How Tuesday’s storms triggered high winds in Maryland and Virginia

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Hundreds of trees were toppled and hundreds of thousands of people lost power Tuesday afternoon and evening as three storm complexes roared through the Washington area. The storms were fueled by warm, moist air sweeping over the region, and were fueled and supported by strong high-level winds along an approaching cold front.

Two complexes in particular were responsible for the lion’s share of the damage. They were both arch-shaped arches that swept from west to east. The first plowed from Washington County to Cecil County in northern Maryland in the late afternoon. The second bombarded Northern Virginia, the District, and central Maryland during the evening. The two knocked down dozens of trees and wires, some on houses and cars.

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A third, somewhat less intense complex moved through north-central Virginia and southern Maryland late in the evening, leaving behind scattered reports of damaging winds and at least one case of flash flooding.

In total, the National Weather Service received approximately 150 damage reports from the three complexes. Pockets of hail also formed in the storms, some as wide as half dollars. PowerOutage.US reported that more than 70,000 outages still remained as of Wednesday afternoon, nearly 24 hours after the storm – the majority in northern Maryland.

All three complexes generated downward bursts of high winds that slammed into the ground and spread outward. The National Weather Service received about 40 reports of winds between 39 and 70 mph. Here are some gust peaks recorded:

  • Downtown: 110 km/h
  • Leesburg: 100 km/h
  • Washington Dulles International Airport: 100 km/h
  • Quantico: 90 km/h
  • Reagan National Airport: 82 km/h

Within a one-county Washington radius, concentrated areas of damage were reported near Chantilly, Olney, College Park and Bowie. Olney appears to be the hardest hit, where trees have damaged up to 20 homes and around half a dozen families have been displaced.

Damage was also extensive in College Park, where a man was seriously injured when a tree fell on a house. The weather service concluded that peak winds in the area reached 80 to 90 mph and the winds were blowing in a straight line, ruling out a tornado.

“Straight-line winds from thunderstorms can produce tornado-like damage,” the weather service wrote in a survey of the storms. “Last night’s estimated winds in College Park are equivalent to those of a low-end EF1 tornado.”

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The storm that caused the severe wind damage at Olney and College Park is known as the Long Track Bow Clapper. An arc echo, unlike a supercell which organizes around a rotating updraft, is powered by a powerful downdraft. The intense downslope thrust causes the front of the storm line to arc outward, rushing past adjacent regions. Bow echoes travel frequently at high speeds of at least 40-50 mph.

The radar image below shows the bow originated over West Virginia, south of Romney, between 3 and 4 p.m., as monitored by the Weather Service’s Storm Prediction Center. The Meteorological Service office in Sterling, Va. issued a continuing series of severe thunderstorm warnings ahead of this storm all the way to Chesapeake Bay.

The following figure shows the storm at 5:30 p.m., as it approached Interstate 95. The arc shape is unmistakable. The northern end of the bow echo had evolved into a comma-head shaped structure and may contain a large vortex, as typically occurs in these types of storms.

The strong downdraft along the storm’s leading edge has generated a menacing-looking shelf cloud – where moist air is lifted a few hundred feet and then condenses into a low cloud.

This northern comma head generated a powerful downburst that affected the subdivision just south of downtown Olney at 6 p.m. A zoom in of the cell is shown in the following figure. The heavy rain echo mass at the northern end of the bow is shown in the left panel. The right panel is the Doppler velocity and reveals hidden air currents in the rain.

The white circle surrounds the area affected by the downburst, but the most extreme winds were contained in the pink colored sub-region just southwest of Olney. It’s a strong flow away from the radar, to the northeast. The adjacent green region represents a strong southwesterly flow.

Taken together, these two patches mean strongly divergent airflow. The premise here is that an intense downdraft core struck the surface at a point of impact indicated by the white “X”. White arrows indicate bi-directional flow from the point of impact. Winds in the pink region must have generated winds of at least 60 mph, likely stronger given the size of the downed trees.

Many people hear a sudden roar as the wind quickly picks up to high speeds in a downburst. After the storm, the degree of tree damage can seem unbelievable. Based on the level of damage and the roar, many assume that a tornado has occurred. But similar to the situation in College Park, radar indicates a straight-line divergent wind pattern, rather than the rotating signature that would be associated with a tornado.

As the echo from the bow approached, the ominous cloud from the shelf was a sight to behold. We received dozens of cloud photos from readers and share a sample below:


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