Squall Lines, Bow Echoes & Derechos

In short: most severe thunderstorm wind comes from lines of storms, not from tornadoes. Rain-cooled air pours down out of the storms, hits the ground, and spreads out ahead of them. When that cold air is deep and a jet of air from behind the line feeds it, part of the line bulges forward into a bow, and the wind under the bulge can do tornado-like damage over a wide area. A long chain of those is a derecho.

Lines of storms

Forecasters call an organized line a QLCS, a quasi-linear convective system. It forms when storms are close together and the shear lines up so they merge. A line can be a few dozen miles long or stretch across several states.

The engine of a line is its cold pool: a dome of air chilled by rain and evaporation that collects under and behind the storms. The front edge of the cold pool is the gust front. As it plows forward, it lifts warm, humid air ahead of it, which builds new storm cells along the front of the line. That is why a line keeps going long after any one cell inside it has died. A shelf cloud, the dark wedge in the photo on Watches, Warnings & Outlooks, is the gust front seen from the ground.

Downbursts and microbursts

A downburst is a strong downdraft that hits the ground and spreads out in all directions. The damage it leaves fans outward, which is how surveyors tell it from a tornado, where debris is thrown in a twisting pattern.

• Microbursts are small, about 2.5 miles (4 km) across or less. They can produce winds over 100 mph in a few minutes, and they are a serious hazard to aircraft.
• Wet microbursts come out of heavy rain cores in humid summer air. Heavy rain, and especially hail, melting and evaporating, chills the air and drags it down.
• Dry microbursts come from high-based storms over very dry air near the ground, common in the West. Light rain evaporating on the way down chills the air hard. The radar may show little more than virga. On a Skew-T, the setup is the inverted V.

On velocity, a downburst is a divergence signature: inbound winds directly beside outbound winds along the same beam, the air spreading apart near the ground. Doppler Velocity, Explained covers how to read it.

The environment number for downdraft strength is DCAPE, covered in CAPE & Instability.

In MesoRadar: DCAPE is under the Skew-T tool's More fold (Pro).

The bow echo

A bow echo is a segment of a line that bulges forward. It happens when the cold pool is strong and a stream of air from behind the line, the rear inflow jet, descends toward the ground and pushes the middle of the line ahead.

What to look for on radar:

• The bow shape itself, on base reflectivity: a line segment curving out ahead of the rest.
• A rear inflow notch: a notch of weaker echo cut into the back of the line just behind the apex. It is drier air from behind the line eating into the rain, a sign the rear inflow jet is strong.
• Strong velocity behind the leading edge, on base velocity at the lowest tilt. Look for a band of strong winds aimed along the beam, toward or away from the radar, just behind the apex. The beam measures only the part of the wind along its own direction, so a line moving straight at or away from the radar shows its full speed, and one moving across the beam shows much less.

Bookend vortices

As a bow matures, rotating circulations often form at its ends. In the Northern Hemisphere the northern one is usually the stronger, turning counterclockwise, and it can curl the north end of the line into a comma head. The circulation focuses the rear inflow toward the apex, and it can last for hours.

Tornadoes in a line

Squall lines produce tornadoes too. They form from small circulations, called mesovortices, along the leading edge of the line, most often near and just north of the apex of a bow or where the line kinks.

They are hard to warn on:

• They form quickly. In squall lines the tight rotation signature has been found to appear only around five minutes before the tornado (Trapp et al. 1999).
• They are shallow, so a beam that is already a few thousand feet up at moderate range can pass over them.
• The numbers show it. Brotzge and colleagues (2013) found tornado warnings for non-supercell storms caught fewer tornadoes, with shorter lead times, than warnings for supercells.

The practical rule: treat every warning on a line as real, even when nothing dramatic shows on radar. On velocity, scan the leading edge frame by frame for small, tight pairs of inbound and outbound winds, especially near the bulges. On radar, that pair is strong rotation. It is a reason for a warning, not proof of a tornado.

Many NWS offices use the three-ingredients method (Schaumann and Przybylinski 2012) to pick the stretch of line to watch: the cold pool and the low-level shear ahead of it are about balanced, so the leading edge stays upright; the 0 to 3 km shear across the line (perpendicular to it) is about 30 kt or more; and a rear inflow jet surge or bow is pushing the line forward. Where all three meet, mesovortices are most likely, usually just north of where the surge hits, so watch velocity there closely. An NWS evaluation found the method reasonably skillful at short lead times, best for clearly linear systems (Gibbs 2021), so treat the method as a guide to where to look, not a forecast.

Read the bottom of a Severe Thunderstorm Warning, too. When forecasters see a chance of a quick spin-up but not enough to issue a Tornado Warning, they add the tag TORNADO…POSSIBLE. On a line, treat that warning as if a tornado could form with little notice: get to your shelter spot, not just indoors.

Derechos

A derecho, from the Spanish for “straight,” is the big version: a long-lived system that keeps producing downbursts along a path hundreds of miles long. SPC’s working definition, in plain words:

• a swath of wind damage at least 250 miles (about 400 km) long,
• gusts of at least 58 mph along most of its length,
• several well-separated gusts of 75 mph or more,
• from a storm system moving faster than the mean wind, driven by its own cold pool.

There are two broad kinds. A progressive derecho is a relatively short line, often a single large bow, that races along a nearly straight path, usually in the warm season. A serial derecho is a long squall line with several bows along it, often tied to a strong low-pressure system.

On August 10, 2020, a derecho crossed Iowa and northern Illinois and into Indiana. NWS Des Moines measured an official gust of 99 mph at Marshalltown, Iowa, and NWS Quad Cities estimated winds near 140 mph from damage in Cedar Rapids.

What makes wind "severe"

The NWS issues a Severe Thunderstorm Warning for gusts of 58 mph or more, or hail 1 inch or larger. Warnings carry a damage threat tag. Considerable is for gusts of 70 mph or more or hail 1.75 inches (golf ball) or larger. Destructive is for gusts of 80 mph or more or hail 2.75 inches (baseball) or larger, and only that tag sets off Wireless Emergency Alerts on phones, the same way a Tornado Warning does. A warning can also carry the tag TORNADO…POSSIBLE (see Tornadoes in a line). The full ladder is in Watches, Warnings & Outlooks.

In MesoRadar: warnings are drawn on the map as the shapes the NWS issued. The Outlooks sheet's Severe outlook has a Wind chip, the SPC's probability of damaging wind (Day 1 Risk is free; the probability chips are Pro). Storm reports show wind gusts and damage with a blue badge. Map symbols.