**A preliminary study of severe wind-producing MCSs**

**in environments of limited moisture**

Stephen F. Corfidi\*

Sarah J. Corfidi

David A. Imy

*NOAA/NWS/NCEP/Storm Prediction Center, Norman, OK*

Allen L. Logan

*Lincoln University, Jefferson City, MO*

Submitted to *Weather and Forecasting*

15 January 2006

*\*Corresponding author address:*

Stephen F. Corfidi

NOAA/NWS/NCEP/Storm Prediction Center

1313 Halley Avenue

Norman, OK 73069

E-Mail:
[stephen.corfidi@noaa.gov](mailto:stephen.corfidi@noaa.gov%0CAbstract)

[**Abstract**](mailto:stephen.corfidi@noaa.gov%0CAbstract)

An examination of severe wind-producing mesoscale convective systems
that occur in environments of very limited moisture is presented. Such
systems, herein referred to as low dewpoint derechos (LDDs), are
difficult to forecast as they form in regions where the level of
convective instability is well below that normally associated with
severe convective weather. Using a data set consisting of twelve LDDs
that affected various parts of the continental United States, composite
surface and upper level analyses are constructed. These are used to
identify factors that appear to be associated with LDD initiation and
sustenance.

It is shown that LDDs occur in mean kinematic and thermodynamic patterns
notably different from those associated with most derechos. LDDs
typically form along or just ahead of cold fronts, in the exit region of
strong, upper level jet streaks. Based on the juxtaposition of features
in our composite analysis, it appears that linear forcing for ascent
provided by the front, and/or ageostrophic circulations associated with
the jet streak, induce the initial convective development where the
lower levels are relatively dry, but lapse rates are steep. This
convection subsequently grows upscale as storm downdrafts merge. The
data further suggest that downstream cell propagation follows in the
form of sequential, downwind-directed microbursts. Largely
unidirectional wind profiles promote additional downwind-directed storm
development and system sustenance until the LDD ultimately moves beyond
the region supportive of forced convective initiation.

**1. Introduction**

Because of the widespread severe weather threats with which they are
associated, high wind or derecho‑producing mesoscale convective systems
(MCSs) pose an important challenge to forecasters (Wakimoto 2001). In
late spring and summer, derecho MCSs typically occur in environments of
substantial convective instability, with very moist boundary layer
inflow (Johns and Hirt 1987; Johns et al. 1990). During the cool season
(October through March), when derechos are more commonly associated with
amplifying disturbances in the westerlies, they occasionally occur in
environments of only modest convective instability (e.g., Wolf 1998;
Bentley and Mote 2000; Evans and Doswell 2001; Burke and Schultz 2004;
Van Den Broeke et al. 2005). Nevertheless, lower tropospheric moisture
content in such situations typically is above seasonal norms.

More rarely, high-wind-producing MCSs occur in environments of very
limited moisture, with surface dewpoints at or below 50 ^o^F (10 ^o^C)
and/or precipitable water less than 0.5 inches (1.25 cm). Systems
forming in such environments, herein referred to as "low dewpoint
derechos" (LDDs), have been observed throughout much of the year and
over much of the United States from the Great Basin to the East Coast.
Because they develop in environments of very low convective available
potential energy (CAPE) not commonly associated with widespread severe
convective weather (i.e., CAPE below 500 J kg^-1^), LDDs are a challenge
to forecasters (e.g., Fenelon 1998; Corfidi 2003).

This paper examines the synoptic and meso-alpha scale (Orlanski 1975)
environment associated with twelve LDDs that have been identified over
the continental United States since the mid 1970s. Emphasis is placed on
those factors that appear to be most strongly associated with LDD
initiation, and how those factors promote system sustenance, in an
attempt to better anticipate these uncommon events.

**2. Data and methodology**

The cases selected (Figure 1 and Table 1) were chosen on the basis of
data availability and knowledge of the event by the authors. The events
included both warm and cool season LDDs that affected wide-ranging parts
of the United States. These systems collectively caused scores of
injuries and millions of dollars in property damage, as recorded in both
the Storm Prediction Center (SPC) and *Storm Data* databases. Measured
gusts in three cases exceeded 80 kts (40 m s^-1^). Half of the events
also produced hail, although most of this was marginally severe in size
(diameter at or below one inch or 2.54 cm); several also produced brief
tornadoes. All produced lightning, despite system cloud tops that
averaged less than 30,000 ft (9 km) and in several cases were as low as
20,000 ft (6 km) AGL.

Surface and upper air data were hand-analyzed for each event to verify
that the convective systems were surface-based. Radar and satellite data
were used (where available) to facilitate this process and to assist in
the identification of thermal and moisture gradients. With one
exception, each case had areal averaged surface dewpoints at or below 50
^o^F (10 ^o^C). This effectively excluded events involving strongly
forced convective bands in environments of intense low level shear and
nearly moist adiabatic thermodynamic profiles along wintertime cold
fronts (e.g., Van Den Broeke et al. 2005); dewpoints in such situations
typically are at or above 50 ^o^F (10 ^o^C). However, average surface
dewpoints in one case (5 July 1997 in North Carolina) were near 62 ^o^F
(17 ^o^C). This system was nevertheless included as moisture was
unusually limited (precipitable water at or below 1.25 cm) and dewpoint
depressions were unusually large (around 25 ^o^F (12 ^o^C)) for a day
with widespread severe convection in that part of the country at that
time of year. Radar data, ranging from manually-digitized displays
available from the National Climatic Data Center to single-site data
from the WSR-88D network, were examined to ascertain LDD initiation and
motion. "Initiation" was defined as the time to the nearest half hour of
the appearance of the first convective cells directly associated with
the event. Visible and infrared satellite imagery provided valuable
input for several of the more recent cases.

In accordance with Johns and Hirt (1987) and Coniglio et al. (2004),
each LDD produced a continuous swath of non-random, convective wind
damage and/or measured convective gusts in excess of severe limits (≥ 58
kts (26 m s^-1^)). The path length criterion, however, was reduced from
400 km to 200 km to allow for the inclusion of shorter-lived yet
significant events (i.e., systems that would still require the issuance
of watches and warnings). While this represents a significant deviation
from the definition of derecho used in most previous studies, for the
sake of simplicity, all of the systems in the present work will be
referred to as LDDs. Because of known areal and temporal inconsistencies
in the convective wind report data base (Weiss et al. 2002; Doswell et
al. 2005), a specific number of reports was not used as a minimum
threshold criterion. That notwithstanding, it is worth noting that each
case included was associated with at least 10 separate reports of
damaging wind and/or measured severe wind gusts; several produced more
than 100 (Table 1).

Similar to the methodology followed by Johns et al. (1990), composite
charts were prepared to depict mean observed conditions at the surface
and at the 925, 850, 700, 500 and 250 mb levels.[^1] While it would be
advantageous to obtain composite data with greater vertical resolution,
only the mandatory levels were sampled to keep the manual compositing
process manageable. The data obtained for each event and for each level
were collected using a 6 x 6 (36 point), 2400 X 2400 km grid overlay.
The resulting 450 km grid spacing roughly approximates that of the North
American radiosonde network. This spacing was believed sufficient to
resolve the salient features of the synoptic scale LDD environment being
investigated.

In contrast to Johns et al. (1990), the grid overlay was *aligned along
the direction of predominant forward motion of the MCS*, with the center
of the grid placed on the centroid of damage and gust reports. This was
done to account for the fact that LDDs, like all derechos, occur with a
wide range of upper jet orientations (e.g., southwesterly to
northwesterly; Coniglio et al 2004). Because LDDs tend to move in the
direction of the mean cloud layer flow as do other fast-moving,
forward-propagating convective systems (Corfidi 2003), this methodology
resulted in the grids being oriented roughly parallel to the mid
tropospheric wind at the system centroids. Such orientation also
minimized loss of detail in the averaging process; it further serves to
enhance the universal applicability of the results.

The composite data were analyzed by hand to ensure maintenance of
relevant thermal and moisture gradients. Average values of geopotential
height, temperature, dewpoint, wind speed and direction were calculated
for every grid point at each level examined. Values were also tabulated
for an additional point, the "centroid," located at the center of the
grid. This location marked the midpoint of observed wind damage and/or
severe gust reports for each event as contained in the SPC and *Storm
Data* databases. A line drawn through the major axis of the reports was
used to identify path direction, while the time and location of the
first and last reports defined path length and duration. For the two
cases which occurred over the Great Basin (31 May 1994 and 1 June 2002),
it appeared that low population density may have negatively impacted the
reporting of severe weather in parts of Utah and southwest Wyoming. As a
result, radar and satellite data were used to extend event path length
and duration. Wind compositing in all cases was accomplished by
calculating arithmetic means of direction and speed (Schaefer and
Doswell 1979).

The radiosonde data times (0000 or 1200 UTC) selected for creation of
the upper air analyses were those believed to be most representative of
the MCS initiation environment. The midpoint time of each event was less
than three hours from the selected radiosonde time for eight of the
twelve events. For four cases (31 May 1994, 21 November 1994, 1 June
2002 and 5 March 2005), the selected radiosonde time preceded the event
midpoint by more than three but less than six hours. The data,
nevertheless, were still deemed representative of upper level conditions
at event initiation. Surface charts valid closer to the actual midpoint
time of the LDD were used in these cases to better depict low level
conditions associated with MCS initiation.

The LDDs in this study exhibited comparatively short lifetimes
(approximately 5 hours) relative to the twice daily radiosonde cycle and
to the duration of most derechos (e.g., Coniglio et al. 2004). Because
of this, no attempt was made to determine separate "beginning,"
"midpoint" or "end time" conditions as was done by Johns et al. (1990).
Note, however, that five of the systems that affected the eastern United
States were producing damaging winds as they moved into the Atlantic,
and that two of the events persisted for more than six hours.

**3. Results**

*a. The synoptic environment*

The composite charts given in Figure 2 depict mean kinematic and
thermodynamic patterns that differ notably from those normally
associated with long-lived, warm season derechos (e.g., Johns et al.
1990). In particular, the flow is decidedly cyclonic at all levels in
the LDD region. In this sense the synoptic environment most resembles
the "upstream trough pattern" identified by Coniglio et al. (2004) in
their observational study of derecho-producing convective systems.
Environments of this type are characterized by the presence of a
well-defined, progressive shortwave trough immediately upstream from the
derecho location. Typically, such convective systems occur in close
proximity to a maximum in the mid tropospheric flow. The 500 and 250 mb
composite charts (Figures 2e and f) reveal that this is indeed true of
LDDs, with the mean centroid located in the exit region of a mid to
upper tropospheric jet streak.

While all of the LDDs studied occurred with cyclonic upper flow, the
direction of orientation of the associated jet axis relative to the
ground was quite variable. For example, the jet was oriented nearly
meridionally (south southwest to north northeast) in the two cases which
occurred over the Great Basin, while the background flow was west to
northwesterly in the cases which affected the East Coast. The
association of LDDs with west or northwesterly flow in the East may
reflect the fact that low to mid level lapse rates (discussed in section
3b) tend to be greatest in that region during periods of persistent west
to northwest flow. It is at these times that the lower tropospheric
environment is likely to be continental in origin. At the very least,
the variability in jet orientations observed indicates that the
kinematic and thermodynamic ingredients necessary for LDD genesis may be
brought together by a wide range of large scale upper level flow
regimes, much as is the case for derechos in general (e.g., Coniglio et
al. 2004).

The surface, 925 and 850 mb composites (Figures 2a, b and c) indicate
that LDDs tend to occur within areas of enhanced low level flow and
thermal ridging ahead of strong cold fronts. These fronts appeared to
provide the necessary forced uplift that fostered initial convective
development in each case examined. As might be expected given the
amplified nature of the large scale pattern, a well-defined couplet is
apparent in the low level thermal advection field. The convective
systems occur near the center of the couplet, immediately downstream
from a pronounced maximum of cold advection. The magnitude of the cold
advection maximum is greater than that of the corresponding warm
advection area located downstream from the LDD. This imbalance is also
apparent at 700 and 500 mb (Figures 2d and e), suggesting that the
upstream shortwave in an LDD environment typically is undergoing
amplification via quasi-geostrophic processes. Pronounced drying is also
evident immediately upstream from the LDD location at both 700 and 500
mb.

The lower tropospheric composite charts (Figures 2a, b, and c) reveal
the recent passage of a trough or wind shift line in the vicinity of the
system centroid. The trough may in part reflect the presence of the
secondary or "southern stream" shortwave disturbance that is apparent at
both 700 and 500 mb (Figures 2d and e), with the low level flow veering
to a more system-parallel (generally westerly) direction in the wake of
the trough. The feature appears to mark the onset of neutral or slight
cold advection at low levels, and is therefore reminiscent of the
prefrontal troughs described by Schultz (2005). In several individual
cases, the trough appears to be thermal in origin, marking a maximum in
the lower tropospheric temperature field. The horizontal extent of the
feature is much greater than that of most of the LDDs examined,
suggesting that it is not simply a reflection of the convective systems
themselves. The composite maps also indicate that the trough is
associated with the leading edge of comparatively dry air advection at
lower levels (Figures 2a and b). The higher moisture values present
downstream may, however, simply be an artifact of the data, as the cases
included several eastward-moving systems over the Mid Atlantic region
where a more maritime environment existed to the south and east.

*b. Sounding and hodograph data*

A mean sounding constructed from surface and mandatory level data for
the LDD centroid location is shown in Figure 3. The "smoothness" of the
display reflects the limited vertical resolution of the data used in its
creation as well as the averaging process. While the sounding must be
interpreted with caution, major features of the LDD environment appear
to have been preserved. In particular, the moisture profile depicts a
lower tropospheric environment that is quite dry relative to other
organized severe convective weather situations; the mean relative
humidity in the lowest 300 mb is 45%. As a result, there is considerable
convective inhibition. The temperature profile, nevertheless, is one of
notable conditional instability owing to the presence of lapse rates
that are considerably greater than the climatological norm (Bluestein
and Banacos 2002). This is especially apparent in the 850 to 500 mb
layer, where the mean lapse rate exceeds 7.0 ^o^C per km. Assuming that
moist convection is able to breach the weak inversion present at 850 mb,
the combination of steep low to mid level lapse rates and large
temperature-dewpoint spreads suggests a mean thermodynamic environment
that is amply suited for strong convective downdraft development
(Wakimoto 1985).

To further investigate the thermodynamic environment of LDDs, plan-view
plots of mean low to mid level lapse rates are provided in Figure 4.
These charts indicate that while the low level LDD environment is indeed
unsaturated, there exists a considerable degree of conditional
instability. For example, in the 850-700 mb layer (Figure 4a), lapse
rates reach a maximum of 7.2 ^o^C per km at the LDD centroid. A rather
sharp gradient in the lapse rate field is also apparent to the left of
the direction of LDD motion (facing downstream), with steeper lapse
rates to the right (i.e., in the general direction of the 850 and 700 mb
inflow (Figures 2c and 2d)), and substantially more stable conditions to
the left. The lower tropospheric instability is surmounted by
instability in the 700--500 mb layer (Figure 4b), although the
horizontal lapse rate gradients in this layer are diminished. A lapse
rate maximum is also apparent in the vicinity of the system centroid
through the depth of the entire lower troposphere (Figure 4c). By
comparison, however, the degree of instability in this deeper layer is
rather weak, owing to the weaker lapse rates present below 850 mb (see
Figure 3).

The steep lapse rates associated with LDDs are noteworthy considering
that (1) most of the cases examined occurred in areas far removed from
the strongly-mixed boundary layer environments of the western United
States (typical source region of elevated mixed layers over the central
and eastern states), and that (2) half of the events took place during
the cool season (October through March). The degree of conditional
instability associated with the 850-700 mb layer is also notable
considering that the climatological areal-averaged lapse rates in that
part of the troposphere range from near moist adiabatic to isothermal
over the central and eastern United States (Bluestein and Banacos 2002).

Figure 5 is a mean hodograph constructed from surface and mandatory
level data for the LDD centroid. Recall from the compositing procedure
that wind directions are relative to system movement, with the x axis
oriented parallel to the observed system motion (denoted by white dot).
It is apparent that the systems move slightly to the right of the wind
at all levels. This reflects the contribution of propagation (i.e., the
development of new convective cells relative to existing activity) to
total system motion, and is consistent with Figure 4, which shows that
the greatest low level instability is typically located to the right of
LDD motion. The hodograph is rather linear, exhibiting moderate to
strong shear that increases monotonically through 250 mb (end of
hodograph). While shear in the lowest 1 km is quite modest (14 kts (7 m
s^-1^)), the mean surface-to-6 km shear is nearly 75 kts (38 m s^-1^).
The strength of the deep shear reflects the baroclinic nature of the
environments in which LDDs occur. By comparison, Coniglio et al. (2004)
found the mean 0-1 km and 0-6 km shear to be 30 kts (15 m s^-1^) and 54
kts (27 m s^-1^), respectively, for strongly-forced derechos. That a
greater degree of shear is distributed aloft in LDD environments perhaps
reflects that the boundary layer is more deeply mixed with LDDs than is
the case with other strongly forced derechos.

*c. Discussion*

The white dot in Figure 5 depicts the speed of average forward motion of
the LDDs studied: 46 kts (23 m s^-1^). This is faster than the mean
environmental flow (43 kts; (22 m s^-1^)) and represents unusually deep
(about 4 km) front-to-rear inflow, considering that the average tops of
the convective systems was approximately 9 km. The presence of deep
front-to-rear flow (relative to cloud depth) sets LDDs apart from other
strongly forced derecho-producing systems which have, by comparison,
shallower system-relative inflow (Evans and Doswell 2001; Coniglio et
al. 2004). *Deep front-to-rear flow, thermodynamic environments that are
favorable for the development of strong, low-level convective
downdrafts, and the fact that LDDs move faster than the mean wind all
suggest that propagation likely plays a disproportionate role in LDD
movement (relative to advection) compared to other strongly forced
MCSs.*[^2]

The notion that propagation plays a particularly important role in LDD
motion is supported by examination of radar reflectivity data from
several of the LDDs in this study. Individual storms within the LDDs
typically are weak (radar reflectivities less than 40 dBZ) and often
persist for less than one hour. But the convective systems as a whole
appear to be maintained by the sequential development of new cells that
form in the downstream direction (forward propagation) along gust fronts
produced by existing storms. This process is most readily apparent in
animated radar imagery, although it is sometimes also evident in
sequences of still images such as those in Figure 6. The images in
Figure 6 depict part of the evolution of an LDD that moved across
coastal South Carolina (Case 12 in Table 1). Especially in the region
just east of Charleston, individual convective elements may be tracked
as they consecutively are "undercut" by the system's east
southeast-moving outflow boundary (downwind from white arrows in Figures
6a--c). The boundary, in turn, then serves to initiate a new band of
storms as it moves off the South Carolina coast after 2205 UTC (Figure 6
d -- f).

Occasionally, the role of forward propagation in LDD motion also is
apparent in geostationary satellite imagery. The visible data sequence
in Figure 7 shows old convective cells being "left behind" atop post-LDD
outflow as new storms form on the northeast-moving gust front associated
with the 31 May 1994 Utah LDD (Case 4 in Table 4). While the degree of
forward propagation apparent in this example is particularly striking,
it is recognized that forward propagation occurs to some extent in many
types of MCSs, and that not all systems exhibiting rapid
forward-propagators are LDDs. Instead, the low ambient relative humidity
of the LDD environment tends to minimize coverage of auxiliary clouds.
As a result, it may be that cell propagation is simply more readily
observable in LDDs than is the case with systems that occur in more
humid regimes.

Another characteristic of the LDD environment that fosters forward
propagation is apparent in the wind profile shown in Figure 3. The
profile is not only strong, but also nearly unidirectional. Strong,
unidirectional wind fields are known to favor coherent motion of
storm-scale downdrafts and to promote rapid elongation of the resulting
MCS cold pool in the downstream direction (Corfidi 2003). This can
strengthen and deepen low-level system-relative inflow and therefore
promote convective initiation and sustenance in relatively dry
conditions by forcing parcels to the level of free convection.

Considering the overall kinematic and thermodynamic environment that has
been presented, a picture of LDD development and sustenance begins to
emerge. First, forced ascent promotes convective initiation along a
sharp cold front or pre-frontal wind shift line. Forward propagation of
this activity is then fostered by a combination of linear forcing
(provided by the front) and the presence of moderate to strong
unidirectional flow. Coupled with the presence of a thermodynamic
environment that is conducive to the formation of strong convective
downdrafts, the setup is one that appears to support an "organized
microburst" convective mode. Successive microburst production allows for
discrete downstream propagation of the nascent convective system. *In
short, the evidence suggests that LDDs may, in fact, be bands of
downwind-directed microbursts. .*With new cell development focused in
the same direction as the mean flow, the advective and propagational
components of system motion are additive. As a result, depending upon
the rate of downstream development, LDD movement may exceed that of the
mean wind. It is further conjectured that the LDD regeneration process
via forward propagation continues until the system ultimately moves into
a region that is no longer supportive of forced convective initiation
and subsequent cold downdraft production.

As might be expected given the rapid motion of the organized downdrafts,
radar data show that the cells comprising LDDs frequently assume a bow
configuration (e.g., Przybylinski 1995). This configuration can occur on
various length scales ranging from that of individual cells to bands of
storms extending 100 km or more (e.g., Figure 6). In several of the
events examined, bowing appeared to follow periods of especially rapid
downstream propagation and/or episodes of locally-enhanced downward
momentum transfer. Bow structures are certainly characteristic of LDDs.
It should be emphasized, however, that LDDs are probably most noteworthy
for the relatively weak reflectivities (usually less than 40 dBZ;
sometimes less than 20 dBZ) displayed through the duration of the event.
Another radar aspect is the absence of long-lived, embedded storms.
Considering what has been said regarding the role of discrete
propagation and sparse moisture in contributing to LDD behavior, these
two observations are not surprising. The radar presentations of two
recent LDDs will be discussed briefly in the next section.

**4. "Cool" and "warm" LDDs**

*a. Comparison data*

A review of the individual upper air and sounding analyses used in this
study reveals that while the LDD environments are similar in many
respects, two broad classes of events may be distinguished. The first of
these most closely resembles the mean pattern described in the previous
section, with decidedly cyclonic flow present at all levels. Nine of the
twelve cases examined were of this type which, for the purpose of
discussion, will be referred to as "cool" events. The second pattern,
represented by the remaining three cases, is characterized by somewhat
weaker and less cyclonic upper level flow. More significantly, systems
of the latter type occur in comparatively warm tropospheric regimes with
very deep convective boundary layers. These LDDs herein will be referred
to as "warm" events. Table 1 provides a listing of the "cool" and "warm"
cases identified.

Differences in the two classes of events are evident in the comparison
soundings and hodographs given in Figures 8 and 9, respectively. While
the limited sample size and compositing process once again dictate that
the data be interpreted with caution, notable differences between the
two subgroups are apparent. For example, the "cool" LDD environment
exhibits a weak inversion between 925 to 850 mb, while the "warm"
temperature profile depicts a mixed layer that extends to at least 700
mb. Since four of the "cool" event soundings were taken at 1200 UTC, the
inversion could reflect, in part, the influence of nocturnal cooling.
The "warm" composite exhibits a moisture discontinuity at 925 mb. Such a
feature does not support the deeply mixed environment suggested by the
temperature profile, and may be an artifact of the small sample size
and/or the use of mandatory level data. Observed warm event proximity
soundings (e.g., Figure 11b) show only slight departures from a deeply
mixed T~d~ profile. Interestingly, however, weak low level moisture
discontinuities are also apparent in the evening soundings presented by
Wakimoto (1985; his Figure 6) for dry microburst days in warm, deeply
mixed boundary layers over the High Plains.

The comparison hodographs (Figure 9) reveal that "warm" LDDs move more
nearly in the direction of the mean flow than do "cool" systems (the
"warm" hodograph lies close to the x-axis). This suggests that "warm"
events tend to occur in the presence of more expansive surface-based
instability relative to the strongly cyclonic cases, resulting in less
propagation to the right of the mean flow. Indeed, the warm sector was
comparatively broad in the vicinity of the three "warm" systems relative
to most of the "cool" events. The small size of the data subset,
however, precludes a definitive statement. The hodographs also imply
that "warm" LDDs experience deeper front-to-rear flow than do "cool"
events. This is apparent from the fact that the "warm" systems move
faster than the environmental flow at all levels except the highest,
whereas the "cool" LDDs move with the speed of the flow in the 3.5-4.0
km layer. The more rapid relative motion of the "warm" systems implies
that propagation accounts for a greater part of system motion than is
the case for "cool" events. This is consistent with Evans and Doswell
(2001) who suggested that propagation plays an increasingly significant
role in MCS motion the weaker the large scale forcing. Coupled with the
modest degree of low level veering and implied warm advection indicated
in the hodograph, "warm" LDDs may be regarded as drier versions of the
"progressive" derechos described by Johns (1993).

*b. Example cases*

An example of a "cool" or strongly cyclonic LDD that affected parts of
North Carolina and southern Virginia on the evening of 7 March 2004
(Case 10 in Table 1) is given in Figure 10. The convective system
preceded an intense Ohio Valley disturbance (Figure 10a) that was
accompanied by 500 mb wind speeds in excess of 120 kts (60 m s^-1^).
Surface dewpoints ahead of the associated cold front in central North
Carolina were between 0 and 5 ^o^C. Coupled with afternoon dry bulb
readings near 20 ^o^C, this yielded dew point depressions of 15-20 ^o^C
(Figure 10b).

The radar sequence in Figure 10c illustrates the comparatively weak and
short-lived cells that are characteristic of "cool" LDDs. Maximum echo
strengths of the wind-producing cores remained at or below 40 dBZ. The
evolution of this particular event was complicated by the fact that the
original gust front and its associated downward momentum surge evolved
from post-cold frontal (elevated) convection which formed over western
North Carolina and southern West Virginia (not shown). This activity
later merged with prefrontal storms which formed over the North Carolina
Piedmont. The latter storms appear as the isolated, orange-colored
(40-45 dBZ) cores northwest of the radar site at 0031 UTC in Figure 10c.
The LDD continued to produce isolated damaging wind gusts until it moved
off the North Carolina and Virginia coast at 0500 UTC.

Figure 11 depicts a "warm" event that moved southeast across western
Arkansas on the evening of 18 March 2004 (Case 11 in Table 1). This LDD
evolved from a cluster of storms that developed during the late
afternoon over eastern Oklahoma ahead of a weak mid level trough (Figure
11a). Substantial surface heating that occurred prior to MCS development
yielded a deeply mixed boundary layer, with temperature-dewpoint spreads
up to 20 ^o^C in western Arkansas (Figure 11b). This promoted the
development of strong convective downdrafts and new cell formation on
the merging gust fronts produced by the initial storms. The largely
unidirectional west-northwesterly cloud layer flow, in turn, hastened
downstream propagation and consolidation of the activity into a forward
propagating system (Figure 11c). In contrast to the other two "warm"
events examined, the distribution of surface-based instability ahead of
the incipient LDD was non-uniform; CAPE was greatest over eastern
Oklahoma and decreased eastward into Arkansas where both temperatures
and dewpoints were lower (Figure 11c). As a result, there was a westward
component to cell propagation, and the system exhibited more rightward
movement relative to the mean flow than did the two other "warm" events.

Comparison of Figure 10c with Figure 11c shows that average radar
reflectivities during the period of high wind production in western
Arkansas were somewhat stronger (with a few cells greater than 50 dBZ)
than in the North Carolina case. This reflects the fact that some of the
incipient convection in Oklahoma included hail-producing supercells, as
CAPE in that area had been rather substantial (around 1000 J/kg; not
shown) during the afternoon. But the storms had weakened by the time
wind damage began to occur and the convection assumed LDD
characteristics over Arkansas around 0215 UTC.

To examine the origin of the steep lower tropospheric lapse rates
present in both events just discussed, back parcel trajectories for the
LDD initiation locations near the time of system genesis are presented
in Figure 12. The trajectories were computed for parcels at the 1000,
1500 and 3000 m AGL levels using the NOAA Air Resources Laboratory's
HYSPLIT model. The HYSPLIT (HYbrid Single-Particle Lagrangian Integrated
Trajectory) model was developed for computing air parcel trajectories in
dispersion and deposition studies (Draxler and Rolpf 2003). As it may be
accessed on-line (<http://www.arl.noaa.gov/ready/hysplit4.html>), it
provides a convenient way to track air parcel motion for use in severe
weather case studies.

As Figure 12 shows, boundary layer parcels in both events originated
over the elevated terrain of the western United States. The three
parcels sampled show a recent (24 to 48 hour) history of descent in the
8 March "cool" event. This suggests that subsidence on the anticyclonic
side of the associated mid level jet streak (not shown) may have been a
contributing factor in enhancing the steep lower tropospheric lapse
rates observed with this strongly forced event. In contrast, the 1000 m
parcel exhibits minimal height change during the 48 hours prior to the
18 March "warm" event, implying that the steep boundary layer lapse
rates in this case were related more to recent surface heating than to
dynamic effects.

**5. Concluding remarks**

This study has presented data on a subset of forward-propagating,
damaging wind-producing convective systems that we have termed low
dewpoint derechos. These systems occur in relatively dry lower
tropospheric environments characterized by very limited CAPE.

Because the LDD environment is not conducive to deep, moist convective
development, a source of strong mesoscale forcing for ascent (such as a
well-defined cold front or wind shift line) is necessary to initiate the
first storms which subsequently "jump start" LDD formation. Once
developed, we hypothesize that LDDs are then maintained by a
thermodynamic and kinematic environment that supports organized,
downwind cell propagation along storm outflow. Deeply mixed boundary
layers, in conjunction with moderate to strong and largely
unidirectional mean flow above the boundary layer, appear to promote an
"organized microburst" convective mode in which the incipient MCSs are
sustained by a series of downwind-directed microbursts. The microbursts
foster discrete downstream propagation of the convective systems until
the potential for convective initiation and subsequent cold downdraft
production ultimately diminishes.

Given the propensity for microbursts to occur during afternoon or early
evening (e.g., Wakimoto 1985), it is not surprising that a similar
diurnal peak in LDD occurrence appears in the present data set (Table
1). Note, however, that several LDDs developed in mid to late morning,
and that two initiated after sunset. These observations reflect the
conditionally unstable environment associated with LDD genesis, and
illustrate that strong forcing for ascent likely is a necessary
ingredient for achieving convective initiation in LDDs.

As previously noted, the LDDs in this study occurred in the exit region
of mid to upper level jet streaks. The events were, however, associated
with a wide range of large scale jet orientations. Coupled with the data
presented regarding the mean thermodynamic environment conducive to LDD
genesis, it seems reasonable to conclude that the jet orientation most
favorable for LDDs likely varies both seasonally and geographically
across the country. The limited number of cases examined, however,
necessarily precludes a definitive statement on this topic. Similarly,
at best we can only speculate as to the true geographical distribution
of LDDs. But the mean analyses do provide clues as to why the systems
thus far have not been observed over either the Gulf Coast region or the
far western United States. Considering that the primary source region of
steep low to mid-level lapse rates is the Plateau and Rockies, and that
sustained cyclonic upper level flow regimes favor the northern states,
it would appear that both the Far West and the Gulf Coast
climatologically are situated somewhat unfavorably for LDD development.
Similarly, the mean thermodynamic fields suggest that LDDs are probably
least likely to develop during late summer and early fall. Indeed, no
LDDs are known to have occurred during this period in recent years.

While this presentation has described some characteristics of the
synoptic and meso-alpha scale LDD environment, considerably more
information is needed on the storm-scale before the location and
intensity of the hazardous winds that LDDs produce can be forecast
reliably. For example, while it appears that dry air and steep lapse
rates are conducive to LDD development and maintenance, subtle changes
in the distribution and temporal evolution of these factors probably
account for some of the "null" LDD events occasionally observed.
Further, given the apparent importance of convectively-induced
downdrafts on LDD development, complex cloud microphysical processes
must be quantified before accurate forecasts become a reality. A
disproportionate number of the events studied here occurred during the
cool season. Does this simply reflect the fact that atmospheric moisture
content is lower at that time of the year, or perhaps that melting
and/or sublimation processes are important to LDD development? Just
prior to the submission of this manuscript, an LDD that occurred in Iowa
and Illinois on 12 February 2003 came to the attention of the authors.
Synoptically, the system appears to closely fit the LDD pattern
described in Section 3, with the exception that the environmental
profile was so cold that snow rather than rain occurred at the surface.
While this case suggests that the presence of melting is not a necessary
condition for LDD development, it does not preclude the possibility that
considerable sublimation may have occurred. Clearly, additional studies,
preferably those that blend both observational and modeling techniques,
are needed to better understand LDDs.

**Acknowledgements**

The authors would like to thank David Bright for the creation of the
GEMPAK backgrounds used in Figure 2, and for his many comments and
suggestions. We also very much thank Mike Coniglio, Dave Schultz, Steve
Weiss and an anonymous reviewer for insightful comments that improved
the manuscript. John Hart and Jason Levit provided software assistance,
and Jared Guyer obtained radar and satellite data for several cases.

**References**

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Bluestein, H. B., and P. C. Banacos, 2002: The vertical profile of wind
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Meteorology and Forecasting*, P. S. Ray, Ed., Amer. Meteor. Soc.,
289-310.

Coniglio, M. C., D. J. Stensrud, and M. B. Richman, 2004: An
observational study of derecho-producing convective systems. *Wea.
Forecasting*, **19**, 320-337.

Corfidi, S. F., 2003: Cold pools and MCS propagation: Forecasting the
motion of downwind-developing MCSs. *Wea. Forecasting*, **18**,
997-1017.

Doswell, C. A. III, H. E. Brooks, and M. P. Kay, 2005: Climatological
estimates of daily local nontornadic severe thunderstorm probability for
the United States. *Wea. Forecasting*, **20**, 577-595.

Draxler, R. R., and G. D. Rolpf, 2003: HYSPLIT (HYbrid Single-Particle
Lagrangian Integrated Trajectory) model access via NOAA ARL READY
website (<http://www.arl.noaa.gov/ready/hysplit4.html>). NOAA Air
Resources Laboratory, Silver Spring, MD.

Evans, J. S., and C. A. Doswell III, 2001: Examination of derecho
environments using proximity soundings. *Wea. Forecasting*, **16**,
329-342.

Fenelon, E. C., 1998: Doppler radar analysis of a microburst producing
bow echo associated with a high shear and very low CAPE environment.
Preprints, *19^th^ Conf. on Severe Local Storms*, Minneapolis, MN, Amer.
Meteor. Soc., 490-494.

Johns, R. H., 1993: Meteorological conditions associated with bow echo
development in convective storms. *Wea. Forecasting*, **8**, 294-299.

\-\-\-\-\-\--, and W. D. Hirt, 1987: Derechos: Widespread convectively
induced windstorms. *Wea. Forecasting*, **2**, 32-49.

\-\-\-\-\-\--, K. W. Howard, and R. A. Maddox, 1990: Conditions
associated with long-lived derechos -- An examination of the large-scale
environment. Preprints, *16^th^ Conf. on Severe Local Storms,*
Kananaskis Park, Alberta, Canada, Amer. Meteor. Soc., 408-412.

Orlanski, I., 1975: A rational subdivision of scales for atmospheric
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Przybylinski, R. W., 1995: The bow echo: Observations, numerical
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**10**, 203-218.

Schaefer, J. T., and C. A. Doswell III, 1979: On the interpolation of a
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Schultz, D. M., 2005: A review of cold fronts with prefrontal troughs
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Van Den Broeke, M.S., D. M. Schultz, R. H. Johns, J. S. Evans and J. E.
Hales, 2005: Cloud-to-ground lightning production in strongly forced,
low-instability convective lines associated with damaging wind. W*ea.*
*Forecasting,* in press.

Wakimoto, R. M., 1985: Forecasting dry microburst activity over the High
Plains. *Mon. Wea. Rev*., **113**, 1131- 1143.

\-\-\-\-\-\--, 2001: Convectively driven high wind events. *Severe
Convective Storms, Meteor. Monogr*., No. 50, Amer. Meteor. Soc.,
255-298.

Weiss, S. J., J. A. Hart, and P. R. Janish, 2002: An examination of
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Soc., 446-449.

Wolf, R., 1998: Evolution and structure of a low-topped severe squall
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Amer. Meteor. Soc., 480-483.

![](media/image1.png){width="6.559027777777778in"
height="3.959722222222222in"}

Figure 1. Tracks of LDD centroids for the events studied. Numbers refer
to cases listed in Table 1.

Figure 2 (next 3 pages). Composite surface (a) and upper air (b through
f) analyses for the 12 LDD events studied. Thick lines: Surface pressure
(in mb) or geopotential height (in decameters). Thin lines: Temperature
(solid red/blue) and dewpoint (dotted green) in ^o^C, except ^o^F in
(a). Temperature in 2 degree increments, except 4 degrees at surface
(a). Dewpoint not contoured at 700, 500 and 250 mb. Wind speed in knots
(half barb = 5 kts (2.5 m s^-1^), full barb = 10 kts (5 m s^-1^),
pennant = 50 kts (25 m s^-1^)). Isotachs (in knots) shown dashed blue at
850, 700, 500 and 250 mb. Jet axes at 250 mb depicted by dark blue
arrows in (f). Heavy dot is location of LDD centroid. Arrows on right
side indicate time-averaged direction of LDD motion.

![](media/image2.png){width="4.552777777777778in"
height="4.434722222222222in"}

\(a\) Surface

![](media/image3.png){width="4.552777777777778in"
height="4.4430555555555555in"}

\(b\) 925 mb

![](media/image4.png){width="4.552777777777778in"
height="4.426388888888889in"}

\(c\) 850 mb

![](media/image5.png){width="4.552777777777778in"
height="4.4430555555555555in"}

\(d\) 700 mb

![](media/image6.png){width="4.552777777777778in"
height="4.426388888888889in"}

\(e\) 500 mb

![](media/image7.png){width="4.552777777777778in"
height="4.451388888888889in"}

\(f\) 250 mb

![](media/image8.png){width="6.563888888888889in" height="6.625in"}

Figure 3. Mean sounding for LDD centroid location. Temperature red,
dewpoint green, wet bulb temperature cyan. Surface data plotted at 1000
mb. Wind speed in knots; half barb = 5 kts (2.5 m s^-1^); full barb = 10
kts (5 m s^-1^); pennant = 50 kts (25 m s^-1^). Wind directions are
relative to system motion (see also Figure 5).

![](media/image9.png){width="3.1645833333333333in"
height="3.073611111111111in"}![](media/image10.png){width="3.138888888888889in"
height="3.0659722222222223in"}

\(a\) (b)

![](media/image11.png){width="3.1645833333333333in"
height="3.0819444444444444in"}

\(c\)

Figure 4. Mean lapse rates in tenths ^o^C per km (decimal point omitted)
for the (a) 850-700 mb, (b) 700-500 mb, and (c) surface-500 mb layers,
contoured in 1 degree increments.

![](media/image12.png){width="6.495833333333334in" height="6.7125in"}

Figure 5. Mean hodograph for LDD centroid location, with speed rings
labeled in knots. Numbers along hodograph give height in km AGL; red
denotes 0-3 km; green 3-6 km and blue greater than 6 km. The x axis is
oriented parallel to the direction of observed mean LDD motion, with the
speed of mean motion (46 kts (23 m s^-1^)) given by the white dot.

Figure 6 (next two pages). Sequence of WSR-88D 0.5^o^ base reflectivity
data from Charleston, SC, at (a) 2140, (b) 2148, (c) 2157, (d)2205, (e)
2214, and (f) 2222 UTC 5 March 2005. Surface data depicted in standard
station model format: temperature and dewpoint (^o^F); pressure (tenths
of a mb) with leading 10 omitted; wind speed (half barb = 5 kts (2.5 m
s^-1^); full barb = 10 kts (5 m s^-1^)); and sky condition. Blue and
yellow lines delineate a severe thunderstorm watch issued by the NOAA
Storm Prediction Center for this event. See text for discussion of area
highlighted by white arrows.

![](media/image13.png){width="4.802083333333333in"
height="2.9069444444444446in"}

\(a\)

![](media/image14.png){width="4.802083333333333in"
height="2.9180555555555556in"}

\(b\)

![](media/image15.png){width="4.802083333333333in"
height="2.8854166666666665in"}

\(c\)

![](media/image16.png){width="4.802083333333333in"
height="2.890972222222222in"}

\(d\)

![](media/image17.png){width="4.804166666666666in"
height="2.9180555555555556in"}

\(e\)

![](media/image18.png){width="4.802083333333333in"
height="2.9069444444444446in"}

\(f\)

![](media/image19.png){width="2.8041666666666667in"
height="2.0548611111111112in"} ![](media/image20.png){width="2.80625in"
height="2.0548611111111112in"}

\(a\) (b)

![](media/image21.png){width="2.80625in" height="2.064583333333333in"}
![](media/image22.png){width="2.8048611111111112in"
height="2.0881944444444445in"}

\(c\) (d)

Figure 7. Sequence of GOES visible data satellite imagery showing
northeastward motion of LDD over western and northern Utah on at (a)
1800, (b) 1900, (c) 2000, and (d) 2100 UTC 31 May 1994.

![](media/image23.png){width="6.496527777777778in" height="6.54375in"}

Figure 8. Comparison of mean soundings for the nine "cold" events
(temperature solid blue, dewpoint dashed blue and wind barbs blue; wet
bulb temperature cyan) and three "warm" events (temperature solid red,
dewpoint dashed red and wind barbs red). Wind data as in Figure 3.

![](media/image24.png){width="6.496527777777778in"
height="6.692361111111111in"}

Figure 9. As in Figure 5, but for the nine "cold" (blue curve) and three
"warm" (red curve) events. Numbers on hodographs give height in km. Mean
motion for "cool" and "warm" events depicted by blue and red dots,
respectively.

![](media/image25.png){width="5.927083333333333in" height="4.71875in"}

Figure 10a. 500 mb analysis, 0000 UTC 8 March 2004. Standard station
model format used: temperature and dewpoint (^o^C); height (dm); and
wind (half barb = 5 kts (2.5 ms^-1^); full barb = 10 kts (5 m s^-1^);
pennant = 50 kts (25 m s^-1^)). Height contours solid black, 60 m
intervals; temperature contours dashed red, 2 degree intervals.

![](media/image26.png){width="6.486111111111111in"
height="6.513194444444444in"}

Figure 10b. Radiosonde observation, Greensboro, North Carolina, 0000 UTC
8 March 2004. Data display same as in Figure 3.

![](media/image27.png){width="5.475694444444445in"
height="8.758333333333333in"}

![](media/image28.png){width="6.0in" height="4.75in"}

Figure 11a. As in Figure 10a, but for 0000 UTC 18 March 2004.

![](media/image29.png){width="6.496527777777778in"
height="6.533333333333333in"}

Figure 11b. As in Figure 10b, but for Little Rock, Arkansas at 0000 UTC
18 March 2004.

![](media/image30.png){width="6.804861111111111in"
height="5.916666666666667in"}

Figure 11c. As in Figure 10c, but from Fort Smith, Arkansas, 0201 --
0537 UTC 18 March 2004, and with surface data depicted in standard
station model format: temperature and dewpoint (^o^F); pressure (tenths
of a mb) with leading 10 omitted; wind speed (half barb = 5 kts (2.5 m
s^-1^); full barb = 10 kts (5 m s^-1^)); and sky condition. Areas of
largely uniform, blue-shaded returns in western and southern sectors of
the displays are ground clutter.

![](media/image31.png){width="6.745138888888889in"
height="5.803472222222222in"}

Figure 11c, continued.

![](media/image32.png){width="3.5659722222222223in"
height="4.129166666666666in"}

![](media/image33.png){width="3.5256944444444445in"
height="4.054861111111111in"}

Figure 12. NOAA Air Resources Laboratory HYSPLIT model back trajectories
of the 1000 m (red), 1500 m (blue) and 3000 m (green) AGL parcels ending
at 0000 UTC on 8 March 2004 (top) and 18 March 2004 (bottom) for the LDD
initiation locations indicated by black stars. Panels at bottom depict
parcel height in mb (right) at the indicated month, date and hour (UTC).
Large circles, squares and triangles used at 24 hour intervals.

<table>
<colgroup>
<col style="width: 9%" />
<col style="width: 11%" />
<col style="width: 12%" />
<col style="width: 6%" />
<col style="width: 15%" />
<col style="width: 9%" />
<col style="width: 9%" />
<col style="width: 7%" />
<col style="width: 8%" />
<col style="width: 8%" />
</colgroup>
<tbody>
<tr class="odd">
<td><strong>Case</strong></td>
<td><p><strong>Sounding</strong></p>
<p><strong>Date (yymmdd)</strong></p></td>
<td><p><strong>Sounding</strong></p>
<p><strong>Time (UTC)</strong></p></td>
<td><p><strong>LDD</strong></p>
<p><strong>Type</strong></p></td>
<td><p><strong>Location</strong></p>
<p><strong>(states affected)</strong></p></td>
<td><p><strong>Initiation</strong></p>
<p><strong>Time (UTC)</strong></p></td>
<td><p><strong>Duration</strong></p>
<p><strong>(hr)</strong></p></td>
<td><p><strong>Speed</strong></p>
<p><strong>(m s<sup>-1</sup>)</strong></p></td>
<td><strong>Length (km)</strong></td>
<td><p><strong>Number</strong></p>
<p><strong>of</strong></p>
<p><strong>Reports</strong></p></td>
</tr>
<tr class="even">
<td><strong>1</strong></td>
<td>770509</td>
<td>0000</td>
<td>C</td>
<td>PA / MD</td>
<td>2300</td>
<td>5+</td>
<td>14</td>
<td>250+</td>
<td>10</td>
</tr>
<tr class="odd">
<td><strong>2</strong></td>
<td>891121</td>
<td>0000</td>
<td>C</td>
<td>PA / NJ</td>
<td>2100</td>
<td>6+</td>
<td>21</td>
<td>450+</td>
<td><em><strong>150</strong></em></td>
</tr>
<tr class="even">
<td><strong>3</strong></td>
<td>940419</td>
<td>0000</td>
<td>W</td>
<td>WI / MI</td>
<td>2230</td>
<td>4</td>
<td>21</td>
<td>300</td>
<td>14</td>
</tr>
<tr class="odd">
<td><strong>4</strong></td>
<td>940531</td>
<td>1200</td>
<td>C</td>
<td>UT</td>
<td>1500</td>
<td>8</td>
<td>13</td>
<td>350</td>
<td><strong>16</strong></td>
</tr>
<tr class="even">
<td><strong>5</strong></td>
<td>941121</td>
<td>0000</td>
<td>C</td>
<td>MO / IL</td>
<td>0400</td>
<td>4</td>
<td>25</td>
<td>350</td>
<td><strong>16</strong></td>
</tr>
<tr class="odd">
<td><strong>6</strong></td>
<td>950404</td>
<td>1200</td>
<td>C</td>
<td>PA / NY</td>
<td>1230</td>
<td>7+</td>
<td>26</td>
<td>650+</td>
<td><em><strong>100</strong></em></td>
</tr>
<tr class="even">
<td><strong>7</strong></td>
<td>970705</td>
<td>0000</td>
<td>W</td>
<td>TN / NC</td>
<td>1830</td>
<td>10</td>
<td>25</td>
<td>900</td>
<td><em>155</em></td>
</tr>
<tr class="odd">
<td><strong>8</strong></td>
<td>010314</td>
<td>0000</td>
<td>C</td>
<td>VA / MD</td>
<td>0030</td>
<td>2</td>
<td>28</td>
<td>200</td>
<td>43</td>
</tr>
<tr class="even">
<td><strong>9</strong></td>
<td>020601</td>
<td>1200</td>
<td>C</td>
<td>UT</td>
<td>1400</td>
<td>7</td>
<td>28</td>
<td>700</td>
<td><em><strong>44</strong></em></td>
</tr>
<tr class="odd">
<td><strong>10</strong></td>
<td>040308</td>
<td>0000</td>
<td>C</td>
<td>NC</td>
<td>2300</td>
<td>5+</td>
<td>20</td>
<td>350+</td>
<td><em><strong>22</strong></em></td>
</tr>
<tr class="even">
<td><strong>11</strong></td>
<td>040318</td>
<td>0000</td>
<td>W</td>
<td>OK / AR</td>
<td>2300</td>
<td>4</td>
<td>28</td>
<td>300</td>
<td><em>18</em></td>
</tr>
<tr class="odd">
<td><strong>12</strong></td>
<td>050305</td>
<td>1200</td>
<td>C</td>
<td>SC</td>
<td>1700</td>
<td>3+</td>
<td>23</td>
<td>250+</td>
<td><em>14</em></td>
</tr>
<tr class="even">
<td colspan="5"><strong>AVERAGES</strong></td>
<td>2000</td>
<td>5+</td>
<td>23</td>
<td>420+</td>
<td>50</td>
</tr>
</tbody>
</table>

**Table 1.** Data for LDD events studied. Sounding date and time refer
to that of radiosonde observation used in creation of the mandatory
level composite maps. Initiation time is for the calendar date closest
to sounding date/time for the first storms directly associated with the
LDD. Symbol "+" used under length and duration to denote cases for which
severe weather was occurring as convective system moved beyond the
continental United States. Symbols "C" and "W" under "LDD type" refer to
"cool" and "warm" types, respectively (see text for details).
**Boldface** in "Number of Reports" signifies occurrence of at least one
significant (65+ kt (32 m s^-1^)) measured wind gust; *italics* denote
occurrence of at least one report of severe hail (diameter greater than
or equal to 1.9 cm).

[^1]: "Subterranean" data were omitted from the two cases that affected
    the Great Basin.

[^2]: Recall (e.g., Chappell 1986, and references therein) that total
    MCS motion may be decomposed into two parts: (1) An advective
    component associated with movement of embedded convective cells by
    the mean wind, and (2) a propagational component reflecting the
    development of new cells relative to existing storms.