A squall is a rapid meteorological event characterized by a swift, sharp increase in wind speed that lasts for a few minutes before quickly decreasing. Because this phenomenon is sudden and intense, the National Weather Service (NWS) issues high-priority alerts to warn the public of the imminent danger. These warnings prompt quick action, as the destructive winds arrive with little advance notice.
Defining the Squall and Its Warning
A squall is a meteorological event defined by a sudden, sustained increase in wind speed of at least 18 miles per hour, reaching a minimum of 25 miles per hour, and lasting for at least one minute. In warmer months, this wind event is commonly associated with a squall line, a long, linear formation of thunderstorms. This line often produces damaging straight-line winds that rush out ahead of the main rain shaft, creating a distinct, powerful wind front.
The official alert for this dangerous, non-winter event is typically issued as a Severe Thunderstorm Warning by the National Weather Service. This warning is triggered when a storm is observed or indicated by radar to be producing wind gusts of 58 miles per hour or greater, or hail one inch in diameter or larger. The warning specifically addresses the high-speed winds expected from an advancing squall line. Warnings are short-fused and focus on a specific area, delivered as a polygon alert to pinpoint the region directly in the storm’s path.
The force behind a squall is often a concentrated downburst of air from the thunderstorm, which is why the winds are referred to as straight-line winds. These powerful air currents spread out rapidly when they reach the ground and can cause damage similar to a weak tornado. The sudden onset and force of these winds mean that structures, trees, and vehicles can be affected almost instantly.
How Squall Warnings Differ from Other Weather Alerts
The primary difference between the wind-focused Severe Thunderstorm Warning and other alerts lies in the specific hazard being emphasized. This warning is issued when winds reach 58 mph or more, or hail reaches one inch, covering the destructive potential of a squall. The NWS introduced damage threat tags for these warnings, with the highest “destructive” tag reserved for wind gusts of 80 mph or greater, characteristic of a severe squall line.
The Tornado Warning is issued when a rotating column of air has been spotted or indicated by radar, posing a threat of a funnel cloud reaching the ground. While a squall line can sometimes produce brief, weaker tornadoes, the main danger is the straight-line wind, which covers a much larger area than a typical tornado track. The Tornado Warning focuses on rotation, whereas the squall threat emphasizes the sudden, non-rotational wind force.
A Severe Thunderstorm Warning is distinct from a High Wind Warning, which is issued for non-thunderstorm related winds, such as those caused by strong pressure gradients. The squall warning focuses on the immediate, convective threat that is part of a thunderstorm system. For mariners, a Special Marine Warning is specifically issued for squalls when wind gusts of 39 miles per hour or greater are observed or forecast over water.
Immediate Safety Measures During a Squall Warning
When a Severe Thunderstorm Warning is issued for the wind threat posed by a squall, action must be taken immediately due to the rapid arrival of the hazard. The most important step is to move indoors to a sturdy structure and seek shelter on the lowest level. Stay away from all windows, glass doors, and skylights, as these can shatter from the impact of wind-driven debris.
If a basement or underground shelter is not available, move to an interior room or hallway on the ground floor, putting as many walls as possible between you and the outside. Securing loose objects outdoors, such as patio furniture, trampolines, and trash cans, should be done before the storm arrives. These items can quickly become dangerous projectiles in high winds.
For those driving, especially in high-profile vehicles like trucks or vans, pull over immediately to a safe location away from trees, power lines, and tall signs. If caught on open water, boaters should head to shore at the first sign of a squall and never attempt to ride out the storm. The sudden increase in wind speed and wave height associated with a marine squall can quickly overwhelm a small vessel.