1. Gould, S. L., 2016: The Meteorological Impact of Mesoscale Lee Troughs Formed by the Ouachita
Mountains. Electronic J. Severe Storms Meteor., 11 (4), 1–16.
The Meteorological Impact of Mesoscale Lee Troughs Formed by the
Ouachita Mountains
Samuel Lane Gould
The National Storm Channel, Tulsa, OK
(Submitted 16 July 2016; in final form 01 August 2016)
ABSTRACT
Lee side troughs that form along mountain ranges have been thoroughly documented with the most
common example being the lee side trough that forms east of the Rocky Mountains in the United States.
The Appalachians along the East Coast form a similar trough on occasion and have been scientifically
studied and documented. This paper focuses on a smaller more mesoscale feature that forms along the lee
of the Ouachitas, a smaller mountain chain in southeast Oklahoma and western Arkansas. Although this
mountain chain lee side trough is weaker than its aforementioned counterparts, this trough does have a
significant meteorological impact upon the region. This paper examines the conditions in which this lee
side trough forms, the extent of its influence as well as the forecast implications for locations in Oklahoma
and Arkansas. This paper concludes with a summary along with suggested areas of study for similar
mesoscale lee troughs for smaller mountain chains across the United States.
––––––––––––––––––––––––
1. Introduction
The lee side trough is a distinctive
meteorological feature and structure named for
its geographical location associated with
mountains or mountain chains. Lee troughs are
observed on the lee side of mountains as
opposed to the windward side of those same
mountains. Orographic effects are well known
and have been a part of meteorological literature
for hundreds of years. The Glossary of
Meteorology (Glickman 2000) defines a lee
trough as “A pressure trough formed on the lee
side of a mountain range in situations where the
wind is blowing with a substantial component
across the mountain ridge; often seen on United
States weather maps east of the Rocky
Mountains, and sometimes east of the
Appalachians, where it is less pronounced.”
While the focus of lee side troughs have been
primarily upon large mountain chains, other
smaller mountain chains have similar features
and phenomena which impact meteorological
conditions on a mesoscale level. These
mesoscale effects can be quite dramatic and
worthy of attention especially when forecasting
for impacted areas and regions. The Ouachita
Mountains in southeast Oklahoma and western
Arkansas are just one such example (Fujita and
Wakimoto 1982).
Figure 1. shows a map of the Ouachita
mountain range in western Arkansas and eastern
Oklahoma and its geographical location in
respect to other geophysical features in the
region. The highest peak in this mountain range
of study is Magazine Mountain located in
Arkansas rising to a height of 2,753 feet or
839m according to arkansasstateparks.com.
Several other peaks in this mountain chain range
Figure 1: Ouachita Mountains of western
Arkansas and southeastern Oklahoma.
(Credit:University of Arkansas Dept. of
Archaeology).
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anywhere from 400m to 700m in height. The
Ouachitas along with the Ozarks are according
the United States Geological Survey, the only
major highland area between the Rocky
Mountains and the Appalachians.
2. Data and methodology
Mesonet data from the Oklahoma Mesonet
Network was attained and plotted on maps for
specific dates in which southeasterly or southerly
flow was occurring across southeastern
Oklahoma. Dates were selected primarily based
upon surface wind direction across the entire
region. Southerly or southeasterly wind flow was
preferred to investigate the development of lee
troughs caused by the east-west running
Ouachita mountain chain. Data was plotted on
maps for various parameters such as sea-level
pressure, wind direction and speed then
analyzed. Isobars were contoured to reveal
mesoscale troughs in the surface pressure fields.
Wind flags were plotted to indicate any changes
in the wind field across the region and finally
wind speeds were analyzed to determine what
affect different speeds might have on the
aforementioned parameters.
3. Pressure Effects
The first parameter considered was the
pressure field generated by southerly or
southeasterly wind flow. Figure 2 is a map of
Oklahoma showing an example plot of sea-level
pressure and subsequent mesoscale analysis
clearly indicates a weak trough of low pressure
extending from the border of Arkansas to south-
central Oklahoma in the lee of the Ouachita
Mountains. The presence of this trough is
believed to be a direct result of the light
southeasterly flow across the Ouachita
Mountains.
Research indicates that lee troughs have been
found to primarily form by three different
methods; secondary circulations associated with
macro scale frontogenesis, mesoscale lee
troughing that is caused by mountain waves, and
synoptic scale lee troughs associated with a
developing cyclone (Steenburgh and Mass
1994).
It is theorized that the lee troughing in the
Ouachitas are primarily formed by troughing
caused by mountain waves. However, data also
seems to indicate that in certain scenarios, a lee
trough may develop or evolve into an inverted
trough especially in the winter season.
This weak lee side trough is common
throughout the year since the prevailing wind
direction is from the southeast or south in this
region. When the winds are from the southeast or
south, mountain waves are formed and lee
troughing develops caused by the dropping of
the surface pressures on the lee side of the range.
Surface pressure falls have been documented as
a significant factor in lee cyclogenesis and lee
side troughing (Egger 1974). Pressure fields can
be significantly altered by mountain waves as
well as vertical variations (Durran 1990).
Figure 2: Surface pressure plot and contours for
07/08/2016 1440Z. (Credit: Oklahoma Mesonet).
Figure 3 shows an idealized airflow across a
mountain range of approximately 500m in height
which is within the height range of the Ouachita
Mountains. Based upon computer model
simulations, the effects of a mountain range the
size and height of the Ouachitas upon the air
flow can extend well over 36km. Considering
that the Ouachitas actually consists of several
ridges roughly parallel to each other, the impact
and number of mountain waves caused by the
Ouachitas are amplified.
4. Wind effects
Because surface pressure has a direct effect
upon wind direction and speed, sixteen ASOS
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stations in the area of study were selected for
more detailed analysis and their locations are
shown on the map in Figure 4.
Figure 3: Idealized airflow over mountain of
500m. (Credit: Durran 1982).
ASOS serves as the primary surface weather
observation network for the United States and is
a cooperative effort between the National
Weather Service, the Federal Aviation
Administration and the Department of Defense
(NWS 1999).
Figure 4: ASOS station locations within the
Ouachita region used in this study. (Credit:
Gould 2016)
ASOS data was collected and then used to
plot wind data at these sixteen locations within
the Ouachita Mountains region to discover what
if any effects the pressure field and lee troughing
influenced the prevailing wind direction at each
station.
Annual wind roses from each of these
stations were plotted to support the prevailing
wind direction and speeds. These plots were
generated using the Iowa State University ASOS
archived data website.
Figures 5 and 6 show the wind roses at HHW
and GMJ and are reflective of most of the ASOS
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stations within the region of study and confirmed
a prevailing annual average of a southerly or
southeasterly wind direction.
Figure 5: Annual Wind Rose for HHW. (Credit:
Iowa State University).
Figure 6: Annual Wind Rose for GMJ. (Credit:
Iowa State University).
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However, four stations; namely FSM, GZL,
RUE and JSV deviated from the others with a
strong easterly wind bias as shown in Figures 7
and 8.
Figure 7: Annual Wind Rose for FSM. (Credit:
Iowa State University)
Figure 8: Annual Wind Rose for JSV. (Credit:
Iowa State University)
A previous study did mention a prevailing
easterly wind at FSM (Shipman 1925) but did
not elaborate on the aerial extent of the easterly
wind bias. Shipman theorized that topography
indeed was a possible cause for a prevalent
easterly wind at FSM.
Shipman believed that the dominant factor
was the geographic location of the Ozark Plateau
further to the northeast and that nocturnal
radiational cooling forces were primarily
responsible for the prevalent easterly wind at the
Fort Smith station.
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If the primary reason for this easterly wind is
due to the Ozark Plateau being a source of cold
dense air flowing into the Arkansas River
Valley, then this easterly wind would primarily
occur during the winter months or during times
of sharp temperature gradients across the Ozark
Plateau. However, the data indicates that in fact,
this easterly wind occurs annually and so the
dense cold air theory would not explain
summertime or year long occurrences.
This paper speculates that this prevailing
easterly wind is both diurnal and nocturnal and
that this dominant annual easterly wind
component is experienced over a relatively large
area stretching all the way from RUE to GZL.
This easterly wind direction is thought to be
directly influenced by and formed from the lee
side troughing caused by the Ouachita
Mountains.
5. Froude Number
It has already been theorized that the
topography in western Arkansas and southeast
Oklahoma influences the climate of the region in
prevailing wind direction, pressure fields,
precipitation, severe weather and winter storm
climatology.
To help determine if the Ouachita Mountains
do indeed have the ability to create a mesoscale
trough under certain conditions and to gain a
better understanding of why these relatively low
elevation hills and mountains have such a
significant impact; an online model created by
UCAR was used to calculate the Froude number
under a number of different variables and
scenarios including atmospheric stability, wind
speed and direction as well as topographical
heights.
The Froude number is a ratio of the speed of
a wind perpendicular to a barrier to the stability
of a low level air mass by measuring the
potential temperature at both the surface and at
mountain top level. Mathematically, the Froude
number is shown in equation 1,
Fr=
U /h
N (1)
where U represents the perpendicular wind to
the mountain barrier, h is the relative average
height of the barrier, in this case (spine of the
Ouachita Mountains), and N is the Brunt-Vaisala
Frequency, a result of the Brunt-Vaisala
Equation shown in equation 2.
N =√g
θ
∂θ
∂ z (2)
where g is gravitational acceleration, θ is the
potential temperature at the surface, and θ/δz
represents static stability i.e. the change of
potential temperature from the surface to the top
of the mountain(s).
The Froude number equation results in a
unitless number representing whether an air mass
being advected into the Ouachita mountain chain
can make it over and through the range. The
Froude number can also help determine if the air
mass becomes blocked on the windward side of
the mountains because either it is too stable or if
the wind is not strong enough to force it over the
ridges.
When the Froude number is found to be <1,
then the flow is considered to be subcritical and
therefore the flow is or becomes blocked. When
values are calculated to be >1 then they are
considered to be supercritical and therefore the
flow is able to flow freely over the Ouachita
Mountains unimpeded.
The importance of blocked flow is pivotal to
understanding how a lee side trough forms as a
result of relatively weak southeasterly or
southerly winds flowing across this mountain
chain. As the air is blocked, air begins to
converge and pile up on the windward side
thereby increasing surface pressures and
subsequently lowering surface pressures on the
leeward side. Blocking can also retard the
movement of warm air advection from the Gulf
of Mexico to the south and may have a
significant impact on severe weather events as a
result and is discussed later in this paper.
For the first scenario using the UCAR Froude
model, the mountain heights were input to be on
average 810m with a wind speed of 8 m/s and a
wind angle of 45 degrees which would model
and represent a southeasterly wind since the
Ouachita Mountains run roughly west to east.
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For atmospheric stability considerations, the
Brunt-Vaisala Frequency numbers were
calculated by gathering and using data from the
12z sounding from SHV to closely represent the
air mass advecting into the Ouachita Mountains
on July 8, 2016 seen in Figure 9.
Figure 9: 12z 07/08/2016 Sounding for SHV.
(Credit: University of Wyoming)
For upper air temperatures in the model, the
500mb temperature at SHV was used along with
the surface temperature and input into the online
model.
Figure 10 illustrates the Froude number
results of that particular scenario using the data
from July 8, 2016. With a calculated Froude
number of 0.61, the flow was found to be
subcritical and therefore the southeasterly flow is
blocked by the Ouachita Mountains.
However, when the surface winds were
increased just 5 m/s more to 13 m/s in the model,
as shown in Figure 11, the Froude number
becomes critical at 1 and therefore flow at any
higher speeds would be supercritical and as a
result be able to pass over and through the
mountain chain.
A common atmospheric scenario described
and illustrated in Figure 10, shows how a lee
trough could form during the morning hours and
then as the afternoon progresses and winds
increase in speed due to diurnal turbulent
mixing, the Froude numbers would become
supercritical and the air flow would no longer be
blocked and the lee trough would weaken and
then disappear or dissipate.
Indeed, this could very well be the reason
that the easterly wind mentioned in (Shipman
1925) was believed to primarily be a nocturnal
event since wind speeds overnight and early
morning hours would be more likely to be
subcritical and therefore blocking would occur
which would then result in a more easterly
component to locations along the nocturnal lee
side trough.
Additional scenarios were constructed to
determine the results from different flow regimes
and the results were analyzed.
For the second scenario, all parameters were
kept the same except a perpendicular southerly
wind direction was considered for the online
model and the results are shown in Figures 12
and 13. As expected, in a more southerly wind
flow, less blocking takes place at lower winds
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speeds and air flow is then able to more easily
advect northward over the Ouachita Mountains.
Figure 10: Froude results of SE Wind at 8m/s.
(Credit: UCAR)
Figure 11: Froude results of SE Wind at 13m/s.
(Credit: UCAR)
6. Gravity Waves
In the earth's atmosphere, gravity waves are a
physical process that produces the transferal or
exchange of force. The Glossary of Meteorology
(Glickman 2000) defines a gravity wave as “a
wave disturbance in which buoyancy (or reduced
gravity) acts as the restoring force on parcels
displaced from hydrostatic equilibrium.”
Generally, gravity waves in the troposphere are
caused by either mountain or frontal systems.
Lee waves formed by the blocking effect of
mountains can be mathematically represented in
equation 3.
2πV [T /g (γd−γ)]1/ 2 (3)
Where V is the current speed, T the
temperature in Kelvin, g the acceleration of
gravity, and d and  the dry adiabatic and
environmental lapse rates respectively.
There are two types of mountain waves,
trapped or vertical propagating waves. Trapped
waves are more common with the Ouachitas and
this is because the wind speed above the
mountains increases sharply and stability
decreases in the atmosphere above the mountain
top stable layer.
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Figure 12: Froude results of S Wind at 8m/s.
(Credit: UCAR)
Figure 13: Froude results of S Wind at 13m/s.
(Credit: UCAR)
Low level moisture has a greater impact upon
lee waves than changes in moisture content in
upper levels (Durran 1982). Lee wave clouds
that form over the Ouachitas can be used in
satellite imagery to identify areas that are stable
from those areas that are less stable (Bikos
2002).
The tornado outbreak of May 3, 1999, is one
such example. Figure 14 shows a visible satellite
image from the afternoon of that tornado
outbreak showing clouds formed by these
trapped waves occurring as a result of relative
stable airflow across the Ouachitas.
Meanwhile, more cumuliform clouds are
found to the west in central Oklahoma. Severe
storms oftentimes form at the intersection of
these types of boundaries. These waves and their
associated clouds also have an influence on the
cloudiness climatology of the region.
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Figure 14: Wave clouds on visible imagery at
1945Z 05/03/99. (Credit: Bikos 2002)
7. Winter Weather Effects
Since the Froude number can be significantly
impacted by the stability of the atmosphere,
winter time scenarios were considered as well
and the results were quite interesting and
impressive.
For one example, data from a catastrophic ice
storm that affected the entire Ouachita region
from 2007 was analyzed. Surface map plots prior
and during that event shows broad light
southeasterly and easterly winds were blowing
across the Ouachita region and an inverted
trough was analyzed to be present as shown in
Figures 15 and 16.
Figure 15: Surface map plot for 21Z 12/07/07.
(Credit: Iowa State University)
Inverted troughs are often plotted and
depicted on surface maps for this region during
many severe winter weather events and should
be investigated further to determine if these
inverted troughs are actually the result of lee side
troughs or perhaps synoptic scale or vertical
forcing.
Upper air and sounding data from SHV and
LZK were retrieved and plotted for 00Z
December 8, 2007 and were also used for input
into the UCAR Froude model and the the results
indicate that blocking indeed was occurring with
the air mass as it traversed the Ouachita
mountain range as seen in Figure 17.
Prior to severe winter storms in the Southern
Plains, many times a developing surface low will
form across the High Plains of Texas around
MAF, and begin to move eastward. Synoptic
flow ahead of the surface low across the
Ouachitas will be southeasterly and as a result, a
lee trough will form and as surface pressures
continue to drop, the surface low will often
deflect more northward towards the lee
trough/inverted trough than the models would
forecast.
As a result of this surface low deflection, the
heaviest snow band associated with the
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Figure 16: Surface map plot for 09Z 12/08/07.
(Credit: Iowa State University)
baroclinic zone will often set up and fall dozens
of kilometers further to the north and west of the
original predictions.
Figure 17: Froude results of SE wind at 5 m/s
using data from 00Z 12/08/07. (Credit: UCAR)
8. Severe Weather Effects
Some recent research indicates that the
Ozarks and the Ouachitas might have an impact
upon tornado climatology. Some analysis seems
to indicate that there are two distinct “tornado
alleys”, the traditional Southern Plains Tornado
Alley and “Dixie Alley”. It has been noted that
these alley's are only separated by the Ozarks
and Ouachitas (P. Grady Dixon et. al. 2011).
While individual tornadoes and their
direction of movement are not affected by the
height of the Ouachita Mountains, they do seem
to quell overall tornadic activity (Price 1981).
Figure 18 shows a map of Arkansas and the
recorded tornado paths and there is clearly a
reduction in tornado paths in the Ouachita and
Ozark mountain regions.
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Figure 18: Tornado Tracks across AR with
Ozarks and Ouachita's outlined. (Credit: Price)
Blocked flow in the Ouachitas may retard the
return of warm moist air from the Gulf of
Mexico into a developing surface cyclone, but
this same blocked flow could in reality
contribute to a maximum of elevated convection
in this region (Colman 1990). It is theorized that
the regions nocturnal jets may in fact enhance
elevated convection (Bonner 1968). Bonner also
mentions that the Black Hills of South Dakota
might have a local influence on the high number
of northerly jets observed at Rapid City.
When warm moist air is blocked by the
Ouachitas, the surface warm front may get hung
up or stall across north Texas and therefore place
the greatest instability and risk of severe weather
further south into North Texas. This exact
scenario appeared to have taken place during the
Great Red River Outbreak of 1979 along with
cross contour flow contributing to the rapid
development of a strong low level jet. (Fuelberg
1982).
Surface analysis from that particular outbreak
event appears to show that a warm front was
struggling to move northward through the
Ouachitas region shown in Figures 19 and 20.
The surface map at the time of the Wichita
Falls tornado shows a weak lee trough with
southeasterly flow across the Ouachitas. This
blocked southeasterly flow coupled with ongoing
thunderstorms perhaps contributed to the slow
advance of the surface warm front despite
tremendous warm air advection aloft.
9. Precipitation Effects
Blocked flow that is ridge-parallel can lead to
increased precipitation totals by simply
enhancing convergence in the lowest levels
(Rotunno and Ferretti 2001).
Low level blocking caused by the lee trough
generated by the Ouachita Mountains has a fairly
significant impact upon precipitation totals and
can be evidenced by the number of wet days
recorded across the state of Oklahoma.
In Figure 21, there is a tongue of higher
values extending westward from the Arkansas
border westward along the periphery of the
Ouachita Mountains along with a significant
drop in the number of wet days in the far
southeastern corner of the state of Oklahoma.
Other studies have concluded increased
precipitation totals are affected by the Ozark and
Boston mountain chains (Ward 1917) and
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Figure 19: Surface Map at 21Z 04/10/79. (Credit:
Burgess)
Figure 20: Surface Map at 00Z 04/11/79. (Credit:
Burgess)
(Smebye 1958) and the Ouachitas of course have
similar patterns.
Climatological records of cooperative
observers in the Ouachitas region show an
increase in precipitation amounts in relation to
stations just outside the uplifted area (Shipman
1925).
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Figure 21: Map of number of % wet days in OK.
(Credit: Daly 2007)
10. Summary
The Ouachita Mountains in southeast
Oklahoma and western Arkansas have a sizable
impact upon various climate and meteorological
phenomena including wind and temperature
climatology, rainfall patterns, atmospheric
pressure, severe weather and winter weather
climatology.
These effects while small can have
significant impacts and implications in a
forecasting environment where detailed and
mesoscale factors need to be considered when
issuing a forecast or when long term
climatological considerations are taken into
account.
It is hoped that this paper has demonstrated
the need to consider that small mountain ranges
are just as important to making local and
mesoscale forecasts as the synoptic and larger
scale features.
It is suggested that other smaller mountain
ranges such as the Black Hills in South Dakota,
the Ozarks and St. Francois in Missouri, and
Misquah Hills or Lutsen Mountains in Minnesota
be important ranges in the local climatology and
meteorology of those regions worthy of study.
ACKNOWLEDGMENTS
The author would first like to thank my wife
Susan whose unwavering support over these last
few years in the online meteorology program has
been most appreciated. Steve Piltz,
Meteorologist In Charge (MIC) of the Tulsa
National Weather Service Office (NWSFO)
whose help in providing guidance and
suggestions was very gratifying. Kevin Lux of
VeroTech lent helpful ideas and reviews when
this work was first presented. The staff at the
Oklahoma Mesonet helped provide data and
steer this research toward operationally
important issues.
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