Tornado Warning Signs: What to Look For in the Sky
Severe weather can strike with little warning, and tornadoes, with winds that can reach 300 mph, are some of the most unpredictable natural forces. Understanding the warning signs of a potential tornado can be crucial for your safety. While some signs may only be visible up close, knowing what to look for can help you react quickly and take necessary precautions.
Growing up in Oklahoma, where an average of 57 tornadoes occur each year, tornado drills were as familiar as fire drills. We were taught to seek shelter in a bathtub or an interior closet if a dedicated storm shelter wasn’t accessible. But what if you’re outdoors enjoying a summer evening or don’t have access to weather updates? Being aware of the visual cues in the sky is essential.
But can you see a tornado coming? Absolutely. However, as Bill Bunting, deputy director of the National Weather Service (NWS)/National Oceanic and Atmospheric Administration (NOAA) Storm Prediction Center (SPC), points out, “Most of the visual clues of an approaching severe thunderstorm or tornado require you to be close enough to the storm.” This is why immediate action is critical.
This article explores the key signs to watch for, so you can understand what’s happening and how to respond.
The Foundation: The Thunderstorm
Before a tornado can form, you need a thunderstorm. According to Bunting, the atmosphere must be unstable and contain enough moisture to allow a developing thunderstorm updraft to rise vertically.
Warm spring or summer afternoons are ideal conditions for thunderstorm development. Add a lifting mechanism, like a cold front, to push that warm, unstable air upward. As the warm, rising air encounters cooler air, it condenses, leading to precipitation. These processes, while complex, are fundamental to thunderstorm formation.
Only a small percentage of thunderstorms will actually produce tornadoes. However, certain visual indicators suggest that a tornado is possible. Here are some key signs:
Towering Vertical Clouds
As a thunderstorm intensifies, the clouds can grow into a massive, towering column. What begins as a fluffy cumulus cloud rapidly expands into a towering cumulonimbus cloud, often reaching miles into the atmosphere. This vertical growth is driven by the updraft, providing the storm with its power.
In certain instances, the updraft can twist or tilt as the storm develops. A tilted updraft can be a signal that the storm could become a supercell, which is a powerful type of thunderstorm known to produce the majority of significant tornadoes. According to Bunting, “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. 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].”
The Anvil-Shaped Top
Mature thunderstorms may develop a distinctive anvil-shaped extension at the top. This occurs when the powerful updraft reaches the tropopause, the boundary between the troposphere and the stratosphere. The anvil cloud then spreads horizontally, often extending for miles. Supercells frequently display dramatic, well-defined anvils.
Heavy Rain
Heavy rain can be an early indicator of a potential tornado. You might observe a dark, wispy column called a rain shaft descending from the storm clouds. According to Bunting, “A greenish or dark sky may signal a stronger thunderstorm,” increasing the likelihood of strong winds and hail. However, a rain shaft by itself doesn’t guarantee a tornado.
A significant risk is that tornadoes can be “rain-wrapped,” meaning obscured by heavy rain. This can make them difficult or impossible to see. Therefore, consider the rain as a potential warning and seek shelter.
The Wall Cloud
A wall cloud, a smaller, often circular cloud that forms below the main thunderstorm tower, is a common feature of many thunderstorms. Bunting states that a well-defined and persistently rotating wall cloud is a significant cause for concern.
Keep in mind that factors such as terrain and visibility can influence whether you even see a wall cloud. As Bunting advises, “I wouldn’t wait to see a wall cloud before taking shelter. But a well organized and persistently rotating wall cloud is of immediate concern.”
Rotation: A Key Warning Sign
Not every wall cloud rotates, but a rotating wall cloud is a serious sign. When the wall cloud exhibits persistent rotation, it significantly increases the possibility of a tornado developing. This rotation is an extension of the rotating updraft, also known as a mesocyclone, which can be several miles across.
Bunting emphasizes that, “The wind direction and speed throughout the lower portion of the atmosphere must be favorable for rotation to develop within the storm.” If the conditions are right, the mesocyclone can concentrate its rotation and lead to a tornado. If this happens, wind speeds can accelerate to damaging levels.
The Funnel Cloud: Before the Tornado
As the vertical rotation within the thunderstorm intensifies, a funnel cloud may form and extend downwards. It is important to remember that a funnel cloud is not yet a tornado. A tornado is defined as a rotating column of air that extends from the cloud base to the ground.
What you’re seeing is the condensation of water droplets. If the funnel cloud touches the ground, it will also pick up dust and debris from the surface.
Debris: The Final Confirmation
Once a funnel cloud touches the ground, it becomes a tornado. Even if the visible vortex isn’t readily apparent, you can still recognize a tornado by its effects.
According to Bunting, storm spotters look for signs of contact with the ground, such as debris swirling in the winds. Often, those tornadic winds do reach the surface, especially if the funnel cloud is halfway or more to the ground. The NWS uses radar to track a “debris signature,” to help confirm a tornado is on the ground. Since you don’t have access to this, it is crucial to seek shelter long before a tornado touches down.
Bunting stresses the importance of taking immediate action, “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.”
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, which inspired the movie Twister.