The term derecho, coined by Dr. Gustavus Hinrichs in 1888, describes a wind event that does not fit
the definition of a tornado. Derechos are strong straight line wind storms that cover an extensive
area. To be identified as a derecho, the coverage area of the must be at least 240 miles in length
with wind gust speeds exceeding 57 miles per hour. Severe wind reports must progress
chronologically and there must be 3 reports, separated by 40 miles or more, of F1 damage
and/or gusts greater than 74 miles per hour. Also, no more than 3 hours can elapse
between successive severe wind damage reports.
Derechos are associated with bands of rapidly moving thunderstorms, typically taking on the shape of a bow
echo. Bow echoes form when an area of mid-level steering flow (10,000 to 20,000 feet / greater
than 40 knots) is stronger than the surrounding flow, causing the squall line or multicell
storms to surge in the middle. This causes a cyclonic rotation on the northern portion and an
anticyclonic rotation on the southern portion of the cloud formation. See Figure 1. Strong mid-level
flow not only causes the formation to surge, but to tilt forward.
This allows the downdraft to remain separate from the updraft. As moisture precipitates out and
encounters a dry layer of air, evaporational cooling occurs. The drier the air, the more intense
the evaporational cooling, the more intense the negative buoyancy the air will obtain, the harder
the air will hit the ground and the faster it will spread out. Also, due to the tilt of the storm, the downdraft
will hit the surface at an angle matching the direction of the storm. This means stronger wind speeds
will push out and ahead of the actual storm itself.
Wind speeds will not remain constant along the path of a derecho. In fact, there may be areas where
recorded wind speeds are lower than the stated severe weather criteria of 57 miles per hour. Other areas
will record wind speeds well in excess of the stated criteria, sometimes exceeding 100 miles per hour. These
patches of non-severe winds embedded within severe winds point to the fact that a derecho is not
a single wind event, but rather a series of wind events called downbursts. Downbursts are further
classified into macrobursts (>2.5 miles in diameter) and microbursts (<2.5 miles in diameter). When
a covey of downbursts occurs along a path 240 miles in length, then a
derecho has occurred. See Figure 2.
There are three different types of derechos. The first type is called the serial derecho. This type
of derecho is associated with strong, migrating low pressure systems in the mid-latitudes. Since
these storms are large and generally impact an area many hundreds of miles long, the chance of
squall lines and multicell thunderstorms developing into numerous bow echoes exists, further
heightening the chance of the development of a derecho. The second type is called a progressive
derecho. This type of derecho is associated with a relatively short line of thunderstorms
that will take the shape of a bow echo. The width of this derecho can vary, staying with a narrow
width across its traveled distance to starting with a narrow width, but gradually expanding
that width as it advances. It may start out as a single bow echo, but it can develop into a
full-blown squall line. The third type of derecho is called a hybrid. This type of derecho
contains traits of the first two derechos. For example, a derecho which is associated with a
strong migrating low pressure center but whose path starts narrow and gradually expands
with progression is typically called a hybrid derecho.
The chances of a derecho occurring in Mississippi are significantly lower than that of the Midwest. The best
chances for the occurrence of derechos in Mississippi are from September through April, when the battle
between continental polar air and maritime tropical area moves further south with the jet stream. During
this time frame, derechos can be experienced from east Texas through the Gulf Coast states. Two other
major axes of occurrence exist to the north during the warm season, when the maritime tropical air mass
can penetrate further inland. From May through August, derechos will occur on an axis extending from
the Upper Mississippi River Valley to the Ohio River Valley, with a second axis of occurrence existing
from the mid-Mississippi River Valley into the Southern Plains.
Forecasting derechos is extremely difficult. While we can get an idea synoptically where to expect severe
weather outbreaks, translating this into the mesoscale is the problem. When thunderstorms do start
developing, it is impossible to say which cells will become severe and which ones will not. This is due
to the little differences in temperature and moisture where the particular cell is developing. These
idiosyncrasies can make the difference between a colder cell core with fairly warm temperatures outside
of the cell producing enough of a thermal gradient to increase the mid-level steering flow which
initiates a bow echo. Downbursts can be very localized. Microbursts tend to be less than
2.5 miles in diameter. Macrobursts, although larger, will soon lose their steam before
reaching derecho criteria on their own. Trying to forecast these conditions over a 240 mile span
is a tremendous task.
The advent and gradual development of Doppler radar has increased our ability to identify threatening
weather signatures and nowcast the possibility of derechos. The ability of Doppler to peer into a storm
and root out some of its secrets can help us to localize warnings to those areas we see bow echoes
developing in. If there has been a previous history with the approaching convection of downbursts
meeting NWS severe wind gust criteria, then those trends can be utilized to provide advance warning
to people in the path of these storms. As good as satellite imagery and Doppler radar are, old
fashioned weather observing may be the best means of nowcasting for the immediate area and forecasting
for those areas to later be impacted by the approaching storm, especially in the near future.
Our current inability to forecast this mesoscale phenomenon can have dire consequences for the individuals
who encounter it unprepared. Due to the nature of a downburst, campers and boaters may be able to see
the weather getting bad, but have no idea of fast it is going to strike and in some cases, will be unable
to return to a place of safety and be left to the mercy of the elements. Boats can be overturned and
campers can be crushed by falling trees. In residential areas, mobile homes, with their large
sail areas, can be overturned easily. Houses surrounded by trees also face the danger of being
damaged or crushed by falling trees. In metropolitan areas, large power outages can occur, producing
food, water, and ice shortages impacting thousands of people. The 16 February 2001 derecho which
moved through central Mississippi and Alabama provided extensive damage. Wind gusts were recorded
over 110 miles per hour, 6,100 homes were damaged or destroyed, and 400,000 homes were left without
power in Alabama. The Mid South Derecho of 22 July 2003 significantly impacted the Memphis
metropolitan area and northern Mississippi, leaving 750,000 people without power for almost
a week. Damaged homes and lack of electricity severely hindered people from conducting
their day to day lives.
In order for forecasting shortfalls to be resolved when it comes to predicting mesoscale events such as
derechos, more data needs to be collected. Unfortunately, that takes time. However, research is being
conducted at the National Severe Storms Laboratory in Norman, Oklahoma. Much of their research has
been recorded in American Meteorological Society (AMS) papers detailing the steps and theories to
their research. Research such as that found in Simulation of a Progressive Derecho Using Composite
Initial Conditions by Michael Coniglio and David Stensrud will ultimately lead to better forecasting
of these events to provide advance warning. These gentlemen took a composite of the data to several
previous derecho events and loaded them into the MM5 model. This research delved into a
convective environment and what needed to happen to produce a derecho. The ability to
manipulate the model data allowed them play with environmental conditions enough to see
how those changes impacted the development of derechos.
In conclusion, much research has yet to be completed in order to for us to accurately forecast the
occurrence of a derecho. Our ability to look at a synoptic situation and warn the general population
of impending severe weather is good, but translating that into a mesoscale forecast is the challenge
we face. Higher resolution models that are provided consistent data over derecho prone areas is
one of the keys to success. But in the meantime, utilization of Doppler radar and our observation
networks will have to serve us in identifying this deadly weather event
and for warning the American public.
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United States. Preprints, 22nd Conf. on Severe Local Storms, Hyannis, MA, Amer. Meteor. Soc,
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Congilio, M. C., and D. J. Stensrud, 2001: Simulation of a progressive derecho using composite initial
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Coniglio, M. C., D. J. Stensrud, and M. B. Richman, 2004: An observational study of derecho-producing
convective systems. Wea. Forecasting.
Coniglio, M. C., and D. J. Stensrud, 2004: Interpreting the climatology
of derechos. Wea. Forecasting.
Evans, J. S., and C. A. Doswell III, 2001: Examination of derecho environments using proximity
soundings. Wea. Forecasting.
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