed to produce supercell thunderstorms, which are responsible for the vast majority of strong and violent tornadoes [EF-2 or greater].”

Flat, Anvil-Shaped Top

An anvil is a blacksmith’s tool: a big block of iron with a flat, extended top. As thunderstorms mature, they can develop an anvil-shaped extension that shoots sideways when the soaring updraft reaches the layer of the atmosphere called the tropopause. Supercells often feature dramatic anvils that go on for miles.

Heavy Rain

Heavy rain can be a precursor to a tornado. Even if it’s not raining where you are, often you can see a dark, wispy column called the rain shaft underneath the storm clouds.

“A greenish or dark sky may signal a stronger thunderstorm,” Bunting says. That means it’s more likely to produce gusty winds and hail.

The presence of a rain shaft (which can also be hail or even snow) doesn’t indicate a tornado by itself, but conditions in a severe thunderstorm can change quickly. Not to mention, if there is a tornado, it’s likely to be behind the rain, so you may not see these “rain-wrapped” tornadoes as the storm churns along.

Lowering, Smaller Cloud (Wall Cloud)

If you’ve heard meteorologists talking about wall clouds, you know they mean business. This smaller, often circular cloud forms below the main thunderstorm tower.

Bunting says wall clouds are a common feature of many (but not all) thunderstorms, and can determine where warm air is being drawn into the storm.

Whether or not you can even see the wall cloud depends on the terrain, visibility and where you are in relation to the storm, according to Bunting. “In short, I wouldn’t wait to see a wall cloud before taking shelter,” Bunting says. “But a well organized and persistently rotating wall cloud is of immediate concern.”

Rotation

Not all wall clouds rotate. But if they do, it could be a tornado forming. “When the wall cloud exhibits persistent rotation,” Bunting says, “then it becomes more of a concern for the development of a tornado.”

The rotating wall cloud is an extension of the rotating updraft, which can be several miles in diameter. This updraft is called a mesocyclone.

“The wind direction and speed throughout the lower portion of the atmosphere must be favorable for rotation to develop within the storm,” Bunting says. Then it becomes a supercell. Under the right conditions, Bunting says, “that rotation can concentrate into a tornado and accelerate wind speeds to damaging levels.”

Funnel Cloud

As the vertical rotation within the thunderstorm becomes tighter and stronger, a funnel cloud can form and drop down toward the ground. But funnel clouds are not tornadoes — yet. Tornadoes, by definition, must extend all the way from the cloud to the earth.

“If the violent winds are not in contact with the ground, then it’s a funnel cloud,” Bunting says.

What you’re seeing is condensation. Water droplets, then dust and debris from the ground path if it makes it to the Earth, make up the visible portion of these funnels.

Debris

Once the funnel cloud makes contact with the Earth, it becomes a tornado. Sometimes, though, it’s not easy to see the vortex all the way to the ground, even if it’s actually touching.

“Storm spotters are taught to look beneath a funnel cloud for signs of contact with the ground,” Bunting says — things like debris from the swirling winds. Often, Bunting says, “those tornadic winds do reach the surface,” especially if the funnel cloud is halfway or more to the ground.

The NWS uses special radar to track the “debris signature” of a tornado, among other things, to help meteorologists know if there’s a tornado on the ground. You don’t have that luxury, so get to shelter long before a tornado touches down.

“When a warning is issued for your area, it is extremely important to take immediate action by moving into a sturdy, reinforced structure before the storm approaches,” Bunting says.

About the Expert

Bill Bunting is the deputy director of the NOAA/NWS Storm Prediction Center in Norman, Oklahoma. As a meteorology student, Bunting participated in the original TOTO (Totable Tornado Observatory) project, cited in the movie Twister.