Squall Line
Also called: quasi-linear convective system, QLCS
A squall line is a line of active thunderstorms, either continuous or with breaks, together with the rain areas the storms produce. Squall lines can extend for hundreds of miles and last for many hours. Their main hazard is straight-line wind damage, though tornadoes sometimes form along the leading edge.
Key Points
- A shelf cloud often forms along the leading edge, marking the arrival of the gust front.
- A pre-frontal squall line is a line of thunderstorms that runs ahead of an advancing cold front.
- When part of the line surges ahead of the rest, it forms a bow echo, the radar signature of the strongest straight-line winds.
- A long-lived squall line that keeps producing damaging wind over hundreds of miles is a derecho.
Related Terms
Bow Echo
A bow or crescent-shaped radar signature indicating strong straight-line winds. Bow echoes form when the center of a squall line accelerates ahead of its ends, often producing damaging winds exceeding 75 mph.
Derecho
A derecho is a widespread, long-lived windstorm produced by a fast-moving line or cluster of severe thunderstorms. Under the criteria the Storm Prediction Center uses, the swath of wind damage must be at least 250 miles (about 400 km) long, with gusts of at least 58 mph along its length and several well-separated gusts of 75 mph or greater.
Shelf Cloud
A shelf cloud is a low, horizontal, wedge-shaped cloud attached to the base of a thunderstorm along its gust front, the leading edge of the storm's rain-cooled outflow. Rising cloud motion is often visible on its leading edge, while the underside looks turbulent and wind-torn. It means gusty winds are about to arrive.
Cold Front
A cold front is the boundary where a cooler, denser air mass is advancing and replacing warmer air. The cold air wedges under the warm air and forces it upward, which often produces a band of showers or thunderstorms, followed by a wind shift and cooler, drier air.
Downburst
A strong downdraft that induces an outward burst of damaging winds at or near the surface. Downbursts can be either microbursts (less than 2.5 miles) or macrobursts (larger than 2.5 miles) and can cause damage similar to a tornado.
Supercell
A supercell is a thunderstorm with a deep, persistently rotating updraft called a mesocyclone. Supercells are the most dangerous type of thunderstorm: they produce nearly all significant tornadoes in the United States and most hail larger than golf balls, along with damaging winds and flash flooding.
Squall Line: Guides and Explainers
These explainers connect this definition to real-world forecasting and decision-making.
Weather Education
How Tornadoes Form: Types, EF Scale, and Safety
Learn how tornadoes form from supercell thunderstorms, the Enhanced Fujita scale, types of tornadoes, and critical safety steps to protect yourself.
Weather Education
What Is a Derecho? The Inland Hurricane Explained
Learn what a derecho is, how it differs from tornadoes and hurricanes, why these long-lived windstorms are so destructive, and how to stay safe when one approaches.
Guides
Weather Radar Colors Explained: How to Read Radar Like a Pro
Learn what weather radar colors mean, from light green to purple. Understand reflectivity, precipitation intensity, and how to interpret radar images.
Helpful Tools and Hubs
Use these pages to apply the concept, compare conditions, or follow related weather workflows.
Feature
Severe Weather
Follow outlooks, watches, warnings, and storm signals in one severe-weather workflow.
Guide
What Causes Thunder and Lightning?
How lightning forms through charge separation, what causes thunder, and how to stay safe.
Guide
Lightning Safety Guide
Know when lightning becomes dangerous, how far away it can strike, and when to move indoors.
Glossary
Tornado
A tornado is a violently rotating column of air in contact with the ground, extending from a thunderstorm.
Sources
Explore Other Terms
Ice Storm
A storm that produces significant ice accumulation from freezing rain, typically 0.25 inches or more. Ice storms can down trees and power lines, causing widespread outages lasting days to weeks. They are among the most damaging winter weather events.
Small Craft Advisory
A Small Craft Advisory is a National Weather Service alert for winds or seas that are hazardous to small boats in coastal and Great Lakes waters. The wind and sea thresholds are set separately for each geographic area. There is no precise definition of a small craft: any vessel that the conditions could affect counts.
Red Flag Warning
A Red Flag Warning is a National Weather Service alert that weather conditions could cause extreme fire behavior. It is issued when those conditions are occurring or expected within 24 hours. Typical criteria combine dry fuels with sustained winds of 15 mph or more and relative humidity of 25% or less, and exact thresholds vary by state and local office.
El Niño and La Niña
El Niño and La Niña are the warm and cool phases of a recurring climate pattern in the tropical Pacific called the El Niño-Southern Oscillation (ENSO). During El Niño, the trade winds weaken and the central and eastern tropical Pacific runs warmer than average. During La Niña, the trade winds strengthen and the same waters run cooler. Both shift weather patterns worldwide.
Kp Index
The Kp index is a 0 to 9 scale of global geomagnetic activity, reported in 3-hour intervals from ground-based magnetometers. A Kp of 5 or higher indicates a geomagnetic storm, and the values map onto NOAA's G-scale from G1 (Kp 5) to G5 (Kp 9). The higher the Kp, the farther from the poles the aurora can be seen.
Aftershock
A smaller earthquake following a mainshock in the same area, caused by the crust adjusting to redistributed stress. Aftershocks are most frequent in the first hours and days and taper off over time, but can continue for months or years after a large event. If one turns out to be larger than the original earthquake, it is reclassified as the mainshock.
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