Thunderstorms are localized weather systems characterized by lightning and thunder. These powerful, short-lived events occur within cumulonimbus clouds, often bringing heavy precipitation, strong winds, and sometimes hail. Understanding a thunderstorm’s path is important for public safety, as the speed and direction of its movement dictate who will be affected and when. The movement of these storms is governed by larger forces high above the surface.
The Predominant Direction of Storm Movement
In the middle latitudes, including much of North America and Europe, thunderstorms adhere to a general movement pattern. The vast majority of weather systems in these regions are propelled from a westerly direction toward the east. This tendency is due to the prevailing global wind circulation known as the Westerlies.
Consequently, most thunderstorms travel from the west toward the east or from the southwest toward the northeast. This pattern follows the large-scale atmospheric flow that dominates continental weather. While this is the expected rule for storm motion, the specific speed and trajectory can vary significantly based on surrounding atmospheric conditions.
Understanding Atmospheric Steering Currents
The mechanism that dictates a thunderstorm’s path is the concept of “steering currents.” A thunderstorm is essentially carried along by the momentum of the wind field in which it is embedded. This steering effect is driven by winds at higher altitudes, not surface winds.
Meteorologists look at the wind flow between the 700-millibar (mb) and 500-mb pressure levels to determine the steering current. These levels correspond roughly to altitudes between 10,000 and 18,000 feet above the ground. Winds at this height are stronger and more stable than those closer to the surface, providing a reliable guide for the storm’s track.
A thunderstorm’s movement speed is typically about half the speed of the wind flow at the 500-mb level. The storm moves in the same direction as the wind at this altitude, following the general contours of the upper-level flow. For example, if the 500-mb winds blow to the northeast, the storm is likely to track in that direction, often moving at speeds ranging from 20 to 40 miles per hour.
Factors That Cause Movement Variation
While the upper-level steering flow provides the general direction, several factors can cause a storm’s movement to deviate from the predicted path.
Weak Steering Currents
A common variation occurs when the steering currents are weak, meaning the winds aloft are moving slowly. When this happens, a thunderstorm can become nearly stationary or move at a very slow pace. This slow movement sometimes leads to significant localized flash flooding due to continuous heavy rainfall over one area.
Propagation
The process known as “propagation” can make a storm complex appear to move against the wind or in an unexpected direction. This occurs when the storm’s outflow boundary—a surge of cool air—pushes up warm, moist air ahead of it. New storm cells constantly form on the leading edge, even as original cells dissipate. The continual development of new cells gives the impression that the entire complex is rapidly moving forward.
Local Boundaries and Terrain
Local boundaries also influence a storm’s immediate trajectory and intensity. An outflow boundary from a cluster of storms can act like a miniature cold front, lifting the surrounding air to trigger new convection. Terrain features or local fronts can temporarily alter the wind field near the surface. Stronger storms like supercells can also deviate by turning slightly to the right of the mean steering flow due to interactions with vertical wind shear.