During the afternoon and evening of Friday June 29, 2012, an
intense, long-lived line of thunderstorms raced eastward at nearly 60 mph from
Mid-Atlantic coast. In its wake, these storms left behind a swath of destruction
that killed at least 20 people, caused millions in property damage, and caused
massive power outages in major urban areas along the storm’s path. Meteorologists
use the term “derecho” to
describe this special type of violent and long-lived windstorm.
Owing to the long-lived and destructive nature, and their immense
distances travelled, meteorologists ascribe the term “derecho” (pronounced "deh-REY-cho")
to these infrequent windstorms. The meaning of “derecho” is
better known to those living in the central and southern plains of the
where derechos occur more frequently.
Because derechos are not common in our region, the term is relatively
unknown to residents in the Mid-Atlantic. Typically, our region averages
about once every 2 to 4 years, as shown in the graphic “Derecho Climatology” from
the National Weather Service (NWS) Storm Prediction Center (SPC). The
last significant derecho to hit the Washington DC-Baltimore metro region
on June 4, 2008.
This summary, using a question-answer format, provides general background
on derechos and gives an overview of the derecho that struck the Mid-Atlantic
on the evening of Friday, June 29, 2012. A more complete and thorough reference
on derechos is available from the NWS Storm Prediction Center’s web
Some of the following information is derived from this SPC web site.
What is a “derecho”?
Essentially, a "derecho" is a long-lived, rapidly moving line
of intense thunderstorms that produces widespread damaging winds in a nearly
By definition (according Johns and Hirt, 1987), the term “derecho” applies
to a complex line of thunderstorms that travels a minimum distance
of 240 miles (~400 km) or more, and produces a nearly continuous and
widespread swath of damaging winds over that distance, with concentrated
wind speeds over 58 mph (93 km/hr). Surface wind gusts accompanying
can often approach or exceed 100 mph.
The term “derecho” was originally coined over a century ago
by a physics professor at the University of Iowa, named Dr. Gustavus Hinrichs,
a paper he published in the American Meteorological Journal in 1888. The
term “derecho” is
Spanish and means “straight ahead”, an attribute Dr. Hinrichs
applied to the storm’s ability to produce damage from essentially
straight-line winds. (Note: a fascinating description
on how this
term was developed by Dr. Hinrichs, and how it recently was revived can
be found here: http://www.crh.noaa.gov/images/dvn/hinrichs/JohnsDerechoStory.pdf).
Why haven’t I heard of “derecho” before?
For a variety of reasons, the term “derecho” as defined in Dr. Hinrich’s
original paper in 1888, never caught on with meteorologists of his time or in
the years afterward. But a century later, in 1987, two forecasters, Robert Johns
and Bill Hirt, with the NWS National Severe Storms Forecast Center (predecessor
to today’s Storm Prediction Center), published a scientific paper that
revived use of the term “derecho”. In their paper, Johns and Hirt
properly used the term “derecho” to describe a number of long-path,
non-tornadic damaging wind events they were studying.
Since publication of Johns and Hirt (1987), use of the term “derecho” has
gained acceptance within the meteorological community. But because derechos
are uncommon, the general public is not often exposed to hearing this term.
those living outside of the Central U.S. and southern plains have probably
never heard the term used. However, that all changed after June 29, 2012. Based
on the recent
experiences of massive power outages and widespread destruction resulting from
the June 29 derecho, many residents in the Mid-Atlantic region and elsewhere
are now more familiar with the term derecho.
How far can derechos travel?
Derechos can travel distances well over 250 miles (400 km). The
recent derecho on June 29th travelled over 700 miles from its start
to the East Coast.
A derecho on July 4-5, 1999 (known as the “Boundary Waters-Canadian
travelled a distance of 1300 miles stretching from northern Minnesota across
southern Canada before weakening as it exited off the coast of Maine. Derechos
are best-recognized for their propensity to produce damaging winds over a
fairly widespread region. They are also commonly prolific lightning producers.
What about those damaging winds from derechos?
Surface wind gusts of
50 to 75 mph are common with many derechos; some even produce
surface wind gusts of over 100 mph!
While derechos characteristically
produce widespread wind damage in a nearly continuous swath, actual
surface winds vary along the track of
Not all locations within the path of a derecho will experience damage.
The highest wind gusts occur with clusters of downbursts from the many
that comprise the derecho. Often, narrow corridors, or channels, of
higher wind gusts are observed within a derecho. Somewhat analogous
to the discontinuous
damage observed with tornadoes, where, e.g., structures on one side
of a residential street might experience damage.
Are all derechos similar?
In short, no. Although derechos share some common characteristics, derechos
occur in many variations. Consult SPC’s reference “About
more details. Although not discussed here, there are two basic types of derechos:
serial and progressive. For Friday’s event on the June 29th, this derecho
is classified as a progressive derecho.
Note: A derecho is not to be confused with a long-lived
supercell thunderstorm, as the processes that maintain the longevity of a
supercell are different than
those that maintain a long-lived derecho.
What did the June
29, 2012 derecho look like on radar?
Viewed on radar, a derecho can easily cover large portions of a state or multiple
states. They often take on a curved or bowed shape. Friday’s derecho
of the 29th appears to have originated from thunderstorms that developed along
an east-west oriented stationary front over Iowa and northern Illinois on Friday
morning (between 09:00-11:00 AM EDT).
Below is an animated radar mosaic of NWS and DoD radars for the CONUS (continental
United States). The loop runs from 02:08 AM EDT (0608 UTC) 29 June 2012 to
02:48 AM EDT (0648 UTC) on 30 June with a 10 minute interval.
As the derecho crossed through the western Washington DC suburbs, here’s
what it looked like on a regional NWS radar mosaic (at 10:38 PM EDT- Friday,
June 29, 2012):
Courtesy of Rich Grumm (SOO, NWS WFO State College), here is an animation from
8:57 PM - 11:35 PM EDT (0057-0335 UTC) on June 29, 2012 of the derecho as it
(KLWX 0.5 base reflectivity on the left and 0.5 base velocity on the right).
In the radar images below, the small circular hole in the center of the image
is the KLWX radar location in Sterling, VA; reflectivity values (left image)
are in units of dBz; velocity values (right) are in units of knots, with the
green and cyan
strong inbound velocities; bright red to orange colors representing strong
What did the June
29, 2012 derecho look like on satellite?
Viewed by satellite, derechos present an extensive cloud shield produced
by the strong updrafts of the thunderstorms comprising a derecho. Below
is an enhanced infra-red (IR) satellite image taken at 9:40 PM EDT Friday
evening (0240 UTC-June 30, 2012) near the time of the above radar image
(9:38 PM EDT). Overlaid on this image is the 15 minute sum of lightning
cloud-to-ground strikes between 9:30 PM and 9:45 PM EDT. Nearly 1400 strikes
are depicted on this image.
Here’s a link to an animation compiled
by researchers at CIMMS (Cooperative Institute for Meteorological Satellite
Studies; a cooperative institute between NOAA and the University of Wisconsin-Madison):
The animation contains data at roughly 15 to 30 minute intervals from the
GOES 13 satellite imager. The loop depicts the 10.7 micro infrared channel,
with the color scale depicting cloud-top temperatures (thunderstorms show
up in the darker red-to black-to gray). The loop extents covers
much of the lifetime of the derecho from its inception around 11:15 AM
EDT (1515 UTC) Friday, June 29, 2012, through 3:30 AM EDT (0732 UTC) Saturday,
June 30, 2012.
What are some typical characteristics of derechos?
Derechos can travel at speeds upwards of 65 mph.
The derecho from June 29th produced
wind gusts of 91 mph at Fort Wayne, Indiana, 82 mph at Dayton, Ohio.
At the three DC-Baltimore metro region airports, Dulles gusted to
71 mph at 1023 PM EDT, Reagan National recorded
a peak wind gust of 70 mph at 1048 PM, and BWI measured a peak gust
of 66 mph 1102 PM.
Nearly three-quarters of derechos occurring in the United States form
during the warm season months between April and October (however, derechos
observed anytime in the year).
Derechos move in roughly a linear fashion, although the can take sharp
turns to the right or left. They often take on a bow-like appearance
Derechos often have two regions of damaging winds. The first area of
damaging winds is associated with the gust front. This front, which is
on the leading
edge of the derecho, typically races out ahead of the main part of the
Damaging winds from passage of the gust front can occur but usually only
for a brief time (less than 1 minute). The second area of damaging winds
occurs in the core of the derecho where typically the highest winds associated
with the descent of moist air from upper levels of the storm rushes down
to the surface in what is called a “rear inflow jet”. This “jet” of
high winds can actually last for several minutes and typically produces
the most destructive damage. The derecho of June 29th featured such
a "rear inflow jet" that caused winds to blow strong (>40 mph) even 10
to 15 minutes after the main storm system had passed.
Why/How do derechos form?
Derechos move through areas where the atmosphere is very unstable. During
the summer months, derechos can form along the northern periphery of
a hot and humid high pressure system.
A measure of atmospheric instability
commonly used by meteorologists is CAPE (Convective Available Potential
Energy). Research on derechos shows that CAPE values greater
of ~4500 Joules/kg over a large area is needed to sustain a derecho during
the summer months. On June 29, 2012,
CAPE values of 4,000 to 6,000 Joules/kg were observed across the entire
path the derecho travelled.
One particular aspect of the unstable atmospheric conditions experienced
on June 29th was the occurrence of an "elevated mixed layer"
(EML) between roughly 10,000 and 20,000 feet above ground
over the Mid-Atlantic region. The vertical change of temperature, or lapse
rate, measured in this layer
aloft was roughly 9 degrees Celcius per 1 km in height. This "steepness"
of this lapse rate resulted in vigorous thunderstorm updrafts associated
with the derecho.
Why are derechos dangerous?
From SPC’s “About
Derechos” web site:
“… Because derechos are most common in the warm season, those
involved in outdoor activities are most at risk. Campers or hikers in forested
are vulnerable to being injured or killed by falling trees. People in boats
risk injury or drowning from storm winds and high waves that can overturn
Those in cars and trucks also are vulnerable to being hit by falling trees
and utility poles. High profile vehicles such as semi-trailer trucks, buses,
and sport utility vehicles may be blown over. At outside events such as
fairs and festivals, people may be killed or injured by collapsing tents
and flying debris.
Even those indoors may be at risk for death or injury. Mobile homes, in
particular, may be overturned or destroyed, while barns and similar buildings
can collapse. People inside homes, businesses, and schools are sometimes
victims of falling trees and branches that crash through walls and roofs;
they also may be injured by flying glass from broken windows. Finally,
structural damage to the building itself (for example, removal of a roof)
could pose danger to those inside.
Another reason those outdoors are especially vulnerable to derechos is
the rapid movement of the derecho. Typically, derecho-producing storm systems
move at speeds of 50 mph or greater, and a few have been clocked at 70
mph. For someone caught outside, such rapid movement means that darkening
skies and other visual cues that serve to alert one to the impending danger
In summary, the advance notice given by a derecho often is not sufficient
for one to take protective action. …”.
In addition, because falling trees and limbs bring down
live power lines during a derecho, there's an enhanced threat of electrocution
from those venturing outside immediately after the storm passes to come
in contact with downed power lines. Seeing and/or avoiding downed power
lines that might still be electrified is especially difficult in the darkness
of night when the power is out (e.g., no street lighting).
What damage can a derecho cause in urbanized areas?
From SPC’s “About
Derechos” web site:
“…Of particular significance in
urban areas is the vulnerability of electrical lines to high winds and
falling trees. In addition to posing a direct hazard to anyone caught below
the falling lines, derecho damage to overhead electric lines sometimes
results in massive, long-lasting power outages. Hundreds of thousands of
people may be affected; in the worst events, power may not be restored
for many days. …”.
In the immediate aftermath of the June 29th, media reports estimated somewhere
between 3.5 to 4 million customers were without power, with approximately
1.2 million customers in the DC metro region alone.
In addition, with this derecho, communications were disrupted across large
areas, including the national Capital/DC region. In northern Virginia,
loss of power to a key communications facility knocked out the 911 service
for a period of time. Other communications issues were loss of telephone
land lines, disruptions to celluar network calling, and scattered outages
to in internet service among private, government and commercial sectors.
What watches and
warnings from the NWS were issued for the June 29, 2012 derecho?
The NWS Storm Predicition Center worked with NWS field offices to issue
four Severe Thunderstorm Watches a few hours in advance of the derecho
to notify everyone for the potential of damaging winds. Warnings were
when that potential turned to expectation. The majority of warnings issued
from local NWS offices on Friday were severe thunderstorms warnings.
Although a handful of tornado warnings were issued, there were only 2
confirmed reports of tornadoes associated with Friday’s derecho.
Local NWS offices issue warnings in the form of polygons. Here is
a plot of NWS office warnings for this derecho. When compared to the
CONUS radar loop, it’s obvious where the bulk of warnings were
The NWS Baltimore/Washington Weather Forecast Office (WFO)
severe thunderstorm warnings, 2
tornado warnings, and 6
special marine warnings.The average lead time for all
15 severe thunderstorm warnings issued was approximately 37 minutes.
Severe Thunderstorm Warnings and Tornado Warnings both mean that there will be damage somewhere within that warned threat zone (by verification over 90% of the time). Anytime you have one of these warnings issued for where you are, you should seek shelter in a sturdy structure away from windows, in a low and central location (hopefully like a interior basement bathroom or closet). In our area, people most often are killed or injured by falling trees either landing on their vehicle or falling into the top floor of their home. A NOAA Weather Radio, which you can pick up at many big box stores or electronics stores, is a great way to receive life-threatening weather warnings that are affecting your county 24/7 (you program your county into it). While there are many ways to get warnings during the day, a weather radio is a great way to be alerted to these warnings when you aren't connected to the outside world - such as at night. You can also sign up for text alerts from many county governments, your local broadcast meteorologists, and/or private weather companies.
What is the extent of damage from the June 29, 2012 derecho?
This derecho caused widespread damage covering multiple states. Much of
the damage occurred when trees toppled on to structures, vehicles and
powerlines. Here is a plot from the SPC showing all severe reports for
29 June 2012. There
continuous path of severe wind and wind damage reports extending from Iowa
to northern Illinois across Indiana to Ohio to West Virginia to the Mid-Atlantic
before exiting off the East Coast.
Nearly all of the reports extending from Iowa eastward to the Mid-Atlantic
coast were caused by a single derecho. In the NWS Baltimore-Washington
WFO area of responsibility, we received over 300 individual
reports of damage; and continue to compile reports.
How predictable are derechos?
Forecasting derechos is still a challenge and an active area of research
within the meteorological community. While meteorologists are better
able to anticipate atmospheric conditions conducive
to formation of derechos, forecasting the detailed timing of the onset
of a derecho, along with its duration and
and even its path, remains a challenge to forecasters today.
From SPC’s “About
Derechos” web site:
“…Many meteorological factors, some acting synergistically
and some seemingly at odds, may come together to yield an environment
conducive to derechos. The variation of these factors over space and
time also is important in fostering or hindering development. Two nearly
identical meteorological settings might yield vastly different outcomes
--- that is, a derecho or no derecho --- depending upon how the atmosphere
evolved to that point in time. …”.
Better observational networks (including satellite, radar and surface observations),
coupled with the running ensembles of high resolution numerical weather
prediction models, offer hope that derechos will be forecast with more
accuracy in the future.
Derechos are relatively uncommon to this region, especially
ones of this magnitude. Typically, the Mid-Atlantic region might experience
a derecho once every two to four years. The last significant derecho in
our region occurred on June 4, 2008 (See Zubrick, et. al., 2009, for a
review of the June 4,2008 event).
Realize that the vast majority of thunderstorms that impact our region
will not be associated with a derecho. But, derechos can
occur here and they could occur at any time; tomorrow, next week or next
month, or not until
several years from now. Monitoring your local NWS forecasts and media outlets
the best way to
know if any
future derecho or other types of severe weather could impact your location.
Steven Zubrick, Science and Operations Officer (SOO)
NOAA/National Weather Service, Baltimore-Washington DC Weather Forecast
Hinrichs, G., 1888: Tornadoes and derechos. Amer. Meteor. J., 5, 306-317,
Johns, R. H., and W. D. Hirt, 1987: Derechos: widespread convectively
induced windstorms. Wea. Forecasting, 2, 32-49.
Zubrick, S.M, G. M. Schoor, and M. Eckert, 2009: Analysis of the severe
Mid-Atlantic Derecho of 04 June 2008. Presentation at the National Weather
Association 2009 Annual Meeting, October 17-22, 2009, Norfolk, VA [abstract
available here: http://www.nwas.org/meetings/abstracts/display.php?id=666]
The author wishes to thank Greg Carbin
at NWS Storm Prediction Center, Jonathan Blaes, SOO at NWS-WFO Raleigh,
State College, and Chris Strong and Jared Klein at NWS-WFO Sterling.