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2003WEATHER AND FORECASTING
Cold Pools and MCS Propagation: Forecasting the Motion of
NOAA/NWS/NCEP/Storm Prediction Center, Norman, Oklahoma
(Manuscript received 13 March 2003, in ﬁnal form 20 May 2003)
The primary factors that affect the direction of propagation and overall movement of surface-based mesoscale
convective systems (MCSs) are discussed. It is shown that although propagation is indeed related to the strength
and direction of the low-level jet as previous studies have shown, it is more speciﬁcally dependent upon the
degree of cold-pool-relative ﬂow and to the distribution of conditional instability present along a system’s gust
front. An updated technique that may be used to forecast the short-term (3–6 h) motion of MCS centroids based
on these concepts is introduced. The procedure builds on the long-established observation that MCS motion is
a function of 1) the advection of existing cells by the mean wind and 2) the propagation of new convection
relative to existing storms. Observed wind and thermodynamic data, in conjunction with anticipated cold-pool
motion and orientation, are used to assess the speed and direction of cell propagation, that is, whetherpropagation
will be upwind, downwind, or some combination of the two. The technique ultimately yields an estimate of
overall system movement and has application regardless of scale, season, or synoptic regime.
Thunderstorms are frequently organized in lines or
clusters known as mesoscale convective systems
(MCSs). The term MCS is generally reserved for en-
sembles of storms that satisfy certain spatial or temporal
criteria (see, e.g., Houze 1993, p. 334; Parker and John-
son 2000). In a less restrictive sense, however, any me-
soalpha- or mesobeta-scale (Orlanski 1975) area of
moist convection might be considered an MCS (Ray
Because MCSs produce a disproportionate share of
signiﬁcant convective weather (high winds, ﬂash ﬂood-
ing, etc.) and because their evolution is often not pre-
dicted well by operational numerical guidance, fore-
casting MCS motion is of considerable importance to
operational meteorologists. Forecasts of MCS motion
are dependent upon anticipation of the predominant
propagational mode or modes likely during an event. In
particular, it is important to distinguish between MCS
environments conducive to upwind propagation and
those that exhibit downwind storm development and
sometimes evolve into derecho-producing squall lines.
Attempts to forecast MCS movement have met with
mixed results. Merritt and Fritsch (1984) examined the
motion of more than 100 MCSs, most of which were
mesoscale convective complexes (MCCs; Maddox
Corresponding author address: Stephen F. Corﬁdi, Storm Predic-
tion Center, 1313 Halley Circle, Norman, OK 73069.
1980). They were among the ﬁrst to recognize that
though no true ‘‘steering level’’ exists for MCC motion,
such systems typically move approximately parallel to
the contours of the 1000–500-hPa thickness. They also
noted that although most convective systems move
downshear along the contours, others inexplicably move
upshear. The speed of MCC motion was found to be
modulated by the location of maximum low-level mois-
ture convergence relative to existing convection.
Newton and Katz (1958) and Chappell (1986), among
others, showed that the motion of a convective system
can be thought of as being the vector sum of 1) an
advective component approximated by the direction and
magnitude of the mean cloud-layer wind and 2) a prop-
agation component governed by the rate and location
of new cell formation relative to existing convection.
Building on this idea, and extending the work of Merritt
and Fritsch, Corﬁdi et al. (1996) showed that the prop-
agation component is, in many cases, directly propor-
tional but opposite in direction to the low-level jet. This
ﬁnding is somewhat surprising given that MCS prop-
agation can be inﬂuenced by a myriad of factors such
as the distribution of convective available potential en-
ergy (CAPE), convective inhibition, gravity waves, out-
ﬂow boundaries, and orographic effects.
This paper discusses MCS motion, with emphasis on
those factors related to a system’s cold pool that most
inﬂuence cell propagation and, ultimately, overall sys-
tem movement. Based on this presentation, a vector-
based forecast technique is developed for predicting the
18WEATHER AND FORECASTING
. 1. Schematic of the original vector technique, with MCS core
motion (thick dotted arrow) expressed as the vector sum of 1) ad-
vection of cells by the mean cloud-layer wind (arrow pointing to
upper right) and 2) cell propagation directed into the low-level jet
(arrow pointing to bottom of page). MCS centroid is depicted by the
cross symbol (after Corﬁdi et al. 1996).
motion of MCSs characterized by downwind propaga-
a. The original vector technique
Corﬁdi et al. (1996) developed a simple technique to
predict the short-term (3–6 h) motion of the mesobeta-
scale cores or ‘‘centroids’’ of MCSs using the low-level
jet to estimate the direction and rate of storm propa-
gation. Forecast centroid motion is taken to be the sum
of 1) a vector that represents cell advection by themean
cloud-layer wind (with ‘‘cloud layer’’ taken to be the
and 2) a vector that represents
storm propagation, that is, new cell development, equal
in magnitude but directed opposite to the low-level jet
(Fig. 1). In practice, the 850-hPa wind is used to ap-
proximate the low-level jet, although it is recognized
that this approach may not identify the true jet in all
cases. In the absence of a distinct low-level speed max-
imum in the vertical direction, the strongest wind in the
lowest 5000 ft (1.5 km) is generally used, in accordance
with Bonner (1968).
The vector technique is applicable in any kind of
environmental wind regime and requires knowledge of
only the 850-hPa and mean cloud-layer winds. The pro-
cedure is especially useful in identifying those kinematic
situations conducive to the development of quasi-sta-
tionary and ‘‘back building’’ MCSs (Bluestein and Jain
1985). Quasi-stationary systems arise when the wind
proﬁle is unidirectional and cell advection is exactly
offset by cell propagation. Back building occurs under
similar conditions but when propagation exceeds ad-
vection, resulting in overall upwind motion. Identiﬁ-
cation of such events is important because they fre-
quently are associated with excessive rainfall (Chappell
The original vector concept, while useful, is never-
theless subject to several limitations. First, the scheme
does not account for spatial and temporal changes in
the environmental wind that, in altering both cell ad-
The speed and direction of the mean cloud-layer wind are cal-
culated using the following relationship: Vmean 5(V850 1V700
1V500 1V300)/4, where V850 is the 850-hPa vector wind, etc.
vection and propagation, can affect MCS movement.
Therefore, motion estimates must be updated frequently
when the wind ﬁeld exhibits signiﬁcant spatial or tem-
poral variability. Second, there is no accounting for the
inﬂuence of terrain on convective development and low-
level ﬂow. As a result, the concept is more difﬁcult to
apply in cases of orographically forced convection(e.g.,
Pontrelli et al. 1999).
A more serious shortcoming of the original vector
approach follows from its assumption that new cell de-
velopment and, therefore, system propagation always
occur in the direction opposite that of the low-level jet,
or, more generally, the low-level ﬂow. To be sure, many
warm-season MCSs over the central United States in-
deed do exhibit propagation in that direction (see, e.g.,
Moore et al. 1993; and Junker et al. 1999) at a rate
approximated by the speed of the jet.
It is clear, how-
ever, that this is not always the case. For example, de-
recho-producing squall lines often move at a substantial
angle to the low-level ﬂow, especially during their ini-
tiation (Johns et al. 1990). Radar data reveal that, al-
though propagation is largely responsible for the ob-
served motion of these systems, new cell development
is not necessarily directed into the low-level ﬂow but
rather occurs on the leading (downwind) edge of the
system cold pool. For this reason, to be more universally
applicable, the vector concept must be modiﬁed to ac-
count for the presence of cold pools and the potential
for propagation away from the low-level ﬂow.
b. Cold-pool and gust-front motion
One of the more distinctive features of a well-orga-
nized MCS is the cold pool that develops at lower levels
beneath or just behind the strongest convection. Cold
pools represent the collective outﬂow of individual con-
vective cells and the negative buoyancy of parcels with-
in or beneath the convection. Sublimation and/or melt-
ing and evaporation of precipitation falling through un-
saturated air, precipitation drag, and vertical perturba-
tion pressure gradients are all factors that may enhance
downdraft development and cold-pool strength.
The periphery of a cold pool, that is, the gust front
or outﬂow boundary, is marked by low-level conver-
gence and ascent (Purdom 1973; Charba 1974; Goff
1976). As a result, gust fronts are often the site of new
cell development. Such activity typically is not distrib-
uted evenly along such boundaries. Instead, storm ini-
tiation tends to occur in discrete zones, within which
kinematic and/or thermodynamic factors are most fa-
vorable for development. Observation suggests that new
cell development occurs most readily where the ambient,
The original dataset used by Corﬁdi et al. (1996) was composed
primarily of nocturnal MCCs that were associated with well-deﬁned
low-level jets east of the Rockies. In retrospect, therefore, it is not
surprising that propagation was found to be correlated well with the
. 2. Plan-view schematic depicting temporal elongation of a
cold pool and gust front associated with the hypothetical quasi-uni-
directional wind proﬁle shown at right. Motion of boundary relative
to ground is depicted by conventional frontal symbols. Dashed lines
indicate gust-front positions at earlier times. Indicated spatial scale
is for illustrative purposes only.
low-level inﬂow relative to the boundary is greatest.
This result follows because areas of strong relative in-
ﬂow will also be regions of maximum lower tropo-
spheric convergence. As previously noted, these regions
are often governed by the position of the low-level jet.
If a signiﬁcant degree of relative motion exists between
a gust front and the low-level environmental wind, how-
ever, convergence maxima may develop along the
boundary in locations away from the low-level jet.
It is obvious that motion of a gust front must be
known if the pattern and intensity of relative inﬂow
along it are to be assessed. Many studies (see, e.g.,
Charba 1974; Goff 1976; Wakimoto1982; Droegemeier
and Wilhelmson 1987; Rotunno et al. 1988) have ex-
amined the motion of gust fronts. These investigations
determined that storm outﬂow behaves more or less as
a gravity current. Indeed, observational studies (e.g.,
Wakimoto 1982) have conﬁrmed that downstream gust-
front speed is governed largely by the density difference
between the downdraft air and that of the surrounding
environment and by the depth of the outﬂow. Because
evaporative cooling and precipitation drag often vary
markedly over time and space, however, downdraft pro-
duction is both temporally and spatially unsteady. In
addition, cold-pool depth and horizontal density differ-
ences ordinarily cannot be measured in real time. For
these reasons, gravity-current theory has proven to be
of limited operational value in forecasting gust-front
Given the limited utility of gravity-current theory, it
is worthwhile to consider the role of momentum transfer
in determining gust-front motion, because lower-tro-
pospheric wind data are generally readily available in
an operational setting. It is clear that, because of mo-
mentum transfer, a gust front will move preferentially
in the direction of the motion associated with the parcels
that contributed to the parent cold pool. Momentum
transfer largely explains, for example, why derecho-pro-
ducing MCSs embedded in northwesterly midtropo-
spheric ﬂow typically move southeastward (Johns and
Hirt 1987). The systems move southeast because their
gust fronts advance primarily in that direction. With
boundary layer convergence maximized along the gust
front on the southeast (downwind) side of the cold pools,
new cell development and, therefore, overall system mo-
tion are toward the southeast (assuming the existence
of a favorable thermodynamic environment).
Perhaps more surprising is that observation suggests
that momentum transfer may also be used to estimate
gust-front speed. Many processes, of course, can inﬂu-
ence gust-front motion on the local (i.e., mesogamma)
scale. Because these processes are often nonlinear,gust-
front speed is typically unsteady over periods on the
order of tens of minutes. Over longer intervals, however,
downwind gust-front behavior is observed to be more
uniform (e.g., Fovell and Ogura 1989). In fact, subjec-
tive examination of nearly 50 forward-propagating
MCSs over the central and eastern United States during
the last two decades has determined that, at least to a
ﬁrst approximation, average downwind gust-front speed
may be estimated by the mean cloud-layer wind or, more
speciﬁc, the speed of the parcels that contribute to the
parent cold pool. As will be shown in later sections,
this ﬁnding may be used to help to estimate the motion
of a forward-propagating MCS.
c. The role of gust-front orientation
Implicit in the observation that cold-pool motion is
determined to a large extent by momentum transfer is
the fact that, over time, cold pools tend to elongate in
the direction of the mean wind. This tendency is most
pronounced when the ﬂow is unidirectional. As a cold
pool elongates, some parts of its associated gust front
necessarily become oriented perpendicular to the mean
wind while other portions come to lie parallel to it.
Continued production of storm outﬂow forces boundary
segments oriented perpendicular to the mean wind to
progress downwind with time while ﬂow-parallel por-
tions move very slowly or not at all (Fig. 2).
The orientation of a gust front relative to the mean
wind is important in determining the direction of cell
propagation and, therefore, the kind of MCS that will
be most favored along it. For example, if sufﬁcient sur-
face-based instability is present along those segments
aligned parallel to the mean ﬂow, new storm develop-
ment is likely to occur repeatedly where low-level con-
vergence along the boundary is greatest. This, of course,
is often in the direction of the low-level jet. In a typical
situation, a component of the propagation will be up-
wind relative to the mean ﬂow. As a result, cells sub-
sequently track downwind in succession (‘‘train’’)along
the front (Fig. 3, top). Because the boundary does not
move, extended periods of such upwind development
can yield excessive precipitation as long as the wind
proﬁle remains unchanged. Indeed, this scenario de-
18WEATHER AND FORECASTING
. 3. (right) Plan view of an elongating cold pool, with cross sections perpendicular to the gust front along (top)
a quasi-stationary segment and (bottom) a progressive segment, showing direction of cell propagation. Hypothetical
wind proﬁles at left are for illustrative purposes only.
scribes the ‘‘mesohigh’’ ﬂash-ﬂood MCS pattern of
Maddox et al. (1979).
If instability is present along those portions of the
gust front oriented perpendicular to the mean wind, ther-
modynamics are favorable for the formation of strong
convective-scale downdrafts, and sufﬁcient conver-
gence is present to initiate storms, downwind or ‘‘for-
ward’’ propagation is likely to occur. Assuming that
these conditions are maintained for some period of time,
a bow echo or derecho-producing MCS may develop
(Fig. 3, bottom). Because cell advection and propagation
are additive, some degree of front-to-rear ﬂow will nec-
essarily be present relative to the gust front. Such sys-
tems occasionally move much faster than the mean wind
when the propagation rate is great.
d. Concurrent upwind and downwind propagation
The role of gust-front orientation in determining prop-
agation direction and MCS type is perhaps best dem-
onstrated by the occasional observation of concurrent
back-building and forward-propagating convective sys-
tems in environments of largely unidirectional mean
ﬂow and minimal cloud-layer shear. As Chappell(1986)
noted, environments that are kinematically supportive
of quasi-stationary or back-building MCSs may also
yield fast-moving, forward-propagating squall lines. In-
deed, the implied wind proﬁle in Maddox et al.’s(1979)
schematic depicting a back-building ‘‘synoptic’’ ﬂash-
ﬂood-producing MCS (their Fig. 6) is similar to that
found by Johns et al. (1990) to be associated with de-
recho-producing squall lines (their Figs. 4–8). What dis-
tinguishes between the two propagational modes is the
orientation of the gust front relative to the mean wind.
The radar evolution of two concurrent bow echo/
back-building MCS events is depicted in Fig. 4. The
ﬁrst occurred in moderate westerly ﬂow on the northern
edge of a subtropical ridge on 24 August 1998 (Fig.
4a). A small bow-shaped MCS moved across northern
Illinois and Indiana, producing wind gusts to 80 kt (40.0
) near Chicago, Illinois. The bow MCS was fol-
lowed by a back-building convective cluster that sub-
sequently caused heavy rain over neighboring parts of
northern Illinois. The latter system developed along and
just behind the trailing outﬂow boundary (gust front)
associated with the bow MCS as the boundary became
quasi-stationary and parallel to the westerly unidirec-
tional mean wind.
A similar event affected the Kansas City, Missouri,
area several weeks later (Fig. 4b). Thunderstorms de-
. 4. Composite Doppler radar depiction of reﬂectivity over (a) northern Illinois, 1700–2000 UTC 24 Aug 1998
and (b) northern Missouri, 0030–0200 UTC 5 Oct 1998.
18WEATHER AND FORECASTING
. 5. Skew T–logpplot of radiosonde observations for (a) Lincoln, IL (near Springﬁeld), at 1800 UTC 24 Aug 1998, and (b) Topeka,
KS, at 1800 UTC 4 Oct 1998. Winds are in knots [barb 510 kt (5 m s
); ﬂag 550 kt (25 m s
)]. Lifted parcel ascent curve (large
dashed line) is for most unstable parcel, including correction for virtual temperature (rightmost small dashed line). Wet-bulb zero line is
shown as a dashed line between environmental temperature and dewpoint proﬁles. Numbers on the hodograph in the upper-right-hand side
depict the altitude above ground level in kilometers.
veloped during the afternoon along a stationary front
oriented parallel to a zone of strong unidirectional south-
west ﬂow aloft. By evening, the activity evolved into a
linear MCS containing an embedded bow echo. The
bowed segment of the system produced damaging winds
in northern Missouri. Of more signiﬁcance was the ﬂash
ﬂooding in Kansas City that accompanied the trailing
southwestern part of the same complex. The ﬂooding
occurred as storm cells repeatedly developed and moved
northeast along the stalled outﬂow boundary left by the
The MCS genesis region in both the Illinois and Mis-
souri events was characterized by unidirectional low-
to-midtropospheric ﬂow, with limited shear in thecloud-
bearing layer [Figs. 5a,b; note that the Topeka, Kansas,
sounding (Fig. 5b) was taken just west of the surface
front mentioned in the previous paragraph; the portion
of sounding above 850 hPa is believed to be represen-
tative of warm-sector conditions east of the front]. Sim-
ilar conditions prevailed farther downstream, along the
paths taken by the forward-propagating members of
each event. As the cold pools elongated, the gust-front
segments oriented perpendicular to the mean wind be-
came the site of downwind convective development
while upwind propagation persisted on those portions
of the boundary that became quasi stationary.
Although not recognized as such at the time, one of the ﬁrst
documented concurrent bow-echo/back-building MCS events was the
Independence Day storm of 4–5 July 1969. Widespread damage from
high winds followed by ﬂash ﬂooding left 41 dead across Michigan,
Ohio, and Lake Erie (Hamilton 1970).
e. The role of dry air
Previous work has suggested that, in addition to gust-
front orientation and motion, thermodynamic factors
might also play a role in determining the primary mode
of MCS propagation. For example, Corﬁdi (1998) con-
ducted a preliminary examination of proximity sound-
ings from MCSs that occurred in environments of large-
ly unidirectional ﬂow over the central and eastern Unit-
ed States between 1980 and 1998. The results suggest
that a characteristic common to those systems that
evolved into bow echoes and/or derechos was the pres-
ence of relatively dry air, either at midlevels or in the
subcloud layer, ahead of the developing convectivesys-
tem. This air appeared to be associated with the for-
mation of a strong cold pool. In converse, quasi-sta-
tionary and back-building MCSs were found to occur
in moister or nearly saturated lower-tropospheric en-
vironments, with comparatively weak cold pools. In
short, the potential to produce cold convective-scale
downdrafts (and, therefore, a strong cold pool) appeared
to distinguish forward-propagating environments from
those more conducive to upwind development.
Dry air is, of course, clearly associated with the oc-
currence of derechos and bows. Johns et al. (1990) noted
the presence of large dewpoint depressions at 700 and
500 hPa in the vicinity of long-lived derechos, and the
ingestion of dry air from the prestorm environment can
assist in the formation and maintenance of surface me-
sohighs by enhancing storm-scale buoyant pressure
ﬁelds and their associated gust-front circulations.
More recent analysis, however, using a dataset of 48
forward-propagating MCSs associated with damaging
surface winds, along with examination of quasi-station-
ary systems that produced major ﬂash ﬂoods in recent
decades, suggests that the relationship between cold-
pool strength and forward-propagating MCS develop-
ment is not so clear. Cold pools are not necessarily weak
in all cases of quasi-stationary or back-building con-
vection; indeed, some quasi-stationary MCSs exhibit
prominent cold pools. For example, the system that pro-
duced the Johnstown, Pennsylvania, ﬂood in July of
1977 (Hoxit et al. 1978; Bosart and Sanders 1981) had
a strong cold pool, and a similarly strong cold pool was
present in the Kansas City ﬂood case just discussed.
Cold-pool strength and, therefore, expansion rate are
certainly positively correlated with downwind MCS de-
velopment, but it is clear that the potential to produce
a strong cold pool cannot alone be used to distinguish
between environments conducive to upwind versus
downwind development; gust-front orientation and mo-
tion are also important.
3. A vector technique for downwind-propagating
a. Development of a vector scheme for downwind-
In this section, a scheme similar to that presented in
Corﬁdi et al. (1996) to estimate the short-term motion
of forward-propagating MCSs is described. Using the
original (1996) technique as a starting point, the ap-
proach applies the concepts discussed in the previous
section to account for cell propagation away from the
low-level jet, along the downwind side of a cold pool.
It has been noted that momentum transfer forces gust-
front segments oriented perpendicular to the mean ﬂow
to move downwind over time. It has been noted also
that the rate of downwind gust-front motion is strongly
correlated with the speed of the mean cloud-layer wind.
Because the gust front is the mobile locus of new con-
vective development in a forward-propagating MCS, a
motion estimate for the boundary (i.e., the cloud-layer
wind) can serve as a proxy for the advective component
of forward-propagating MCS motion.
If one accepts that the advective component of a for-
ward-propagating convective system is given by the
mean cloud-layer wind, examination of the schematic
depicting the original vector technique (Fig. 1) reveals
that the MCS motion vector provided by that scheme
is, in fact, the propagation vector of a forward-propa-
gating system. This result follows because the motion
vector of the original scheme represents the vector dif-
ference between a gust front moving at the speed of the
mean cloud-layer wind and the low-level ﬂow. In other
words, the motion vector provided by the original tech-
nique is, in fact, the negative of the gust-front-relative
low-level ﬂow for a boundary moving with the speed
and direction of the mean cloud-layer wind.
The length of the motion vector provided by the orig-
inal technique is directly proportional to the degree of
convergence and rate of new cell development along
the gust front. Addition of this vector representing cell
propagation along the gust front to that representing the
downwind motion of the boundary (i.e., the mean cloud-
layer wind) can therefore provide an estimate of the
overall motion of a forward-propagating MCS. In short,
the vector approach for a forward-propagating system
requires just one extra vector addition beyond the two
used in the original method (where upwind cell devel-
opment is assumed) and can yield a drastically different
forecast motion, as shown in Fig. 6.
Table 1 presents the results of applying the forward-
propagating vector technique to 48 convective systems
associated with damaging surface winds. The events
occurred throughout the central and eastern United
States, predominantly during the spring and summer.
They were selected on the availability of a sounding
representative of the inﬂow environment [uncontami-
nated, and within 100 n mi (185 km) and 2 h of the
event] and composite radar data. Forecasts were made
for the 3-h motion of the strongest MCS radar reﬂec-
tivity core (the MCS centroid).
As the table shows, successful forecasts [deﬁned as
direction and speed of motion within 208and 10 kt (5.0
), respectively, of observed] were produced for 38
of the 48 events. On average, the speed errors are ran-
dom, although there appears to be a tendency to un-
derestimate the forward motion of systems containing
embedded supercells and/or strong rear-inﬂow jets (la-
beled ‘‘SPRCL/RIJ’’ in right-most column of Table 1)
Enhanced and/or otherwise altered downstream propa-
gation rates associated with the presence of these fea-
tures are believed to be responsible for the errors.
The directions of motion forecast by the downwind
technique display a small left bias (negative directional
errors in Table 1). This observation most likely reﬂects
the large-scale warm-advection environment within
which the MCSs occurred. Because the lower-tropo-
spheric shear typically turns right (clockwise) down-
stream from warm-advection maxima, there is a ten-
dency for forward-propagating MCSs to turn right with
time (e.g., Johns et al. 1990). Of course, these systems
do not physically change direction per se; the ‘‘turning’’
reﬂects a gradual rightward shift in the area most fa-
vored for new cell development as the systems move
downwind. Because application of the vector technique
uses instantaneous wind data obtained at a given point
in time, it is impossible to account for such longer-term
rightward deviation in any one forecast. The effect is,
however, seen easily if simultaneous forecast motions
are plotted spatially on a regional grid. Note also that,
for longer-lasting systems, Coriolis accelerations acting
on the rear-to-front and front-to-rear ﬂows may also bias
motion to the right (Skamarock et al. 1994).
A surface boundary external to the convective system
that forced propagation to occur away from the purely
downwind direction resulted in signiﬁcant directional
18WEATHER AND FORECASTING
. 6. Comparison schematics of (top) original (upwind) and (bottom) downwind versions
of the vector technique to forecast short-term motion of upwind-developing systems (MCSmotion
given by thick red arrows) and downwind-developing systems (MCS motion given by thick green
arrow at bottom of ﬁgure), respectively. Vector lengths are proportional to wind speed; MCS
centroids are denoted by the cross symbol.
errors in three warm-season cases (11, 32, and 36). It
is clear that surface data in the vicinity of a developing
MCS must be examined carefully to identify any syn-
optic or mesoscale boundaries that might have such an
effect. Such situations will require modiﬁcation of the
vector technique (namely, rotation of the propagation
vector) to account for the altered direction of propa-
Three of the 10 unsuccessful forecasts in Table 1 ap-
pear to have been related to the large-scale environment
in which the events occurred. Each was associated with
a serial bow MCS along a cold front (Johns and Hirt
1987), and forecast speed was overestimated in each case.
In one instance (case 16), overall system motion was
slowed because surface-based instability, present both
upwind and downwind from the initial convective area,
enabled the MCS to exhibit simultaneous upwind and
downwind propagation. In the remaining two cases (20
and 29), system motion appeared to be overestimated
because propagation was very limited relative to advec-
tion. The squall lines moved downstream roughly at the
speed of the mean wind and associated cold front, ap-
parently as a result of nearly saturated conditions in the
surface-to-700-hPa layer (not shown). Further discussion
of this topic is provided in section 5.
The factors associated with two of the remaining un-
successful forecasts (cases 1 and 12) were not readily
apparent and await further investigation.
4. Case applications of the downwind vector
a. Comparison application of the original and
downwind techniques: 16 August 1997
Figure 7a shows a proximity thermodynamic sounding
and wind proﬁle associated with the incipient stage of a
forward-propagating MCS that subsequently moved
across northern Ohio and Pennsylvania on 16 August
1997 (case 48 in Table 1). The system developed in an
environment of moderate, unidirectional westerly ﬂow in
the warm sector of a surface wave crossing southern
Quebec, Canada (Fig. 7b). Large-scale forcing was weak,
similar to situations described by Coniglio and Stensrud
(2001) and Evans and Doswell (2001). Considerable sur-
face-based instability was present, however, throughout
the warm sector, within which afternoon temperatures
warmed to above 908F (308C; not shown).
Using a mean wind vector of 2608/35 kt (18 m s
and a low-level ‘‘jet’’ of 2508/32 kt (16 m s
cation of the original vector technique yields a system
movement toward the east-southeast at approximately
1. Forward-propagating MCS events used to test the downwind vector technique. Data include date/time (year 11900, month,
day, UTC hour), three-letter identiﬁcation of sounding (raob) site used to calculate pertinent wind vectors, forecast (FCST) and observed
(OBSVD) MCS motion [direction (DIR)/speed (SPD); speed is in knots; 1 kt is about 0.5 m s
], and direction/speed errors. Direction and
speed errors exceeding 208and 10 kt (5.0 m s
), respectively, are in boldface. The apparent source of failure for those events exceeding
the above criteria is indicated in right-most column (Boundary means external boundary was present; SPRCL/RIJ means system contained
supercells and/or a rear-inﬂow jet; Translational means system was strongly affected by the translational motion of a mesoalpha-scale
environment conducive to storm initiation; Unknown means the source of error was not readily apparent). See text for details.
Case no. Date/time
(YYMMDDHH) Raob site
(DIR/SPD) Observed motion
(DIR/SPD) Direction error
(FCST2OBSVD) Speed error
(FCST2OBSVD) Apparent failure
). As Fig. 7c (top) shows, the original
vector approach depicts a scenario in which cell ad-
vection is offset almost totally by cell propagation.
However, as might be expected given the availability
of dry air at midlevels (Fig. 7a), the MCS began to
produce strong convective downdrafts early in its life
cycle; by 1400 UTC, a well-deﬁned cold pool was pres-
ent beneath it (not shown).
Because the associated
downdrafts brought strong westerly winds to the sur-
Note that the sounding in Fig. 7a was taken around local sunrise;
insolation after this time resulted in substantial boundary layer warm-
ing downwind from the incipient convective system, enhancing both
updraft strength and downdraft production.
18WEATHER AND FORECASTING
. 7. (a) Same as Fig. 5a, but for White Lake, MI (near Detroit), 1200 UTC 16 Aug 1997. (b) Regional surface
mesoanalysis, valid for the same time as in (a): thermodynamic data (8F), wind (kt), and pressure (hPa, with ﬁrst
two digits omitted); synoptic-scale boundaries are depicted with large pips, mesoalpha-scale gust front is shown with
small pips, and center of the MCS mesohigh is indicated by the ‘‘B’’ (‘‘bubble high’’) over southeast Michigan. (c)
(top) Application of original and (bottom) downwind versions of the vector technique to the 16 Aug 1997 MCS,
based on sounding data in (a). Forecast motions are depicted by heavy solid arrows, with the MCS centroid depicted
by the cross symbol. Directions are in degrees azimuth, and speeds are in knots. (d) Three-hourly radar-observed
positions of leading convective line (solid lines) and severe-weather reports (damaging winds are crosses, hail is
dots, and tornadoes are small squares) associated with the forward-propagating MCS of 16 Aug 1997.
face, the cold pool elongated toward the east. At the
same time, capping prohibited the development of new
convection toward the west (i.e., in the upwind direc-
tion), despite the fact that the near-surface ﬂow wasfrom
the west. As a consequence, with strong system-relative
convergence and instability both present in the down-
wind (east) direction, ascent along the progressive part
of the gust front readily led to new cell development in
the downwind direction, and the system propagated to
the east. Because cell advection was also toward the
east, however, advection and propagation were nearly
directly additive (Fig. 7c, bottom). Thus, the MCS did
not remain nearly stationary as the original vector tech-
nique would suggest but rather accelerated eastward as
a forward-propagating squall line that moved at a speed
faster than that of the mean wind (Fig. 7d). The down-
wind vector technique’s motion estimate of 2658at 40
kt (20 m s
; Fig. 7c, bottom) compares favorably to the
9-h observed mean motion of 2758at 45 kt (22 m s
The original vector technique seriously underesti-
mated the motion of the squall line because it failed to
account for the fact that propagation would occur down-
wind rather than upwind. This case demonstrates the
need to identify the region of greatest system-relative
convergence and the distribution of surface-based con-
ditional instability along the gust front when determin-
ing the preferred direction of propagation. Use of the
850-hPa wind or some other estimate of the low-level
ﬂow to represent propagation will yield erroneous re-
sults when convergence is maximized in a direction
away from the low-level jet in the presence of condi-
18WEATHER AND FORECASTING
. 8. (a) Same as Fig. 7d, but for the forward-propagating MCS of 19–20 Jul 1983. (b) Same
as Fig. 5a, but for Bismarck, ND, 1200 UTC 19 Jul 1983. (c) Same as Fig. 7b, except valid for
the same time as in (b). The center of the MCS mesohigh is indicated by the ‘‘B’’ (bubble high)
over northwest North Dakota. (d) Application of the downwind vector technique for the the 19
Jul 1983 MCS, based on sounding data in (b). Directions are in degrees azimuth, and speeds are
in knots. The MCS centroid is depicted by the cross symbol.
b. Application of downwind technique to a derecho:
19–20 July 1983
In the middle (low) latitudes, where the mean tro-
pospheric ﬂow is typically westerly (easterly), gust-
front-relative ﬂow will be enhanced when the boundary
layer winds have an easterly (westerly) component. In
some instances, the magnitude of gust-front-relative
ﬂow sometimes exceeds that of the mean cloud-layer
wind. Depending upon thermodynamic conditions, con-
vective systems developing in this kind of environment
occasionally attain speeds that are more than 2 times
that of the mean wind. The classic derecho of 19 July
1983, which produced a swath of widespread wind dam-
age across the upper Mississippi Valley (Fig. 8a; see
Johns and Hirt 1985), serves as an example of this type
of an event.
Figure 8b, the thermodynamic sounding and wind
proﬁle taken at Bismarck, North Dakota, at 1200 UTC
19 July, is representative of conditions during the ini-
tiation of the MCS. CAPE, calculated by lifting a parcel
from near 850 hPa, is substantial (around 4000 J kg
and nearly dry adiabatic lapse rates are present at mid-
levels to foster strong convective downdraft develop-
ment. As Fig. 8c shows, the system formed in a region
of high boundary layer moisture content [average sur-
face dewpoints around 658F (188C)] on the north side
of a weak west–east front that had become stationary,
parallel to the mid- and upper-tropospheric ﬂow. The
MCS raced east-southeast during the following 15 h,
averaging more than 50 kt (25 m s
), despite the fact
that the mean cloud-layer wind over the region during
the period was westerly at only 25 kt (12 m s
Conditions were favorable for downwind develop-
ment as the boundary layer moisture axis extended east
into Wisconsin, and an easterly component was present
in the lower levels to enhance inﬂow to the gust front.
At the same time, capping associated with ampliﬁcation
of the large-scale ridge upstream from the system (not
shown) prohibited convective initiation on the upwind
side of the cold pool produced by the ﬁrst storms over
northwest North Dakota. Application of the downwind
vector technique (Fig. 8d) readily illustrates how ex-
18WEATHER AND FORECASTING
treme system motions can be attained when cell advec-
tion and propagation are not only additive, but propa-
gation speed is enhanced by an ‘‘opposing’’(in this case,
easterly) component to the boundary layer wind.
Another factor that can contribute to the rapid down-
wind movement and may have been a factor in thiscase
is of thermodynamic origin. It is frequently noted (e.g.,
Johns 1993) that boundary layer moisture tends to
‘‘pool’’ on the poleward side of weak warm-season
fronts, such as the one over South Dakota and lowa
(Fig. 8c). Indeed, evapotranspiration can signiﬁcantly
augment the local boundary layer moisture content, es-
pecially when the mixed-layer depth remains constant
as a result of cloudiness and/or the presence of a frontal
inversion. The added moisture lowers the level of free
convection and assists convective initiation along the
gust front, thereby hastening downwind propagation.
c. Application of downwind technique to a cool-
season derecho: 20–21 November 1989
Although storm-scale downdrafts are believed to orig-
inate primarily above the lifting condensation level
(Wakimoto 2001), some of the cases examined in this
study suggest that forward propagation may also be fos-
tered by mesohigh development associated with the
presence of dry air in the subcloud layer. This ‘‘orga-
nized microburst’’ MCS mode occurs most frequently
in arid regions, although systems of this kind occasion-
ally develop elsewhere when moisture is sparse but steep
lower-tropospheric lapse rates are present to enhance
convective downdraft development. For example, sev-
eral mesosystems of this type have produced signiﬁcant
wind damage in the mid-Atlantic region and over the
Midwest and plains in recent years. Moisture in such
situations is often so limited that thunderstorms can only
develop where sustained convergence is provided by a
gust front, orography, or some other mechanical initi-
ating mechanism. Once storms do form, the resulting
MCS is sustained by a downwind succession of micro-
Operational experience has shown that systems that
form in environments of this kind typically display weak
radar reﬂectivities but can produce devastating winds.
For example, in the 20 November 1989 case discussed
here, thunderstorm echo tops associated with the even-
ing squall line were at or below 20 000 ft (7 km), and
maximum reﬂectivities were less than 30 dBZ. Never-
theless, the storms produced a continuous swath of dam-
aging winds from central Pennsylvania into southeast
New York and southern New England, with measured
gusts in excess of 70 kt (35 m s
; Fig. 9a).
The environment across the mid-Atlantic region on
the afternoon of 20 November was characterized by fast,
largely unidirectional west-northwesterly ﬂow in ad-
vance of a short-wave disturbance and cold front over
the upper Great Lakes (not shown). Modiﬁed, dry polar
air was present ahead of the front. Because boundary
layer moisture was limited [surface dewpoints below
458F(78C)], CAPE was minimal (Figs. 9b,c; note that,
because the sounding site was south of the MCS track
and south of the associated midlevel jet streak, an in-
version is depicted at 700 hPa that was substantially
weaker or nonexistent farther north). Nevertheless, lapse
rates were steep, especially for the time of the year and
the region. Sunshine and westerly (downslope) ﬂow east
of the Appalachians warmed afternoon surface temper-
atures to the mid-60s Fahrenheit (18–208C) over the
lower elevations of eastern Pennsylvania and New Jer-
sey, producing large dewpoint depressions.
air enhanced the buoyancy, fostering late-day thunder-
storm development along the cold front in central Penn-
sylvania. Sustained uplift along the front and ideal con-
ditions for cold downdraft production allowed the
storms to grow quickly into a linear MCS. Because
storm advection and propagation were additive, thesys-
tem accelerated southeastward at nearly 60 kt (30 m
), more than 20 kt (10 m s
) faster than the mean
cloud-layer wind (Fig. 9d).
5. Practical aspects of application
a. Elevated systems
A signiﬁcant forecast problem involving MCS de-
velopment on the cool side of surface boundaries is
determining whether the system will remain elevated or
will at some point become ‘‘rooted’’ in the boundary
layer. Dependent as they are on the existence of surface-
based convection along a gust front, it is clear that nei-
ther the original nor downwind versions of the vector
scheme can be applied to a purely elevated MCS. De-
termining the potential for surface-based development
with an elevated MCS is difﬁcult, although systems with
strong cold pools and relatively warm/moist ‘‘cool’’ sec-
tors are good candidates. In the 19–20 July 1983 event,
for example, daytime heating eroded the shallow skin
layer present in the morning over North Dakota (Fig.
8b), resulting in a deep afternoon mixed layer over Min-
nesota and Wisconsin (not shown). This allowed bound-
ary layer parcels north of the stationary front to be lifted
along the gust front, contributing to the rapid down-
stream propagation observed. It should not be assumed,
however, that an MCS will remain completely elevated
just because the low-level air is cold (e.g., Schmidt and
Cotton 1989). Upon selection of a representative ‘‘in-
ﬂow’’ wind, the vector technique may, of course, always
be used to estimate future system motion if it appears
that an elevated MCS might become surface based.
Such environments, in theory, are characterized by a substantial
degree of downdraft CAPE (DCAPE). Because of the limitations of
parcel theory used in its development DCAPE is often not a reliable
estimator of cold-pool strength, especially in the presence of sub-
stantial shear (Gilmore and Wicker 1998).
b. MCSs containing supercells and mesoscale
The presence of embedded supercells can signiﬁ-
cantly affect MCS evolution and motion. Many MCSs,
especially those that produce derechos, initiate as su-
percells (e.g., Johns and Leftwich 1988; Klimowski et
al. 2000). In other cases, the onset of forward propa-
gation and bow-echo development appears to be related
to the appearance of rotating updrafts in existing con-
vection [e.g., the Texas derecho of 4 May 1989 (Smith
1990) and the 17 August 1994 Lahoma, Oklahoma,
event (Janish et al. 1996)]. At the same time, embedded
supercells sometimes occur in back-building or quasi-
stationary convection [e.g., Texas to Mississippi, 15–16
November 1987 (Corﬁdi et al. 1990) and Arkansas/Ten-
nessee, 1 March 1997 (Rogash et al. 2000)].
As Schmidt and Cotton (1989) and others have
shown, the presence of a supercell can drastically alter
storm-scale ﬂow within an MCS, thereby inﬂuencing its
overall motion, strength, and longevity. For example,
in a case included in the developmental sample for the
downwind vector technique (case 46 in Table 1; Spoden
et al. 1998), forecast speed was signiﬁcantly underes-
timated [forecast: 40 kt (20 m s
); observed: 70 kt (35
)], although the system’s eastward motion was
correctly depicted. The presence of a strong, cyclonic
circulation in the northern part of the MCS may have
hastened the system’s forward movement by increasing
westerly ﬂow in the cold pool. In a case presented by
Schmidt and Cotton (1989), redistribution of the pre-
cipitation cascade by a persistent rotating storm in an
elevated squall line altered the shape of the system’s
cold pool. This not only affected storm propagation, but
also the location of strongest surface winds. It is also
worth noting that the presence of ‘‘book-end vortices’’
can hasten MCS motion by fostering the development
of rear-inﬂow jets (e.g., Weisman 1993).
Long-lasting MCSs sometimes contain larger-scale
convectively induced circulations known as mesoscale
vorticity centers (MCVs). These features, which develop
in response to Coriolis acceleration of the rear-to-front
or front-to-rear ﬂow and/or in response to tilting and
stretching of environmental and system-generated vor-
ticity, also affect MCS motion and longevity (e.g., Bran-
des 1990; Bartels and Maddox 1991; Davis and Weis-
man 1994; Skamarock et al. 1994; Trier et al. 1997;
Weisman and Davis 1998). The original dataset of Cor-
ﬁdi et al. (1996) and the cases examined for the down-
wind vector technique include events with both MCVs
and supercells. In fact, the prominent mesoscale vortex
associated with one of the cases in the original study
(6–7 July 1982) was the subject of detailed investigation
(Menard and Fritsch 1989). Absence of high-resolution
radar data precludes an accurate assessment of the rel-
ative frequency of MCVs and supercells in the datasets
used to develop the vector technique. It is clear that the
inﬂuence of supercells and other vortices is too complex
to be addressed explicitly by the scheme. Nevertheless,
because the collective impact of these features was an
unwitting factor in its development, the presence of a
supercell or MCV in a given MCS does not necessarily
mean that the technique will yield erroneous results.
c. Inﬂuence of the background synoptic-scale
The advective component of MCS motion becomes
increasingly dominant relative to propagation as the
translational motion of the background synoptic-scale
‘‘support’’ for an MCS increases. This effect is most
apparent in conjunction with cool-season serial bow
MCSs (Johns and Hirt 1987). Because the support (usu-
ally a short-wave trough) in such cases often moves
rapidly, and because nearly saturated conditions and/or
inversions are typically present in the lower troposphere
to limit downwind propagation, the vector technique
often overestimates the motion of serial bows, as was
noted in section 3. With the convection conﬁned to a
narrow zone of forced ascent along a front, systems of
this kind essentially move with the speed of the asso-
ciated synoptic-scale disturbance.
Although it is often not obvious to the casual ob-
server, the translational speed of an MCS’s synoptic
support can signiﬁcantly inﬂuence the sensible weather
produced by the system. For example, cold fronts in
environments of strong, largely unidirectional ﬂow are
often accompanied by quasi-linear MCSs (Hobbs and
Persson 1982). These systems sometimes exhibit con-
siderable forward motion because of movement of the
front (and the short-wave trough) and therefore often
do not yield excessive rainfall. Inspection of time-lapse
radar data and application of the original vector tech-
nique reveals, however, that many such MCSs are ac-
tually quasi stationary or back building relative to the
front. The absence of excessive precipitation reﬂects the
‘‘external’’ component of motion that maintains system
An example of this kind of event occurred in con-
junction with an intense cyclone over the Mississippi
Valley on 9–10 November 1998. The linear MCS in
question extended for more than 400 n mi (740 km),
embedded in deep unidirectional southwest ﬂow ahead
of a progressive short-wave trough (Fig. 10a). The nar-
row line of forced convection moved northeast at 30 kt
(15 m s
), roughly with the speed of the cold front/
upper trough responsible for its development. The ther-
modynamic environment (not shown) was such that sur-
face-based storm initiation was prohibited except along
the front, and cold convective downdraft potential was
minimal. As a result, the weak cold pool that did develop
elongated parallel to the mean ﬂow, and individual
storms trained from south to north along the boundary
as the convective system swept northeastward. Rain was
brieﬂy heavy as the line passed, but excessive rainfall
18WEATHER AND FORECASTING
. 9. (a) Same as Fig. 7d, but for the forward-propagating MCS of 20–21 Nov 1989. (b)
Same as Fig. 5a, but for Sterling, VA (near Washington, DC), 0000 UTC 21 Nov 1989. (c) Same
as Fig. 7b, except valid 1800 UTC 20 Nov 1989. (d) Same as Fig. 8d, but for the 20–21 Nov
1989 MCS and based on sounding data in (c).
did not occur because of the external motion provided
by the synoptic-scale trough.
In contrast, extensive ﬂooding accompanied a similar
convective system that moved very slowly across south-
ern California on 6 February 1998. The California MCS,
like the one over the central United States, was also
embedded in large-scale southwest ﬂow ahead of a deep
trough. The translational motion of the region conducive
to thunderstorm development was limited in the Cali-
fornia event, however, because the large-scale pattern
was much less progressive (Fig. 10b). The short-wave
impulse approaching southern California at 1200 UTC
6 February lifted north-northeast to off of the Oregon
coast on 7 February, maintaining deep, unidirectional
southwesterly ﬂow over the affected region for an ex-
tended period. As a result, excessive rainfall did occur,
and the training/back-building nature of the embedded
convection was more readily apparent than in the No-
d. Environments of weak mean ﬂow
In contrast to the systems embedded in strong mean
ﬂow, the motion of convective systems in weak ﬂow is
dominated by propagation. The advective component
nevertheless remains important in determining the most
favored direction for propagation.
The combination of strong propagation and weak
advection accounts for the somewhat unusual behavior
of the convective clusters that occasionally produce
damaging winds in the Phoenix, Arizona, area each
summer. Such systems are typically associated with
modest east to northeasterly midtropospheric ﬂow
(e.g., Maddox et al. 1995; McCollum et al. 1995).
Northeasterly midlevel winds and downslope ﬂow fa-
vor the southwestward motion of gust fronts produced
by diurnal thunderstorms forming over the high terrain
north and east of the city. Convergence along the con-
vective outﬂow, coupled with the presence of steep
lower-tropospheric lapse rates and large dewpoint de-
pressions, fosters additional downdraft development.
This outﬂow drives convective initiation sequentially
southwest across central and southern Arizona through
the day. Depending upon the boundary layer moisture
availability over the lower deserts, such activity some-
times propagates as far southwest as southern Cali-
fornia. The extent to which propagation is involved in
system motion is one of the more unique characteristics
of organized severe convection in Arizona, and sys-
tems of this kind are generally well forecast by the
downwind vector technique.
18WEATHER AND FORECASTING
. 10. (a) National Oceanic and Atmospheric Administration daily weather map of North American 500-hPa height analyses at (top)
1200 UTC 10 Nov and (bottom) 1200 UTC 11 Nov 1998. Heights (dam) are depicted by solid lines, and temperatures (8C) are shown by
dashed lines. Heavy solid line denotes the position of the linear MCS at map time. (b) Same as (a), but for (top) 1200 UTC 6 Feb and
(bottom) 1200 UTC 7 Feb 1998.
e. Computation of mean wind/cold-pool motion and
Little has been said thus far about the depth of the
layer used to compute cloud-layer mean wind (in the
original technique) and cold-pool motion (in the down-
wind version). The layer used in the developmental da-
tasets, 850–300 hPa, was chosen because inclusion of
200-hPa data was found, on average, to overestimate ob-
served cell speed and, hence, the cloud-layer mean winds
computed for the original (Corﬁdi et al. 1996) study.
Examination of several recent forward-propagating
systems that moved faster than forecast by the down-
wind technique suggests, however, that the underesti-
mation may in fact have been due in part to exclusion
of data above 300 hPa. Because each case was char-
acterized by very large (i.e., greater than 5000 J kg
surface-based CAPE, it is speculated that a substantial
amount of cloud material was likely present above 300
hPa and/or that the cold pools were stronger and, there-
fore, faster moving than average. Use of wind data up
to 200 hPa is encouraged when calculating the mean
wind in regions of very high CAPE.
Careful consideration should also be given to the
depth of the layer used to estimate the low-level jet (or,
more proper, the propagation component) in the vector
scheme, because propagation is so sensitive to the lower-
tropospheric ﬂow. Deﬁnitions suggested by Bonner
(1968) were used to identify low-level jets in theoriginal
(Corﬁdi et al. 1996) study. Given our limited under-
standing of the microphysical and cloud-scale aspects
of thunderstorm initiation and given the constraints of
parcel theory (e.g., Ziegler and Rasmussen 1998), it is
clear that selection of the most appropriate inﬂow layer
is best made on a case-by-case basis. For ease of cal-
culation, the maximum wind in the lowest 5000 ft (1.5
km) was found to provide a useful estimate in most of
the events used in the current study, but a somewhat
deeper layer might prove more appropriate when the
lifting condensation level is very high.
6. An MCS continuum
For the purposes of discussion, the MCSs in this pre-
sentation have been referred to as being of either the
forward- or backward-propagating type. In reality, of
course, the interplay of variables that affect MCS prop-
agation is complex and may vary over space and time.
As a result, observed systems typically exhibit a con-
tinuum of MCS propagational modes. Section 2 shows
that a given MCS may simultaneously exhibit both up-
wind and downwind development. The tendency for
downwind or upwind development may also change
over time. Forward-propagating systems, for example,
sometimes assume back-building characteristics later in
their life cycles, or at least periods of diminished down-
wind development. This change may occur as a result
of moistening of the low to midtroposphere by nearby
convection (which decreases negative buoyancy) and/
or as a result of diurnal cooling (which reduces potential
for new cell development). An event that exhibited such
evolution was the 4 October 1998 MCS in Kansas City.
Because propagation in real-life systems is rarelypurely
of one form or another, in general, it is advantageous
to recompute or slightly modify previously calculated
motion vectors to account for varying degrees of for-
ward propagation along the gust front over space and
time. This recomputation may require multiple ‘‘local
reﬁnements’’ to previously computed vector calcula-
tions during the life of an event.
Researchers in recent decades have identiﬁed many
of the synoptic and mesoalpha-scale meteorological pat-
terns associated with MCSs that produce hazardous
weather such as excessive rainfall. These valuable in-
vestigations have enhanced recognition of impending
weather threats and have helped to increase warning lead
times. In some instances, however, it appears that at-
tention has been focused on enumerating minor differ-
ences that might exist between events occurring in dif-
ferent geographical areas or seasons at the expense of
emphasizing those characteristics universal to MCS-in-
duced weather hazards in general. For example, the
Johnstown ﬂood in July of 1977 was one of the most
notorious mesohigh ﬂash ﬂoods to have occurred in re-
cent years. Analysis reveals, however, that the mesoal-
pha-scale meteorological setup of the Johnstown tragedy
was very similar to that of the October 1998 ﬂood in
Kansas City, even though the synoptic environments of
the two events were much different.
Both featured a
mesoscale outﬂow boundary that had become parallel
to the mean cloud-layer ﬂow in a moist, largely uni-
directional wind regime, and, in both cases, the bound-
ary remained stationary for an extended period of time.
In lieu of pattern recognition, it seems advantageous
to focus on the salient processes common to such events
almost universally, regardless of the prevailing synoptic,
geographical, or seasonal environment. This idea is in
accord with the ingredients-based approach to forecasting
advocated by Doswell et al. (1996), and the vector con-
cept can be used to facilitate it. For example, from a
vector perspective, it is apparent that back-building
MCSs, lake-effect convective plumes (Peace and Sykes
1966; Niziol 1987), cool-season convective trains (Rey-
nolds 1998), and many cold-frontal rainbands are, in fact,
regional and/or seasonal variations of a common kine-
matic and thermodynamic theme: the presence of weak,
unidirectional cloud-layer ﬂow in a nearly saturated en-
vironment, with a slow-moving or stationary initiating
mechanism. Lake-effect plumes produce heavy snow for
much the same reason that back-building MCSs produce
ﬂash ﬂoods: system propagation is offset by advection,
and the initiating mechanism (a lake-enhanced boundary
in the case of snowbands; a gust front in the case of a
back-building MCS) remains nearly stationary.
A more complete technique for estimating short-term
MCS motion that builds on the work of Corﬁdi et al.
(1996) has been presented. It is based on the fact that
the preferred direction of system propagation (i.e., the
location of new cell development relative to existing
activity) is not always determined by the low-level jet.
Propagation direction is, instead, more generally dic-
tated by the location of maximum gust-front conver-
gence in the presence of conditional instability. For
some convective systems (in particular, many MCCs),
the location of maximum gust-front convergence is, in-
deed, in the direction of the low-level jet. Because their
advective motion is partially offset by propagation,
MCSs of this kind tend to move more slowly than the
mean cloud-layer ﬂow. For these systems, the original
vector technique may be used to provide a forecast of
In contrast, the greatest gust-front convergence occurs
on the downwind or forward side of bow-echo and de-
recho-producing convective systems. Such systems de-
velop when conditions are supportive of downstream
convective development along a gust front. Because the
advective and propagation components of overall sys-
tem motion are additive, these MCSs sometimes move
The Johnstown ﬂood occurred near the axis of a broad upper-
level anticyclone, whereas the Kansas City event occurred on the
eastern side of a progressive, large-amplitude short-wave trough.
18WEATHER AND FORECASTING
faster than the mean wind. The downwind vector
scheme may be used to estimate their motion.
In part because system cold pools tend to elongate in
the direction of the mean wind over time, environments
of strong ﬂow with minimal cloud-layer shear may be
associated with both forward-propagating MCSs and qua-
si-stationary and/or back-building systems. This situation
is especially true when wind proﬁles are unidirectional.
Portions of the gust front that align parallel to the mean
ﬂow become favorable sites for upstream development,
whereas parts that orient perpendicularly become sup-
portive of downstream development. This observation,
coupled with knowledge of the spatial and temporal dis-
tribution of surface-based instability, may be used to de-
termine whether cell propagation will be directed primarily
upwind or downwind and, therefore, whether a system
will exhibit forward or back-building development (or per-
haps both) during its evolution (appendix).
Based as it is on simple assumptions about cold-pool
behavior and motion, the vector technique can, at best,
provide only a rough estimate of MCS movement. The
scheme could be reﬁned by incorporating more detailed
real-time thermodynamic data that describe the potential
for convective downdraft development than are now
available. The technique, however, may be used with
both observed data and model output, and it can assist
in anticipating the predominant convective mode that
will be assumed by an incipient MCS. The scheme also
may be used to visualize better the constant interplay
between cell advection and propagation that accounts
for observed MCS motion.
Acknowledgments. The author thanks P. Banacos, D.
Blahyj, C. Doswell, J. Evans, S. Goss, D. Imy, R. Johns,
J. Kain, C. Mead, J. Moore, J. Racy, D. Schultz, D.
Stensrud, R. Thompson, and S. Weissfor valuable com-
ments and P. Banacos, G. Carbin, and P. Janish for as-
sistance with ﬁgures. Special thanks are given to J.
Evans for providing data on forward-propagating sys-
tems. Thanks also are given to R. Grumm, R. Maddox,
B. Schwartz, and an anonymous reviewer for providing
substantial constructive criticism of the manuscript.
Summary of Cold-Pool Factors that Affect
1) A cold pool will elongate in the direction of the mean
cloud-layer wind as a result of momentum transfer.
2) The degree of elongation increases as the wind pro-
ﬁle becomes more unidirectional, and this effect oc-
curs on all time- and space scales.
3) Propagation, or new cell development relative to ex-
isting storms, occurs most readily on the periphery
of the cold pool (i.e., along those portions of the gust
front), where the relative inﬂow is strongest and where
surface-based convective instability is present:
(a) upwind-developing MCSs are most favored
along quasi-stationary (mean ﬂow parallel) por-
tions of the gust front, and
(b) downwind-developing MCSs are favored on the
more progressive (mean ﬂow perpendicular)
parts of the boundary.
4) Thermodynamic factors modulate the role played by
gust-front orientation and motion:
(a) upwind-developing environments are character-
ized by comparatively moist conditions through
the low to midtroposphere and, therefore, rela-
tively weak convective-scale downdrafts, and
(b) downwind-developing environments are char-
acterized by comparatively dry conditions at
midlevels and/or in the subcloud layer and,
therefore, a tendency to produce strong convec-
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