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       The violent storms that swept across the D.C. area Saturday afternoon and evening, the worst in more than a decade for some, were sparked by extreme heat and humidity, a strong cold front, and a vigorous disturbance high in the atmosphere.

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       Producing winds over 80 mph, the storms toppled trees onto buildings and cars, dislodged chimneys, tore down signs, peeled off roofs and cut power to more than 200,000 customers. Sections of Arlington and Northwest Washington, where thousands remain without power, were among the hardest hit.

       The storms that blasted the region didn’t contain tornadoes nor was there a derecho, which is a long-lived and large complex that often stretches across multiple states. Saturday’s storms were much more localized. But their winds were just as strong and the damage as devastating in some areas as those produced by some tornadoes and derechos.

       Watch: Many #Arlington neighborhoods had an intense 2 or 3 mins during today's storm around 5:00. Watch these 30 secs in Va Square area. More tomorrow. Check the white sign in the median. From #SafetyVid. @capitalweather @dougkammerer @ARLnowDOTcom @StormHour #weather #stormhour pic.twitter.com/i2Z7tlC0Az

       — Dave Statter (@STATter911) July 30, 2023

       The source of Saturday’s fierce winds was a phenomenon known as downbursts. They are common in summer storms and are simply blasts of wind that originate in the clouds and then slam into the ground and fan out. They can produce gusts more severe than low-end tornadoes and are, by far, the most frequent source of violent winds in D.C.-area storms.

       Very localized downbursts — affecting parts of neighborhoods — are frequently called microbursts, whereas larger ones — that can cover miles — are referred to as macrobursts. Saturday’s storms may have contained both types.

       Saturday marked the third day in a row that excessive heat fueled storms that unleashed strong downbursts in the region. But its storms were given a boost by the temperature contrast incited by the incoming cold front.

       Inside Saturday’s storm

       A narrow but solid line of storms, known as a squall line, began forming in D.C.’s northern and western suburbs during the midafternoon ahead of the front, at the same time a high-altitude atmospheric disturbance was passing by overhead. The effect of this disturbance was to help lift the hot air near the ground up high into the sky where it could cool, condense and form billowing clouds.

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       The radar snapshot of precipitation intensity at 5 p.m. (left side of image below) shows the maturing squall line as it ripped through the District, Arlington, central Fairfax County and Northern Prince George’s County, already having plowed through areas north and northwest of the Beltway.

       The squall line consisted of small clusters of storm cells, called multicells, which kept regenerating as the line advanced forward because of the combination of an unstable air mass (hot air near the ground and colder air aloft) and winds that intensified with altitude.

       The right panel of the figure above shows the radar-estimated winds at the same time. The peach tones embedded within that panel spell trouble. We’ve highlighted one pocket near Bailey’s Crossroads in eastern Fairfax County where the wind speed only a couple thousand feet above the surface was nearly 72 mph.

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       Those embedded, small pockets of intense winds, which generally last less than five to 10 minutes, are the downbursts. As they accelerate downward from storm clouds and then outward upon reaching the ground, they can create a roaring sound.

       View of “The storm” from my balcony in Arlington, VA. (7/29/23) Minutes later, the power went out. @capitalweather pic.twitter.com/c575oqaJ8v

       — Priya Mathew (she/her) (@pmathew814) July 29, 2023

       In these downbursts, sudden gusts over 60 mph can take out even a stout tree, especially those weighed down by full foliage and when the ground is fairly saturated from recent days of thunderstorm rain.

       Why there weren’t tornadoes

       What this squall line did not contain was small pockets of rotation called mesocyclones, which breed tornadoes. Exceptionally strong wind shear — winds that change in direction or speed with altitude — is needed for mesocyclones to develop, but the strongest shear Saturday afternoon remained well north of the region; there were tornadoes reported in New England.

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       Thus, the wind damage, which was widespread and significant across the D.C. area, was most likely driven by non-tornado “straight-line” winds produced by downbursts.

       Severe thunderstorm warning in Washington DC with 80 MPH winds. This is the scene downtown: pic.twitter.com/sE9JCmEzRV

       — Carl Nasman (@CarlNasman) July 29, 2023

       Why this wasn’t a derecho

       Derechos are defined as a long-lived band of curved or bow-shaped thunderstorms producing a swath of continuous wind damage hundreds of miles long. The derechos we see around here typically arrive from very distant locations, such as Ohio and even farther west.

       What came through Saturday was a “homegrown” squall line that cropped up along and east of the Blue Ridge Mountains.

       What was amazing about this squall line was how fast it formed.

       At 2 p.m., skies were mostly sunny and radar showed no storms in the region. Then, between 3 and 4 p.m., small, disorganized storm cells that developed east of Interstate 81 began to quickly organize:

       3:43 pm: First severe storm warning of the day, runs until 430pm. Covers much of area northwest of Beltway from Leesburg to Rockville and surrounding areas. 60mph wind gusts and quarter size hail possible. pic.twitter.com/5v7ST48TDS

       — Capital Weather Gang (@capitalweather) July 29, 2023

       By 4:30 p.m., a solid line of fierce storms had consolidated and was already unleashing damaging gusts in many areas:

       430p: Radar shows nasty line of storms from near Columbia to Gainesville entering northwest part of Beltway and sweeping east and southeast. Recent gust to 69 mph in Germantown. These are serious storms that will affect much of metro area. Stay inside until they pass. pic.twitter.com/wLuDR1bbbH

       — Capital Weather Gang (@capitalweather) July 29, 2023

       The squall line’s rapid development was certainly aided by the extremely hot and humid environment. A measure known as convective available potential energy (CAPE) — which is essentially the fuel for storms — was over 3,000 Saturday afternoon. That’s a very high value for this area.

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       A number of people who endured the worst of Saturday’s storms compared them to the June 29, 2012, derecho in the area, which unleashed widespread 60-to-80-plus mph winds. (That storm produced CAPE that topped 5,000.)

       Saturday’s storm was certainly the worst of the summer so far and, for the hardest hit areas, the worst since at least that infamous derecho. Mercifully, storms aren’t in the forecast again until the end of the upcoming workweek.

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