Two Derechos in Two Days Sweep Through the East

Jonathan Erdman and Brian Donegan
Published: May 16, 2018

A squall line of severe thunderstorms raced through parts of the Northeast on May 15, producing damaging winds over a large-enough swath to satisfy criteria for a second derecho in two days.

(MORE: Northeast Storms Turn Deadly, Snarl Commutes, Knock Out Power)

Beginning as a small broken line of thunderstorms around midday in northwest Pennsylvania, the line eventually congealed and raked across Pennsylvania, southern New York, Connecticut, Massachusetts, Rhode Island, northern Delaware and Maryland through the evening, producing well over 200 reports of high winds or wind damage.

Radar and thunderstorm wind/wind damage reports (blue dots) from the derecho on May 15, 2018 from noon through 9 p.m. EDT.

Among the most notable reports from this squall line included:

  • In Newburgh, New York, several buildings lost portions of their roofs, including the Hotel Imperial. Several trees also crashed onto cars and streets.  
  • A possible tornado struck around 3:30 p.m. EDT near Yulan, New York, or about 40 miles east-northeast of Scranton, Pennsylvania. Multiple people were trapped in vehicles and houses. 
  • Winds toppled trees near Grand Central Station in New York, snarling travel during the afternoon commute.
  • Roughly 600,000 people were without power at the time of peak outages in Pennsylvania, Connecticut, Virginia, New Jersey and New York following the squall line, according to poweroutage.us
  • Roughly 80,000 of those outages come from the Stroudsburg, Pennsylvania area where wind toppled numerous trees onto vehicles, houses, power lines and roads. 
  • Hail as large baseballs fell near the Catskill Mountains and mid-Hudson Valley, as leading thunderstorms punched through southeastern New York. Large hail broke windows on a home and in a car near Elizaville, New York Tuesday afternoon. 

Using the criteria from a 2005 study by Walker Ashley and Thomas Mote, this squall line met the criteria to be deemed a derecho, a term meteorologists use for a widespread straight-line damaging wind event produced by thunderstorms. 

The squall line produced a swath of high winds and wind damage roughly 470 miles long from western Pennsylvania to southern New England to Maryland over a span of roughly eight hours from around midday through early evening. This is well above the 248 mile - 400 kilometer - criterion from the Ashley and Mote study. 

It was the second day in a row a derecho swept through the East, following the low-end derecho in the mid-Atlantic states on May 14.

(MORE: Why Summer's Thunderstorm Clusters are Both Important and Dangerous)

Thunderstorm high wind/wind damage swaths from the May 14 (left) and May 15 (right), 2018 derechos in the East. Each swath exceeded the 248 mile - 400 kilometer - length threshold to qualify as a derecho, per the Ashley and Mote, 2005 study.
(Storm reports: NOAA/NWS/SPC)

National Weather Service post-storm damage surveys and video confirmed 9 tornadoes touched down in Connecticut, New York, and Pennsylvania.

May 14 Derecho

On Monday, May 14, a line of severe thunderstorms tracked over 400 miles, producing high winds or wind damage from Ohio to the Virginia Tidewater over roughly eight hours.

Radar and thunderstorm wind/wind damage reports (blue dots) from the low-end derecho on May 14, 2018. The radar loop was roughly from noon through 10 p.m. EDT.

The line first produced severe weather in the Columbus, Ohio, metro area just before 1 p.m. EDT, including a 59-mph gust at The Ohio State University Airport and downed trees near Galena.

The early-afternoon line of storms produced wind damage in parts of southeastern Ohio and southwestern Pennsylvania, then accelerated into the Appalachians of West Virginia, where widespread trees were downed in Pendleton and Randolph counties.

By early evening, the squall line raced southeastward through the Washington D.C. metro and much of Virginia before fizzling late in the evening off the Virginia Tidewater.

Using the criteria from a 2005 study by Walker Ashley and Thomas Mote, this squall line appeared to have been a low-end derecho, a term meteorologists use for a widespread straight-line damaging wind event produced by thunderstorms. 

(MORE: Summer Derechos Have a Favored Corridor)

Working through Ashley and Mote's (hereafter, A&M) criteria, here's how we arrived at this conclusion:

  • Length: The swath of high winds/wind damage was roughly 430 miles – roughly 692 kilometers – long, easily satisfying the 400-kilometer criterion from A&M. (Note: a proposed change to the derecho criteria from NOAA's Storm Prediction Center would raise the length requirement to 650 kilometers)
  • Chronological progression: The wind reports flowed forward logically with time (see the radar/reports loop above).
  • No large time/space gap between reports: There were no time gaps of over 2.5 hours or spatial gaps of greater than 2 degrees of latitude or longitude between successive wind reports. While the wind damage reports were more concentrated from eastern West Virginia to Virginia, given the time/space gap didn't exceed thresholds, we include the Ohio and Pennsylvania reports in the overall length of the squall line.
  • Origin of wind swath: Damage swaths were all part of the same system, which included some severe thunderstorms ahead of the main squall line in western Virginia.
  • Continuity: Again, the line of thunderstorms responsible for the high winds and wind damage was an easily tracked entity via radar.

As there is an intensity spectrum of tornadoes, tropical storms and hurricanes, such is also the case with derechos.

We would categorize this as a low-end derecho.

The high wind swath wasn't particularly long, there weren't a particularly large number of reports, and, while not one of the A&M derecho criteria, there appear to have been no "higher-end" wind gusts – say, 75 mph or higher.

So, you can think of this derecho as the analog of an EF0 tornado or a 40-mph tropical storm (recognizing EF0 tornadoes and low-end tropical storms can still be dangerous).

The infamous late-June 2012 mid-Atlantic derecho would, therefore, be like an EF5 tornado or a Category 5 hurricane if there was a derecho intensity scale.

The length of the wind damage, high wind report (blue dots) swath from the May 14, 2018, low-end derecho. To be classified as a derecho, Ashley & Mote ('05) require a swath length of at least 400 kilometers (roughly 248 miles).
(Reports: NOAA/NWS/SPC)

Washington D.C. Impact on May 14

Washington's Dulles Airport issued a temporary ground stop early Monday evening as a line of severe thunderstorms approached the area. Runways reopened and ground operations resumed about an hour later.

These storms prompted a tornado warning just west of Washington D.C., including the area around Dulles Airport, as rotation was indicated on Doppler radar imagery. For safety, all passengers at the airport were sent underground to the train tunnel.

A National Weather Service employee spotted a funnel cloud in Ashburn, Virginia, just northwest of Dulles Airport, but there were no reports of it touching down.

In addition, golf ball-size hail – 1.75 inches in diameter – was reported in Reston, Virginia, with this line of severe storms. Wind damage was also observed there, with at least one large tree uprooted onto a garage.

Many people across the Washington D.C. metro area captured stunning photos of a shelf cloud as the thunderstorms moved in.


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