A wake low is a highly localized and potentially hazardous weather event that occurs in the aftermath of a large, organized thunderstorm system. This low-pressure phenomenon often develops behind a powerful line of thunderstorms, such as a squall line or a Mesoscale Convective System (MCS). The presence of a wake low can lead to sudden, damaging winds, often occurring precisely when residents assume the severe weather threat has passed.
Defining the Wake Low Phenomenon
A wake low is formally defined as a mesoscale low-pressure area, also known as a mesolow or wake depression, that forms to the rear of a mature squall line. The term “mesoscale” indicates it is a small-scale feature, typically spanning tens to a few hundred kilometers across and lasting for only a few hours. This low-pressure feature trails a region of higher pressure, called a mesohigh, which is created by the cold, dense air produced by the storm’s downdrafts.
The mesolow is most commonly found in the trailing stratiform precipitation region of the storm, often near the back edge where the steady rain is ending. The wake low is situated in the “wake” of the main storm complex, which is where it derives its name. Its existence is linked to the parent storm and it will typically weaken once new thunderstorm development along the squall line ceases.
The Mechanism of Wake Low Formation
The creation of a wake low is a dynamic process rooted in the vertical movement of air behind the storm complex. The primary mechanism is atmospheric subsidence, or downward-moving air, which causes adiabatic warming. As air sinks, it is compressed, leading to a rise in temperature and a decrease in density in the mid-to-lower atmosphere. This warming and drying air mass above the surface layer leads to a rapid hydrostatic pressure fall at the ground, caused by the reduced weight of the air column.
This sinking motion is frequently driven by the descent of the rear-inflow jet (RIJ), a stream of fast-moving air that rushes into the back of the storm system. The RIJ develops as a response to the contrasting pressure fields within the storm, specifically the surface mesohigh created by the cold pool and a mid-level mesolow. When a portion of this jet descends toward the surface, it causes the air to warm through compression, which directly contributes to the wake low’s formation. The resulting pressure minimum is located precisely where the subsiding, warmer air mass is strongest, often near the very back of the rain shield.
Associated Hazards and Severe Weather Impacts
The meteorological danger posed by a wake low stems from the extreme pressure gradient it creates with the preceding mesohigh. Air rushes from areas of high pressure to low pressure, and the tight spacing between the mesohigh and the wake low generates high-speed, straight-line winds. These winds can be strong, often gusting between 50 and 70 miles per hour, and their damaging force can be mistaken for a weak tornado.
A hallmark of a wake low passage is a sudden, dramatic drop in barometric pressure, sometimes falling by several millibars in minutes, followed by a sharp rise as the feature moves away. The intense winds occur as this low-pressure center passes directly overhead.
A related, though rarer, hazard is the heat burst, which can occur in association with a decaying wake low. A heat burst is characterized by gusty winds alongside a rapid and significant jump in temperature and a decrease in humidity. This event is caused by a parcel of air aloft that descends rapidly, warming dramatically due to compression as it reaches the surface. Temperatures can increase by 20 degrees Fahrenheit or more in minutes.
Challenges in Forecasting and Detection
Anticipating the precise development and track of a wake low presents a significant challenge for forecasters due to its small scale and short lifespan. Because a wake low is a mesoscale feature, it can easily fall below the spatial and temporal resolution capabilities of standard numerical weather models. Models with coarse grid resolutions may not accurately simulate the intricate processes of the rear-inflow jet and the resulting pressure drops.
Meteorologists rely heavily on high-density surface observation networks, or mesonets, to detect the rapid pressure fluctuations that signal a wake low’s presence. Tracking these events in real-time is also dependent on modern Doppler radar systems, which provide high-resolution data on wind fields and precipitation patterns. Even with advanced tools, the sudden nature of a wake low means that forecasters must often issue warnings based on rapid, real-time analysis rather than long-range model guidance.