While most tornadoes travel quickly, typically moving at a forward speed of 20 to 40 miles per hour, some can slow down dramatically or appear to stop entirely. A tornado is considered slow-moving or “stationary” when its forward translation speed drops below five miles per hour. This unusual meteorological scenario does not mean the danger has passed. The tornado continues to spin violently, concentrating its destructive power over a small area for an extended period. This situation requires immediate and specific safety actions because the threat is magnified by the duration of the storm’s presence.
Understanding “Stationary” Tornadoes
The movement of a supercell thunderstorm, and the tornado it produces, is dictated by the environmental steering flow. This flow refers to the average wind speed and direction throughout the layer of the atmosphere where the storm is located. When a strong jet stream or upper-level winds are present, they act like a conveyor belt, pushing the storm system quickly across the landscape.
A lack of well-defined steering flow is the primary cause of a stationary or slow-moving tornado. If the mid- and upper-level winds are weak or disorganized, the parent thunderstorm can lose its forward momentum, causing the entire rotating structure to stall in one location. This meteorological configuration can result in a storm that spins in place without translating significantly.
Local atmospheric features can also cause a storm to briefly stall or move erratically. For instance, a supercell may encounter a boundary, such as an outflow boundary from a previous storm, which temporarily disrupts the storm’s inflow of warm, moist air. This interaction can cause the storm to suddenly turn, loop, or momentarily stop as it attempts to reorganize itself along the boundary.
When a storm stalls, the updraft responsible for sustaining the tornado’s rotation can become vertically stacked, where the rotation aloft aligns directly over the surface rotation. This vertical alignment contributes to the storm’s inability to move forward. The tornado will then appear to whirl violently while remaining fixed over a single area before either dissipating or resuming movement.
The Unique Dangers of Stalled Tornadoes
The lack of forward motion in a tornado significantly increases the danger to any structure caught in its path. A fast-moving tornado exposes a building to maximum wind speeds for only a few seconds. However, a stalled tornado subjects the same structure to tornadic winds and debris for minutes, sometimes even longer. This prolonged exposure dramatically increases the likelihood of catastrophic structural failure.
The sustained force allows the internal wind pressures to work on a building’s connections, leading to progressive failure of the roof, walls, and foundation. This extended duration of maximum stress can cause a moderate-strength tornado, like an EF-3, to produce damage over a concentrated area. This destruction is comparable to that typically associated with a much more intense, but fast-moving, EF-4 or EF-5 tornado.
A stationary tornado also presents a unique challenge for public perception and warning systems. The visual appearance of a tornado remaining in the same place can be confusing, making some people assume the threat has passed or that the storm is weak. However, the duration of the threat is extended, and the National Weather Service warning polygon will remain active for an unusually long time.
A stalled tornado’s path can become highly unpredictable, as the storm may suddenly re-engage with a weak steering wind or boundary and move off in an unexpected direction. This erratic behavior makes any attempt to outrun the storm particularly dangerous. The concentration of damage in a small segment of the path can overwhelm local emergency services and recovery efforts in the immediate aftermath.
Immediate Safety Procedures
When a tornado is stalled or moving extremely slowly nearby, the primary safety action is to immediately seek the safest shelter available and remain there. This shelter should be a basement, a storm cellar, or an interior room on the lowest floor of a sturdy building, away from all windows. The time spent in shelter must be significantly longer than for a typical tornado, which usually passes in less than a minute.
Continuously monitor local alerts from a NOAA weather radio or trusted local media, even while in the shelter. The warning polygon for a stalled storm will remain in effect for an extended period. The threat only dissipates once the National Weather Service issues an all-clear or confirms the storm has moved away or dissipated. Do not emerge from your shelter until the warning has officially expired or has been canceled for your specific location.
Attempting to evacuate in a vehicle when a tornado is stalled in your immediate vicinity is discouraged. The path of a stalled tornado is unpredictable, and the debris field is concentrated, making roadways hazardous. The safest course of action is to shelter in place, as the structure of a home or building offers a much higher probability of survival than a vehicle, which is easily destroyed by tornadic winds.