Low-end derecho may have swept through D.C.’s north and west suburbs Tuesday night

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”.

Figure 1. Damage reports from July 8, 2014 storms (NWS SPC)

Figure 1. Damage reports from July 8, 2014 storms (NWS SPC)

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.

 

figure2

Figure 2.

figure 3

Figure 3.

You can see the time evolution of the line of storms below:

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.

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)…

Damage photos:

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

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