Although there is no official declaration from the National Weather Service (NWS), we believe there is evidence suggesting yesterday’s line of storms may have constituted a minimal, or low-end, derecho.
Our assessment is based on the wind damage pattern, radar images and wind reports from central New Jersey through central Maryland into northern West Virginia. The storms resulted in several hundred thousand power outages through this swath.
In northern Carroll County, a boy was killed and eight children injured when a tree came down at a summer camp (see related news video below).
The apparent derecho unraveled as it moved inside the Beltway and southeast of the District, but – based on the evidence – was legitimate to the north and west.
Figure 1 shows the distribution of damaging wind reports (at the bottom of this post, we’ve included the preliminary set of wind damage reports in our region compiled by the National Weather Service). I’ve circled what appears to be a linear, cohesive corridor of dense wind reports, beginning at the Ohio-Pennsylvania border, and extending to coastal New Jersey. The verbal description for many of these reports is simply “trees and limbs down”.
The NWS definition of derecho is “a widespread, long-lived, convectively-generated wind storm”. Minimum length criteria are 240 miles, with gusts that meet severe criteria (greater than 57 mph). Wind speed may vary considerably along the derecho track.
Derechos are usually generated by one or more bow echoes. Bow echoes are arc-shaped, fast-moving convective lines. They are single or multiple short bows, or a single, extensive, long-track bow. The bow shape is generated when microburst or downburst winds at the surface cause a portion of the convective line’s leading edge to surge ahead. In addition, “bookend vortices” curling up on the ends of the line segment help distort the front of the line into a spearhead or bow shape.
In Figures 2 and 3 below, we show the radar evolution of yesterday’s convective line. The first shows the beginnings of the bow echo (red arrow) over far western West Virginia at 3 p.m. The bow segment lengthened and maintained its integrity as it traveled toward the northeast. Figure 3 shows a mature bow structure crossing the DC region at 7 p.m.
You can see the time evolution of the line of storms below:
A 9 hour mosaic of stronger radar reflectivity evolution over the northeast today. pic.twitter.com/G98o7fX08l— Greg Carbin (@GCarbin) July 9, 2014
Actual wind speed observations are more difficult to come by, due to the spotty nature of reporting stations, particularly over the sparsely-populated Appalachians. Anecdotally, with many reports of “trees and limbs down” (see list below), one can surmise gusts that are considered “severe” i.e. damaging. Martin State airport in Baltimore County recorded a wind gust to 61 mph. Weatherbug station reports in the greater D.C. region reported maximum gusts in the mid- to upper-50 mph range.
Some of the wind gust reports from today’s storms pic.twitter.com/noKoQJToKB— TerpWeather (@TerpWeather) July 9, 2014
Dr. Greg Carbin, warning coordination meteorologist at the NWS Storm Prediction Center (SPC), expresses reservation whether minimum, measured wind speed criteria were met for a derecho, but also cautions that he has not reviewed all available observations.
It’s important to recognize that derechos lie along a spectrum: from marginal, poorly-organized events to very strong systems, in which wind gusts exceed 100 mph. In our assessment, there are many attributes of yesterday’s storm suggestive of low-end derecho, but let’s not throw caution to the wind!
Below is a compilation of damage photos (we received via Twitter) and storm reports (compiled from the National Weather Service)…
Wires down on Tuckerman Ln in North Bethesda. @capitalweather pic.twitter.com/BQyglpcOvS — TerpWeather (@TerpWeather) July 8, 2014
Downed tree with wires down on Wake Dr in Kensington, #MoCo emergency response on scene #WTOP #dctraffic @dsotisWTOP pic.twitter.com/pJdsGeA9f9 — Laurie Cantillo (@lcantilloWTOP) July 8, 2014
@capitalweather witnessed tree fall down on car next to Grovesnor metro. Luckily no one inside. pic.twitter.com/tk0iuvnl2I — Amateur MD weather (@mdamWX) July 8, 2014
@capitalweather Winds blew over the fence around the new Silver Spring library #WindGusts #StormIsComing pic.twitter.com/lvWaUCabfA — Jessie (@Jessie_Curryean) July 8, 2014
@capitalweather Storm damage in Sterling. About half of a large mature maple. Glad it fell away from the house. pic.twitter.com/iFZBJeuDbx — Steve Damerell (@AToneDefTool) July 8, 2014
Tree on house 10621 Lorain Av, Forest Glen structural damage no injuries need bldg inspector — Pete Piringer (@mcfrsPIO) July 8, 2014
Storm reports, from the National Weather Service:
PRELIMINARY LOCAL STORM REPORT...SUMMARY NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC 1143 AM EDT WED JUL 09 2014 ..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON... ..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE.... ..REMARKS.. 0442 PM TSTM WND DMG SENECA ROCKS 38.83N 79.38W 07/08/2014 PENDLETON WV 911 CALL CENTER TREE DOWN NEAR SENECA ROCKS. 0450 PM TSTM WND GST 3 NW NEW CREEK 39.40N 79.07W 07/08/2014 M63 MPH MINERAL WV TRAINED SPOTTER 0450 PM TSTM WND DMG 2 ESE MAYSVILLE 39.10N 79.13W 07/08/2014 GRANT WV TRAINED SPOTTER MULTIPLE TREES DOWN ON RT 42 IN MAYSVILLE 0454 PM TSTM WND DMG PETERSBURG 39.00N 79.13W 07/08/2014 GRANT WV 911 CALL CENTER TREES DOWN THROUGHOUT COUNTY AT MULTIPLE LOCATIONS. 0454 PM TSTM WND GST FROSTBURG 39.65N 78.93W 07/08/2014 M58 MPH ALLEGANY MD MESONET FROSTBURG STATE UNIVERSITY MESONET 0514 PM TSTM WND DMG 2 SSW RIO 39.10N 78.69W 07/08/2014 HARDY WV 911 CALL CENTER TREE DOWN ACROSS ROAD AT 300 BLOCK SPERRYS RUN RD 0514 PM TSTM WND DMG 1 NNE MCCAULEY 39.06N 78.70W 07/08/2014 HARDY WV EMERGENCY MNGR TREE DOWN ON LEISURE ACRES ROAD 0514 PM TSTM WND DMG BAKER 39.04N 78.75W 07/08/2014 HARDY WV 911 CALL CENTER TREE DOWN AT OLD BAKER ROAD AND OLD 55. 0518 PM TSTM WND DMG 3 W WARDENSVILLE 39.08N 78.65W 07/08/2014 HARDY WV EMERGENCY MNGR TREE DOWN ON 800 BLOCK OF PINNACLE DRIVE 0518 PM TSTM WND DMG 2 ESE MATHIAS 38.86N 78.83W 07/08/2014 HARDY WV 911 CALL CENTER TREE DOWN AT UPPER AND MIDDLE COVER ROADS. 0519 PM TSTM WND DMG WARDENSVILLE 39.08N 78.60W 07/08/2014 HARDY WV 911 CALL CENTER TREE DOWN. 29124 WV-55 0529 PM TSTM WND DMG CUMBERLAND 39.65N 78.76W 07/08/2014 ALLEGANY MD CO-OP OBSERVER TREE DOWN AT LOUISIANA AVENUE IN CUMBERLAND MD. 0529 PM TSTM WND DMG GREAT CACAPON 39.62N 78.29W 07/08/2014 MORGAN WV 911 CALL CENTER TREES AND POWER LINES DOWNED 0529 PM TSTM WND DMG 1 SSE SHADY GROVE 39.63N 78.15W 07/08/2014 MORGAN WV 911 CALL CENTER 2 TREES DOWNED NEAR INTERSECTION OF MARTINSBURG AND SPHORS ROADS. 0529 PM TSTM WND DMG GREAT CACAPON 39.62N 78.30W 07/08/2014 MORGAN WV 911 CALL CENTER TREE ON DOWN FENCE AND AGAINST HOME ON WIGGINS RUN ROAD. 0530 PM TSTM WND DMG 3 SW MILL GAP 38.29N 79.75W 07/08/2014 HIGHLAND VA 911 CALL CENTER TREE DOWN ACROSS RD NEAR THE INTERSECTION OF MILL GAP RD AND LOWER BACK CREEK RD 0533 PM TSTM WND DMG 2 SW BERRYVILLE 39.60N 78.25W 07/08/2014 MORGAN WV 911 CALL CENTER TREES DOWNED ON POWER LINES. 0533 PM TSTM WND DMG 1 S BERRYVILLE 39.60N 78.23W 07/08/2014 MORGAN WV 911 CALL CENTER TREE ACROSS BOTH LANES. 0535 PM TSTM WND GST 1 NW BERKELEY SPGS 39.64N 78.24W 07/08/2014 M59 MPH MORGAN WV MESONET WARM SPRINGS INTERMEDIATE SCHOOL MESONET. 0539 PM TSTM WND DMG 2 W CHERRY RUN 39.63N 78.07W 07/08/2014 MORGAN WV PUBLIC TREE ON MASON ROAD. 0539 PM TSTM WND DMG 3 WNW CHERRY RUN 39.64N 78.08W 07/08/2014 MORGAN WV 911 CALL CENTER TREE ON ROAD. WILD ROSE LN AND RIVER ROAD. 0542 PM TSTM WND DMG 2 E TOMAHAWK 39.53N 78.02W 07/08/2014 BERKELEY WV TRAINED SPOTTER MOBILE HOME FLIPPED BY WIND GUST ON CANNON HILL RD 0545 PM TSTM WND DMG 1 NNW CEDAR HILL 39.29N 78.12W 07/08/2014 FREDERICK VA TRAINED SPOTTER TREE DOWN ALONG WITH MULTIPLE LARGE BRANCHES. 0545 PM TSTM WND DMG WINCHESTER 39.17N 78.17W 07/08/2014 CITY OF WINCHESTER VA PUBLIC LARGE TREE BRANCH DOWN ON HOUSE ON E HART ST. 0553 PM TSTM WND DMG 2 ESE STRASBURG 38.98N 78.33W 07/08/2014 WARREN VA 911 CALL CENTER TREE DOWN ACROSS STRASBURG RD 0554 PM TSTM WND DMG FRONT ROYAL 38.92N 78.18W 07/08/2014 WARREN VA 911 CALL CENTER TREE DOWN ACROSS STONEWALL JACKSON HIGHWAY 0558 PM TSTM WND DMG 1 SW KEARNEYSVILLE 39.38N 77.90W 07/08/2014 JEFFERSON WV PUBLIC TREE DOWN ON LEETOWN RD NEAR RT 9. 0600 PM TSTM WND DMG 1 ENE HAGERSTOWN 39.64N 77.70W 07/08/2014 WASHINGTON MD TRAINED SPOTTER MULTIPLE TREES SNAPPED AND DOWN ON CARS IN THE 700 AND 800 BLOCKS OF INTERVAL RD 0600 PM TSTM WND DMG 1 SW SAINT JAMES 39.55N 77.77W 07/08/2014 WASHINGTON MD NEWSPAPER ROOF OF BARN BLOWN OFF. 0603 PM TSTM WND DMG 1 ENE HAGERSTOWN 39.64N 77.70W 07/08/2014 WASHINGTON MD TRAINED SPOTTER FOUR TREES DOWN ON INTERVAL ROAD. 0609 PM TSTM WND DMG 2 N ROUND HILL 39.16N 77.77W 07/08/2014 LOUDOUN VA COUNTY OFFICIAL TREE DOWN BLOCKING WOODGROVE ROAD NEAR ALLDER SCHOOL ROAD. 0610 PM TSTM WND DMG 2 NNE DELAPLANE 38.95N 77.91W 07/08/2014 FAUQUIER VA 911 CALL CENTER TREES DOWN NEAR INTERSECTION OF POSSUM HOLLOW DRIVE AND DELAPLANE GRADE ROAD. 0628 PM TSTM WND DMG 1 ESE MOUNT PLEASANT 39.45N 77.32W 07/08/2014 FREDERICK MD PUBLIC MULTIPLE TREES DOWNED. HOME SIDING DAMAGE ALONG KELLY ROAD. 0635 PM TSTM WND GST 2 WNW NEW MARKET 39.40N 77.31W 07/08/2014 M58 MPH FREDERICK MD MESONET OAKDALE HIGH SCHOOL MESONET. 0639 PM TSTM WND DMG 3 SSW DAMASCUS 39.26N 77.23W 07/08/2014 MONTGOMERY MD CO-OP OBSERVER 6 INCH BRANCHES SNAPPED. INTERSECTION KINGS VALLEY DR AND KINGS VALLEY RD. 0639 PM TSTM WND DMG 1 N HERNDON 38.98N 77.39W 07/08/2014 FAIRFAX VA NWS EMPLOYEE LARGE TREE DOWN NEAR HERNDON PKWY AND MONROE ST 0655 PM TSTM WND DMG 2 SE LINEBORO 39.70N 76.81W 07/08/2014 CARROLL MD NEWSPAPER *** 1 FATAL, 8 INJ *** 1 FATALITY AND 8 INJURIES FROM FALLING TREE BRANCHES AT YOUTH CAMPSITE WHILE TRYING TO SEEK SHELTER. 0658 PM TSTM WND GST 4 NE MILLERS 39.72N 76.80W 07/08/2014 M63 MPH CARROLL MD CO-OP OBSERVER 0700 PM TSTM WND DMG 1 NE FOUR CORNERS 39.03N 77.01W 07/08/2014 MONTGOMERY MD BROADCAST MEDIA LARGE TREE DOWN ON HOUSE ON LORAIN AVE 0705 PM TSTM WND DMG COLUMBIA 39.20N 76.86W 07/08/2014 HOWARD MD PUBLIC FIVE TREES DOWN... ONE LARGE 0708 PM TSTM WND DMG 1 N ALEXANDRIA 38.84N 77.09W 07/08/2014 ARLINGTON VA PUBLIC 3-5 INCH TREE BRANCHES DOWN 0715 PM TSTM WND DMG 1 WSW LINTHICUM 39.20N 76.68W 07/08/2014 ANNE ARUNDEL MD PUBLIC LARGE TREE LIMBS DOWN 0718 PM TSTM WND DMG 2 WSW FORT WASHINGTON 38.73N 77.06W 07/08/2014 FAIRFAX VA PUBLIC LARGE TREE LIMBS DOWN 0721 PM TSTM WND DMG JARRETTSVILLE 39.60N 76.47W 07/08/2014 HARFORD MD PUBLIC MANY LARGE BRANCHES DOWN. 0730 PM TSTM WND GST 1 N BALTIMORE MARTIN ST 39.34N 76.42W 07/08/2014 M61 MPH BALTIMORE MD AWOS 0731 PM TSTM WND DMG 1 NNE WASHINGTON 38.92N 77.01W 07/08/2014 DISTRICT OF COLUMB DC EMERGENCY MNGR TREE DOWN NEAR INTERSECTION OF 1ST STREET AND U STREET NORTHEAST. 0759 PM MARINE TSTM WIND 6 ESE HAVRE DE GRACE 39.51N 75.99W 07/08/2014 M58 MPH ANZ530 MD MESONET NORTH BAY...NORTH EAST MD MESONET. 0900 PM TSTM WND DMG LAMBS CREEK 38.26N 77.26W 07/08/2014 KING GEORGE VA 911 CALL CENTER TREE DOWN ACROSS LAMBS CREEK CH RD NEAR INTERSECTION WITH KINGS HWY && EVENT NUMBER LWX1400676 LWX1400677 LWX1400678 LWX1400674 LWX1400675 LWX1400702 LWX1400705 LWX1400715 LWX1400706 LWX1400714 LWX1400713 LWX1400679 LWX1400716 LWX1400718 LWX1400721 LWX1400695 LWX1400717 LWX1400723 LWX1400680 LWX1400720 LWX1400722 LWX1400683 LWX1400682 LWX1400692 LWX1400700 LWX1400701 LWX1400694 LWX1400690 LWX1400691 LWX1400684 LWX1400696 LWX1400699 LWX1400719 LWX1400681 LWX1400685 LWX1400688 LWX1400712 LWX1400703 LWX1400693 LWX1400711 LWX1400708 LWX1400707 LWX1400709 LWX1400686 LWX1400687 LWX1400697 LWX1400689 LWX1400698 A SEVERE LINE OF THUNDERSTORMS MOVED RAPIDLY EAST ACROSS THE AREA TUESDAY AFTERNOON AND EVENING PRODUCING SEVERE WIND GUSTS AND DOWNED TREES. THE MOST SUBSTANTIAL AREAS IMPACTED WERE EASTERN WEST VIRGINIA...WESTERN AND NORTHERN MARYLAND...AND EXTREME NORTHERN AND NORTHWESTERN VIRGINIA. $$ STRONG