Storms are guided by the flow of air around them, determined by the complex system of winds high in the atmosphere. For most people living in the mid-latitudes, the general rule is that weather systems, including storms, move from west to east across the landscape. This pattern is a direct result of global atmospheric circulation, which distributes heat and moisture across the planet.
The General Rule of Storm Movement
The primary force dictating the trajectory of weather systems in the mid-latitudes is a persistent band of winds known as the Prevailing Westerlies. These winds flow predominantly from west to east, acting as a massive atmospheric conveyor belt for nearly all non-tropical storms, known as extratropical cyclones. This predictable flow is why a storm tracked in the western United States one day is generally expected to arrive in the central or eastern states days later.
The existence of the Prevailing Westerlies is rooted in two fundamental principles of atmospheric science: differential heating and the Coriolis effect. Solar energy warms the Earth unevenly, with the equator receiving significantly more heat than the poles, causing warm air to rise and cold air to sink. This temperature gradient drives the global atmospheric circulation cells, which redistribute this energy.
The flow of air within these cells is then acted upon by the Coriolis effect, a deflective force caused by the Earth’s rotation. In the Northern Hemisphere, this force deflects moving air to the right, transforming poleward flow into a persistent current blowing from the west. The Prevailing Westerlies are a direct consequence of this, providing the baseline expectation for storm movement.
Steering Currents: The Role of Pressure Systems and the Jet Stream
While the Prevailing Westerlies set the general west-to-east direction, the precise path and speed of a storm are controlled by atmospheric features called steering currents. The most influential of these is the Jet Stream, a fast-flowing, narrow river of air located high in the atmosphere. The Jet Stream acts as the primary highway for low-pressure storm systems, which tend to follow its meandering path.
This upper-level wind flow is created by the sharp temperature contrast between cold polar air and warmer subtropical air masses. Storms follow the Jet Stream because its dynamics create areas of divergence aloft, which is crucial for initiating and strengthening low-pressure systems. The path of the Jet Stream is rarely a straight line, instead developing large waves known as Rossby waves, which feature troughs (southward dips) and ridges (northward bulges).
The location of a storm relative to these waves determines its direction; storms often develop and deepen beneath the troughs, while the ridges are associated with clearer, more stable weather. High-pressure centers, or anticyclones, play an important role by acting as walls that block a storm’s path. Since a low-pressure storm cannot easily move into an area of higher pressure, it is forced to skirt around the periphery of the high-pressure system.
These blocking high-pressure systems can cause a storm to slow down, stall completely, or be diverted sharply, leading to prolonged periods of rain or drought in a single region. For example, a high-pressure system that becomes “stuck” over a continent can force the Jet Stream and its associated storms to take a long detour around it. The combined interaction of the Jet Stream’s flow and the location of high-pressure blocks dictates the movement of any given storm.
Major Exceptions: Tropical Storms and Hurricanes
The west-to-east general rule has a significant exception in the movement of tropical cyclones, which include tropical depressions, tropical storms, and hurricanes. These systems form closer to the equator, where they are initially governed by a different set of winds known as the Trade Winds. The Trade Winds blow from the east toward the west, pushing newly formed tropical systems on a westward track across the ocean.
In the Atlantic, storms are commonly carried westward from the coast of Africa toward the Caribbean and North America. This initial movement is relatively slow and continues until the storm moves poleward to higher latitudes. The movement then becomes more complex as the storm begins to interact with the larger atmospheric features of the mid-latitudes.
As a tropical storm moves away from the equator, it often enters a crucial phase known as “recurvature.” This is when the storm’s path bends sharply, turning toward the north and then the northeast. Recurvature occurs because the system leaves the influence of the Trade Winds and begins to encounter the Prevailing Westerlies, which then steer the storm toward the east.
Additionally, large, semi-permanent high-pressure systems, such as the Bermuda High in the Atlantic, act as powerful steering mechanisms. The clockwise rotation of air around these highs forces tropical systems to move around their edges. If the storm is to the south of the high, it is steered westward, and if it reaches the western edge, the flow around the high turns it northward and then northeastward.