What is a derecho?

A derecho is a widespread, long-lived windstorm produced by an organized, fast-moving thunderstorm system, with damage laid down in roughly straight lines for hundreds of miles. It is not a storm type you can identify from one radar image. It is a verdict about what a system did over distance and time, usually confirmed after the damage reports are in.

This page goes deeper than the overview in squall lines, bow echoes and derechos: where the criteria came from, how they have been tightened, and what the radar record of a real derecho looks like from start to finish.

Where the word comes from

Gustavus Hinrichs, a physics professor at the University of Iowa, coined the term in 1888 to separate straight-line wind damage from tornado damage in his studies of Iowa storms. “Derecho” is Spanish; the Storm Prediction Center glosses it as “right,” “direct” or “straight ahead.” The term then dropped out of use for nearly a century, until Robert Johns and William Hirt revived it in their 1987 paper in Weather and Forecasting.

The definition, and how it has changed

Everyone in the literature agrees on the concept: a family of downbursts produced by a mid-latitude mesoscale convective system (MCS). The disagreement has always been about the numbers. Squitieri, Wade and Jirak (2023, BAMS) reviewed the history and found the quantitative thresholds varied enough between studies to change which events counted, and therefore where derechos appear to be most common.

The 2016 proposal, by Corfidi, Coniglio, Cohen and Mead in the Bulletin of the American Meteorological Society, argued that the original definition’s reliance on counting storm reports had let the word spread to wind events it was never meant for. The 2025 revision, by Squitieri, Wade and Jirak of the Storm Prediction Center, defines a derecho as “a widespread severe windstorm characterized by a family of destructive downbursts containing hurricane-force gusts associated with an extratropical, cold-pool-driven mesoscale convective system,” and identifies events with an algorithm rather than by judgment. SPC’s “Facts About Derechos” page now cites that revision.

The common thread across all three is sustained organization, not peak intensity. A single severe gust, however strong, does not make a derecho. A long chain of them from one system does.

Progressive and serial

Johns and Hirt described two types, and SPC still uses the split.

• Progressive derechos come from a relatively short line of storms, which SPC puts at roughly 40 to 250 miles long, that often takes the shape of a single bow echo. They are typically a warm-season, weakly forced phenomenon, travel along a relatively narrow path, and are hard to forecast because the large-scale pattern gives little warning.
• Serial derechos come from an extensive squall line, many hundreds of miles long, with multiple bowing segments embedded in it. They develop in strongly forced environments and sweep across an area both wide and long.

SPC also recognizes hybrids with features of both. The 2016 and 2025 definitions both restrict the term to cold-pool-driven systems; Squitieri et al. (2023) note that whether serial events should count remains debated, partly because their damage can be comparable.

What a derecho looks like on radar over its life

No single frame shows a derecho. What radar shows is the parent system evolving, and the useful skill is recognizing the stages.

1. Upscale growth. Separate cells or small clusters merge into a larger organized system. Klimowski, Hjelmfelt and Bunkers (2004), studying 273 bow echoes, found that 45% grew from weakly organized cells, 40% from squall lines and 15% from supercells, and that storm mergers preceded bow echo formation in roughly half of cases.
2. Bowing. A segment accelerates and bulges forward. The bow is the reflectivity expression of a strong cold pool and a rear-inflow jet pushing the leading edge ahead.
3. The rear-inflow notch. A channel of weak echo cutting into the back of the system, marking the descending jet. The same 2004 study cautions that a notch may only indicate damaging winds are imminent or already occurring; it is not reliable lead time.
4. Maturity. The system maintains itself for hours. SPC notes derecho-producing systems typically move at 50 mph or more, and that a rotating bookend vortex on the poleward side can enhance the wind. Fujita (1979) described bow echoes evolving into a comma-shaped echo with a dominant cyclonic vortex on that poleward side.
5. Decay. The bow loses definition and the swath ends. The June 2012 derecho, for example, weakened when it met maritime air off the Delaware and New Jersey coast.

The whole sequence usually crosses several radar sites. That is why a mosaic loop spanning many hours is the natural way to watch one, and why velocity from individual sites matters for the details: strong outbound or inbound winds behind the leading edge, at low elevation, show momentum close to the surface.

Two documented cases

29 June 2012, Ohio Valley to the Mid-Atlantic. SPC records a progressive derecho that formed in Iowa, moved about 700 miles in twelve hours, and produced a 91 mph gust at Fort Wayne, Indiana, the strongest June or July gust on record there. NWS Baltimore/Washington logged 71 mph at Dulles at 10:23 p.m. EDT. SPC reports 22 deaths and about five million people without power; Squitieri et al. (2023) add 34 heat-related deaths in the heat wave that followed, while power was still out.

10 August 2020, the Midwest. NWS Quad Cities reports estimated winds around 140 mph in Cedar Rapids and an unofficial measured gust of 126 mph at Atkins, Iowa, at 12:31 p.m. CDT, with many locations under damaging winds for 30 to 60 minutes rather than the more usual 10 to 20. Squitieri et al. (2023), citing Bell et al. (2022), call it the costliest thunderstorm event in U.S. history, with costs put at up to $11 billion.

Why so many windstorms get called derechos

After an event like 2020, the word enters general use, and it tends to get attached to any damaging line of storms. Most of those are severe thunderstorm wind events that fall short on length, continuity or the count of significant gusts. That is not a technicality. Corfidi et al. (2016) made the case that loose use of the term leads to miscommunication, and the 2025 revision raised the bar further by requiring multiple measured hurricane-force gusts.

A practical test: if the damage came from one storm, from rotation, or from a swath a county or two long, it is not a derecho under any of the three definitions. None of that makes the storm less dangerous where it hit.

Related: how to read weather radar, for the velocity fundamentals this page assumes.

WeatherStory Radar reads full-resolution Level II data straight from NOAA’s public archive.