1. Severe thunderstorms frequently occur in the Mid-Atlantic U.S.
during the late spring and summer months. The most common
threat posed by severe thunderstorms in this region is damaging
straight-line wind gusts. Many of these storm events produce
elongated or linear reports of wind damage; the longest-lived of
these storm complexes occasionally meet the criteria of a derecho.
We sought to build a general severe weather climatology over the
continental United States over the past 15 years, and to identify
events which had elongated reports of severe weather in the Mid-
Atlantic Region. Specifically, the focus of this study was on the
more localized features of these storm systems, called spearhead
echoes (SHEs; Fujita and Byers, 1977).
Methods
Conclusions
Examination of High-Impact Severe
Weather Events in the Mid Atlantic
Joe Egan1 and Richard Grumm2
1 Valparaiso University, Valparaiso, Indiana
2 NOAA/NWS State College, PA
• SHEs and derechos account for a significant number of Mid
Atlantic severe weather reports
• Average of 27-28 events in Mid Atlantic per year
• SHEs:
• Shorter-lived but locally as damaging as derechos
• Usually associated with a bow echo; can be located within a larger
derecho
• Often account for much of the severe wind reports during many events
• Recognition of SHEs on radar important to ensure timely and
accurate warning decisions
• SHEs often contain an appendage that is associated for much of the
concentrated wind damage caused by the storm
• Forecasters need to know their environments and practice situational
awareness
• Higher Downdraft CAPE = greater likelihood of downbursts and SHE
development (Severe Thunderstorm Warning often needed for storms in this
environment )
Motivation Results
• Fujita, T. T, and H. R. Byers, 1977: Spearhead echo and
downbursts in the crash of an airliner. Mon. Wea. Rev., 105, 129–
146, doi:10.1175/ 1520-0493(1977)105,0129:SEADIT.2.0.CO;2
• Grumm, Richard and Elyse Colbert/NWS State College, 2012:
End of heat-event severe event of 7 July 2012. Accessed 10 June
2015. [Available online at
http://cms.met.psu.edu/sref/severe/2012/07Jul2012.pdf]
• NOAA/NCEP/NCDC, 2015: HDSS Access System. Accessed
19 June 2015. [Available online at
http://www.ncdc.noaa.gov/has/has.dsselect]
• NOAA/NWS/NCEP/SPC, 2015: About Derechoes. Accessed
24 June 2015. [Available online at
http://www.spc.noaa.gov/misc/AbtDerechos/derechofacts.htm]
• NOAA/NWS/NCEP/SPC, 2014: Downdraft CAPE. Accessed
25 June 2015. [Available online at
http://www.spc.noaa.gov/exper/mesoanalysis/help/help_dcape.html]
• NOAA/NWS/NCEP/SPC, 2015: Severe Weather Summaries.
Accessed 10 June 2015. [Available online at
http://www.spc.noaa.gov/climo/online/]
• NOAA/NWS/NCEP/SPC, 2015: SPC Severe Weather Events Archive.
Accessed 10 June 2015. [Available online at
http://www.spc.noaa.gov/exper/archive/events/]
References
Evolution of a
Spearhead Echo (SHE)
• High Downdraft CAPE present (> 800 J/kg)
• Formation of appendage pointed in direction of motion
• “Parent echo” ahead of SHE ingested into larger echo
• Main threat: Straight-line wind damage (from
downbursts/microbursts)
• Too short-lived to be derecho
• Damage swath < 250 miles long
• Created a general severe weather climatology from 2000 to
2015 using Storm Prediction Center (SPC) Storm Reports
• Documented events with linear patterns of thunderstorm wind
damage in the Mid-Atlantic states
– Furthermore, classified Pennsylvania events as Derecho, or
events featuring a classic SHE, frontal SHE, or both
(hybrid)
• Examined radar data from select events with NCEI archive
radar data and GR2Analyst
From Fujita and Byers (1977)
• Average 27-28 SHE/Derecho events per year in Mid Atlantic
• Includes SHEs/derechos along cold fronts
• Climate Period: 2000-2014
• Nearly 90% occur between May and September
• 180 events affected Pennsylvania (11.6 per year)
• 16 Derechos (met Storm Prediction Center criteria)
• 250 + miles of wind damage reports
• Several “well-separated” 65-kt wind gusts
• 86 classic SHEs (non-frontal)
• Remaining events (78) were SHEs associated with cold fronts
• Focus of study was on classic SHEs
Case #1: Central Pennsylvania SHE from 7 July 2012
Base Reflectivity and Base Velocity on the KCCX radar from 20:08 UTC (left), 20:48 UTC (middle), and 21:40 UTC (right) Storm Prediction Center Storm Reports
( represents radar area of interest)
The storm progressed from NW to SE and was responsible for numerous straight-line wind damage reports
(yellow icons on radar and blue dots on SPC map). Notice the appendage (SHE) on the reflectivity display
coincided with the strongest velocities and most of the damage reports. Velocity folding (misplaced green
velocities within area of red) was evident in some locations due to the strong winds (faster than radar’s
Vmax). Range folding (purple haze) also occurred as the storm moved further downstream beyond the
radar’s rmax.
Case #2: Central Pennsylvania SHE from 7 July 2013
Base Reflectivity and Base Velocity on the KCCX radar from 19:24 UTC (left), 20:19 UTC (middle), and 20:55 UTC (right) Storm Prediction Center Storm Reports
( represents radar area of interest)
The storm complex progressed to the NE as a squall line, with the northern portion of the line responsible
for most of the wind damage. Notice the main appendage on the reflectivity display (classic “bow echo”
shape) co-located with the highest velocities and damage reports. Some velocity folding also occurred with
this storm cluster.
• Synoptic and mesoscale conditions favorable for SHEs and
Derechos
• Difficult to forecast, even for the experts at the SPC, especially
in the warm season in weak forcing environments
• Cold pools are beginning to show up on hi-resolution models.
like the HRRR, for some events. Therefore, these could be
used for additional research and simulations
• Communication of threats to life and property
• Outreach events are critical to further the public’s
understanding of the threats posed by severe straight-line
winds
• National Weather Service (NWS) could consider amending
warning bulletin call to action for a SHE-producing storm to
more clearly communicate damage potential and injury risk
• A separate warning type/level could be developed by the NWS
for particularly intense severe thunderstorms containing SHEs
(similar to a Tornado Emergency)
Future Work