Factors governing
MCS movement and behavior
Stephen F. Corfidi
OU / CIMMS
Cooperative Institute for Mesoscale Meteorological Studies
(Formerly, NOAA / NWS / NCEP Storm Prediction Center)
Chinese Meteorological Administration
Forecaster Training
30 October 2019
Presentation overview
● Discuss the major factors that govern MCS motion and behavior
● Introduce a simple technique to forecast short-term MCS motion
● Describe cold pool elongation, concurrent MCSs, and how these
concepts may be used to identify the predominant weather
hazard(s) most likely to occur with an MCS
● Illustrate the role of terrain, mesoscale convective vortices (MCVs),
and other factors on MCS motion and behavior
● Summarize concepts presented
Concurrent MCSs
MCS with an MCV
Some concepts of MCS development and motion
(courtesy of Browning, Chappell, Fritsch, Ludlam, Merritt, Newton, etc., 1950s-1980s)
● MCSs occur in environments of low-level warm advection (veering wind
profiles)
● No true “steering level” exists for MCS motion
● MCSs move to the right of the mean flow, generally close to contours of
constant 1000-500 mb thickness
● MCSs are composed of individual storms or “cells;” thus, total MCS motion can
be considered as the sum of cell advection plus cell propagation
Total MCS motion :
Advection : Movement of storm cells by the mean wind
Propagation : Development of new cells with respect to
existing storms
Individual storm cells:
Small, stippled ovals
t0
t1
t2
Overall MCS:
Large, black ovals
Mean wind: Black arrows
Time: “t,” etc.
Advection
Propagation (dotted)
Total MCS motion (heavy arrow)
Meso-Beta Elements : The “building blocks” of MCSs / MCCs
MBEs: Embedded clusters of stronger, actively-growing storms; the “Hot towers” of an MCS
MBE
Meso-beta: 20-200 km
Mean MCS genesis stage composite hodograph
● MCSs occur in regions of warm advection (veering wind profiles)
● MBEs move along lines of constant thickness (1000-500 mb)
● MBEs move about 30° to right of mean flow (850-300 mb; black arrow)
Speeds in ms-1; directions in degrees azimuth;
Dataset of >100 cases (many were MCCs)
Corfidi, Merritt, and Fritsch (1996)
Graphical plots of the data displayed in
the preceding hodograph
● MBEs move along lines of constant thickness (1000-500 mb)
● MBEs move about 30° to right of mean flow (850-300 mb)
● What accounts for deviant motion of MBEs: Cell advection or propagation?
MBE direction vs.
1000-500 mb
thickness
MBE direction vs.
mean wind
direction
Corroborates Newton and Katz (1958)
Cell advection is not the prime contributor to
deviant MBE motion because cells tend to
move with the mean cloud-layer wind
…i.e. cell advection can be approximated by the cloud-layer mean wind :
Direction
Speed
Corroborates Byers and Braham
Thunderstorm Project findings (1949)
However, care must be taken in selecting layer
used to compute mean cloud-layer wind
…inclusion of 200 mb data in this particular data set resulted in an over-estimation of cell speed :
Same plot as on right side
of previous slide; cell speed
vs. mean wind in 850-300 mb layer
Cell speed vs. mean wind
in the 850 – 200 mb layer
Corroborates, amongst others, H. B. Brooks (1946): A summary of some
radar thunderstorm observations. Bull Amer. Meteo. Soc., 27, 557-563
If cell advection does not account for deviant MBE motion,
what factors affect cell propagation?
● Strength and breadth of the low-level inflow
● System-generated cold pools / outflow boundaries
● Existing mesoscale and synoptic-scale boundaries
● Orographic influences
● Mesoscale convective vortices (MCVs)
● Internal gravity waves / bores
● Hydrodynamic vertical pressure gradients (supercell processes)
● Character and distribution of CAPE
● Character and distribution of CINH
● Character and distribution of moisture
This list is not complete, and these factors change over space and time
But numerous studies and empirical evidence suggest that, to a first
approximation, cell propagation generally occurs in the direction of
maximum system-relative inflow --- and at a rate modulated by the
speed of that flow
This suggests that the low-level jet (LLJ) might be used to estimate
system-relative inflow --- and, therefore, the direction and speed of cell
propagation :
= MBE location
Correlation coefficient (r) increases to .84 with
the exclusion of the two outliers at upper left
The “Vector Method” for estimating MBE motion
● Use the LLJ as an estimate of cell propagation (direction and speed),
with propagation vector (VPROP) directed opposite to the LLJ
● Use the mean cloud-layer wind (VCL) as an estimate of cell advection
● Add the two vectors to yield an estimate of MBE motion, VMBE
= MBE location
Corfidi, Merritt, and Fritsch (1996)
The “Vector Method” for estimating MBE motion
Vector technique provides reasonable estimates of MBE direction and speed :
r = .78
Direction
r = .80
Speed
Application of the Vector Method to the
Maddox et al. (1979) flash flood conceptual models
Wind and thermodynamic environments are favorable for
upshear (regenerative or “back-building”) convective development
in both “Frontal” and “Synoptic” events; in the former, upshear
propagation is episodic; in the latter, continuous :
“Frontal” events
(Episodic
upshear
propagation)
850 mb
500 mb
= MBE location
“Synoptic” events
(Continuous
upshear
propagation)
Application of the Vector Method to
16 November 1987 LA / MS regenerative MCSs
A series of upshear-propagating, nearly stationary MCSs track ENE across LA and MS over 24 hours,
producing 12-18 inches of rain, with major flash flooding and tornadoes :
Surface 1200 UTC
Forecast motion:
~ 260 / 7 (kts)
KLCH 0000 UTC
500 mb 1200 UTC
0200 UTC
0400 UTC
Application of the Vector Method to
16 November 1989 Mid-Atlantic regenerative MCS
Setup appears supportive of another “synoptic-type,” upshear-propagating, quasi-stationary
MCS with heavy rain…
Surface 1200 UTC
KACY 1200 UTC
Forecast motion:
~ 240 / 5 (kts)
500 mb 1200 UTC
Composite radar
1030 UTC
Application of the Vector Method to
16 November 1989 Mid-Atlantic regenerative MCS (contd.)
…but the large -scale pattern was moving W-to-E; the MCS did back-build…but it also progressed W-to-E:
1030 UTC
1230 UTC
1430 UTC
Severe
reports
1200 UTC 16 Nov
1200 UTC 17 Nov
Wind gusts (+);
No flash flooding
Synoptic and mesoscale environment over the lower
Great Lakes, 1200 UTC 16 August 1997
Deep, seasonably strong westerly flow, with rich low-level moisture and the potential for strong surface heating :
Radar / Stlt 1215 UTC
Surface 1200 UTC
KDTX 1200 UTC
500 mb 1200 UTC
16 August 1997 OH / PA derecho MCS
Incipient MCS became a downshear or forward-propagating system rather than upshear or “back-building” one;
MCS produced a series of damaging downbursts (with seven deaths) from northern Ohio to NYC :
3-hourly squall line motion with wind
(+) and hail (•) reports
Composite reflectivity
1300 - 2000 UTC
KBUF 1200 UTC
IR satellite 1215 - 2115 UTC
16 August 1997 OH / PA derecho MCS : Lessons learned
The MCS downshear-propagated because new storm development initially was not focused in the direction
of the LLJ (i.e., on the southwest side of the MCS), but rather was maximized on the
downshear (east) side of its elongating cold pool :
Propagation is not
exclusively in the
direction of the LLJ
ehkfgsklgls
1215 UTC
Temporal elongation of an MCS cold pool
associated with wind profile at left
ehkfgsklgls
2115 UTC
16 Aug 1997 OH / PA derecho…and IL / IN flash flood MCS
The initial MCS did not produce excessive rain because it was fast-moving; cell propagation augmented cell
advection; i.e., propagation and advection were in the same direction (downshear-propagation)
But storms did subsequently form along the quasi-stationary part of the initial MCS’s gust front over northeast
Illinois, northern Indiana, and northern Ohio --- where upshear propagation and echo training with
a new MCS produced up to 8 inches of rain and flash flooding as daytime heating eliminated CIN
Thus, the same wind profile (deep, largely unidirectional flow) ultimately fostered
concurrent downshear and upshear-propagating MCSs :
ehkfgsklgls
Upshear propagating /
“echo training” MCS
(Main threat :
Heavy rain)
Downshear
propagating MCS
(Main threat :
High wind)
Concurrent upshear and downshear-propagating MCSs
…can occur when the thermodynamic environment is favorable for deep convection along both the
stationary and progressive parts of an elongating MCS cold pool / gust front;
such systems are most prominent when deep flow is largely unidirectional :
Upshear-propagating MCS
Plan view;
cross sections
shown at left
(Mainly) Heavy rain threat
QUASI-STATIONARY
GUST FRONT
Downshear-propagating MCS
(Mainly) High wind threat
PROGRESSIVE
GUST FRONT
Corfidi (2003)
Parker and Johnson
(2000)
Typical thermodynamic environments supportive of
upshear and downshear MCS propagation
Weak cold pools more common with upshear propagation; strong cold pools with downshear development
Upshear-propagating MCS
QUASI-STATIONARY
GUST FRONT
Downshear-propagating MCS
PROGRESSIVE
GUST FRONT
However…proximity soundings do not necessarily provide all
the information needed to determine dominant severe threat
Other factors to consider include: Temporal and spatial changes in relevant thermodynamic fields
Presence of anomalous ingredients (LLJ speed and breadth; PW…)
Possibility for unusually efficient processing of large quantities of
very moist and buoyant air
UTC
= Flash flooding
8 May 2009 KS / MO / IL
“Super Derecho”
500 agl wind speed
Coniglio, Corfidi, and Kain (2010)
Precipitable water
An equation for heavy precipitation
…attributable to C. F. Chappell via Doswell, Brooks, and Maddox (WAF, Dec 1996)
At a given point, P = R • D
Where:
P = Total precipitation produced at a point
R = The average precipitation rate
D = Duration of the precipitation
…and R ~ E • w • q
E = Precipitation efficiency of system
w = Ascent rate of air feeding system
q = Mixing ratio of air feeding system
…and D ~ A / C
A = Size of the heavy-precipitation-producing system
C = Speed of the system’s movement with respect to point
Therefore, P = (E • w • q) • (A / C)
MCS behavior and motion forecasts help assess “P”
…mainly by estimating duration, i.e., variables “A” and “C”
The 1977 Johnstown flood was associated with the upshear
propagating member of a concurrent MCS in NW flow :
Increasing moisture / decreasing cold convective downdraft potential over time fostered
back-building / upshear MCS development over SW PA after passage of a forward-propagating MCS :
19-20 July 1977, UTC
2100 UTC
1800 UTC
2100 UTC
0000 UTC
Hoxit et al. (1978): Meteorological analysis of the Johnstown, PA flash flood (NOAA-ERL)
The 20 July 1977 Johnstown, PA flash flood (continued)
Deep flow veered to west-northwesterly --- but moistened and remained largely unidirectional over southwest
Pennsylvania on the afternoon of 19 July in the wake of a forward-propagating MCS
Sustained warm advection with LLJ fostered regenerative / upshear-propagating MCS development along stalled
portion of elongating cold pool / outflow boundary over the region the following evening
PW 1.4 in
Nearly 12” of rain / 10 hrs
PW 1.9 in
1200 UTC
19 July
0000 UTC
20 July
1200 / 0000 UTC overlay
A concurrent MCS in SW flow was responsible for
the 4 October 1998 Kansas City flash flood :
Concurrent flash flood / high wind situations can occur with a variety of synoptic-scale patterns :
KTOP 1800 UTC
Surface 1200 UTC
500 mb 1200 UTC
Reflectivity (KEAX) 2100-0300 UTC
A more complex (but typical) concurrent MCS setup :
Kansas-Nebraska border, 22-23 June 2003
Kinematic and thermodynamic fields vary markedly over space and time…
850 mb, 0000 UTC / 23rd
500 mb, 0000 UTC / 23rd
Visible
satellite,
1200 / 22 –
0100 /23
Surface, 2200 UTC / 22nd
22-23 June 2003 Kansas-Nebraska MCS
Another MCS that exhibited concurrent downshear and upshear development, but with setup “muddied’ by
kinematic and thermodynamic fields that changed markedly in space and time
Upshear development fostered by presence of greater instability / weaker CIN in upstream (west) direction,
and by strengthening of LLJ upstream from existing storms after sunset
Downshear development fostered by presence of supercells that produced copious rain and hail (i.e.,
strong convective outflow), and by a mesoscale pattern that favored storm mergers
“Pivot area” between upshear and downshear propagation segments was hardest hit, with > a foot of rain
Composite
reflectivity,
2200 UTC / 22 –
1300 UTC / 23
Yet another concurrent MCS environment :
Lake effect snow bands of the Great Lakes
Environment characterized by deep, largely unidirectional flow with minimal cloud-layer shear
Cell propagation concurrently offsets and augments cell advection, leading to the simultaneous
development of upshear (back-building) and downshear-propagating (bowing) segments
The major axes of the lakes serve as stationary initiating mechanisms --- and enhance low-level
baroclinity that, in turn, focus low-level uplift
Unidirectional flow enhances precipitation efficiency
Reflectivity, KTYX, 2100-0300 UTC
29-30 January 2004
KBUF, 1200 UTC
The role of terrain in MCS movement
Terrain can provide an additional source of potential low-level uplift to the equation of MCS movement
Certain mesoscale flow patterns (e.g., schematic below) can focus and enhance terrain-induced uplift,
thereby fostering development of upwind / regenerative MCSs
While the details associated with such patterns vary, all are characterized by :
Weak mean flow (Minimizes advective component of MCS motion)
Weak deep-layer shear (Implies that any frontal zones in region will tend to move slowly;
i.e., pattern translation minimized)
Veering winds with height (Warm advection regime; sustained uplift)
Substantial warm rain component (Limits strength of cold air outflow, thereby helping to keep
terrain the main source of low-level ascent)
High PW / minimal dry air (Limits entrainment and maximizes precipitation efficiency)
Figure from Pontrelli et al. (1999):
Wea. Forecasting, 14, 384-404
Soundings from selected terrain-related flash floods
…showing weak mean flow, weak shear, veering wind with height, warm profiles, and rich moisture
(1 Aug 1976 Denver sounding modified for Loveland, CO area, as in Maddox et al., 1977)
Rapid City, SD
1972
Madison County, VA
1995
Big Thompson, CO
1976
Ft Collins, CO
1997
Potential for
stationary
MBEs
increases if
supercells are
involved
(e.g.,
Cheyenne, WY
flash flood,
1 Aug 1985)
Vector method can be used to estimate MCS (MBE) motion
with terrain-related events and with elevated MCSs
by using winds in the cloud-bearing layer :
MCVs and MCS motion
Potential vorticity anomaly associated with MCV produces deformed isentropic surfaces shown in top diagram
Vertical shear
Vortex-relative
environmental flow yields
ascent on downshear
side of vortex, and
descent on upshear side
Side view, looking north
Additional ascent is
provided on east side of
vortex by interaction of
vortex’s cyclonic circulation
with synoptic-scale
baroclinity
Side view, looking west
Figures adapted from Trier and Davis
(2007), and Raymond and Jiang (1990)
MCVs and MCS motion (continued)
In conjunction with the larger scale background environment, MCV-induced vertical motions
contribute to thermodynamic destabilization downshear and thermodynamic
stabilization upshear, and thus help determine location and likelihood for
“secondary” convective development (i.e., propagation component of MCS motion)
Patterns of MCV-induced vertical motions, destabilization, and changes in shear also are
influenced by the tilt and asymmetries of the vortices, by time of day (i.e., nocturnal
vs. diurnal), and antecedent conditions
In short, many factors dictate how an MCV will influence secondary convection and,
therefore, MCS motion and the potential for flash flooding
X = MCV locations
BAMEX (2003) cases presented by Trier and Davis (2007)
“Bow-and-Arrow” MCSs can arise from MCVs
Keene and Schumacher (2013)
Converging flow atop surface cold pool in
wake of forward-propagating MCS / bow
leads to “arrow” that can produce flashflooding / large hail
Source level of the confluent flow and its
orientation relative to the original
(forward-propagating or bow) MCS vary
Effects of dry air aloft on MCSs
Conclusions of James and Markowski (2010)
Dry air aloft reduces total condensate and, therefore, total updraft / downdraft strength --and thereby tends to weaken cold air outflow
Influence of dry air on cold pool strength occurs mainly via changes in diabatic processes in the
phase-change areas of an MCS (i.e. , melting and evaporation of mid-level fall-out)
Lower hydrometeor mixing ratio effect of dry air dominates the evaporative effect --- leading to
reduced diabatic cooling and, consequently, comparatively weak cold pools
Influence of dry air is sensitive to CAPE --- greatest when CAPE is small
With high CAPE, the detrimental effects of drier air on cold pool strength are less marked,
especially well behind MCS gust front --- but near the gust front, the reduction in
melting hail offsets increased evaporation, yielding minimal net change
Changes in cold pool strength / longevity ultimately affect MCS behavior (mode) --- and motion
3000 J kg-1
4500 J kg-1
Moist
Dry
1500 J kg-1
Dry layer in low CAPE (1500 J/kg) simulation
reduced to only 70%; the dry layers in the higher
CAPE simulations had 10% relative humidity
Role of hydrometeor re-cycling
Modified from Seigel and Van Den Heever (2013)
Positive feedback
between hydrometeor
re-cycling and cold pool
strength
Re-cycling of hail by
rear-inflow jet (RIJ)
strengthens mid-level
updraft that, in turn,
increases mid-level fallout and, subsequently,
surface cold pool that
enhances low-level uplift
Smaller hail yields
greater upward mass flux
and greater mid-level
buoyancy (via increased
latent heating)
Other factors affecting MCS motion and behavior
Spatial gradients in vertical shear, and temporal changes in shear…
Diurnal changes in strength, location, depth of LLJ…
Mesoscale gravity waves…
Forecasting MCS motion and behavior will remain challenging for some time to come!
Summary
● MCS motion is a sum of cell advection and cell propagation
Summary
● MCS motion is a sum of cell advection and cell propagation
● Propagation occurs most readily on those portions of the system gust front
where relative inflow is maximized; this is often --- but not always --in the direction of and at a rate proportional to the LLJ
Summary
● MCS motion is a sum of cell advection and cell propagation
● Propagation occurs most readily on those portions of the system gust front
where relative inflow is maximized; this is often --- but not always --in the direction of and at a rate proportional to the LLJ
● Momentum transfer dictates that, over time, MCS cold pools elongate in the
direction of the mean cloud-layer shear; parts of the associated gust front
become nearly stationary, while other parts remain progressive
Summary
● MCS motion is a sum of cell advection and cell propagation
● Propagation occurs most readily on those portions of the system gust front
where relative inflow is maximized; this is often --- but not always --in the direction of and at a rate proportional to the LLJ
● Momentum transfer dictates that, over time, MCS cold pools elongate in the
direction of the mean cloud-layer shear; parts of the associated gust front
become nearly stationary, while other parts remain progressive
● Assuming the presence of sufficient convergence, moisture, and buoyancy to
support storm development along an elongating gust front:
Summary
● MCS motion is a sum of cell advection and cell propagation
● Propagation occurs most readily on those portions of the system gust front
where relative inflow is maximized; this is often --- but not always --in the direction of and at a rate proportional to the LLJ
● Momentum transfer dictates that, over time, MCS cold pools elongate in the
direction of the mean cloud-layer shear; parts of the associated gust front
become nearly stationary, while other parts remain progressive
● Assuming the presence of sufficient convergence, moisture, and buoyancy to
support storm development along an elongating gust front:
Upshear-propagating / regenerative MCSs are favored on the stationary parts;
these systems are most likely to produce flash floods
Summary
● MCS motion is a sum of cell advection and cell propagation
● Propagation occurs most readily on those portions of the system gust front
where relative inflow is maximized; this is often --- but not always --in the direction of and at a rate proportional to the LLJ
● Momentum transfer dictates that, over time, MCS cold pools elongate in the
direction of the mean cloud-layer shear; parts of the associated gust front
become nearly stationary, while other parts remain progressive
● Assuming the presence of sufficient convergence, moisture, and buoyancy to
support storm development along an elongating gust front:
Upshear-propagating / regenerative MCSs are favored on the stationary parts;
these systems are most likely to produce flash floods
Downshear-propagating MCSs are favored on the progressive parts; these
systems are most often associated with damaging winds, but can produce
flash flooding when the environment supports efficient processing of large
quantities of moist, buoyant air
Summary
● MCS motion is a sum of cell advection and cell propagation
● Propagation occurs most readily on those portions of the system gust front
where relative inflow is maximized; this is often --- but not always --in the direction of and at a rate proportional to the LLJ
● Momentum transfer dictates that, over time, MCS cold pools elongate in the
direction of the mean cloud-layer shear; parts of the associated gust front
become nearly stationary, while other parts remain progressive
● Assuming the presence of sufficient convergence, moisture, and buoyancy to
support storm development along an elongating gust front:
Upshear-propagating / regenerative MCSs are favored on the stationary parts;
these systems are most likely to produce flash floods
Downshear-propagating MCSs are favored on the progressive parts; these
systems are most often associated with damaging winds, but can produce
flash flooding when the environment supports efficient processing of large
quantities of moist, buoyant air
● Upshear and downshear-propagating MCSs sometimes occur simultaneously
Summary
● MCS motion is a sum of cell advection and cell propagation
● Propagation occurs most readily on those portions of the system gust front
where relative inflow is maximized; this is often --- but not always --in the direction of and at a rate proportional to the LLJ
● Momentum transfer dictates that, over time, MCS cold pools elongate in the
direction of the mean cloud-layer shear; parts of the associated gust front
become nearly stationary, while other parts remain progressive
● Assuming the presence of sufficient convergence, moisture, and buoyancy to
support storm development along an elongating gust front:
Upshear-propagating / regenerative MCSs are favored on the stationary parts;
these systems are most likely to produce flash floods
Downshear-propagating MCSs are favored on the progressive parts; these
systems are most often associated with damaging winds, but can produce
flash flooding when the environment supports efficient processing of large
quantities of moist, buoyant air
● Upshear and downshear-propagating MCSs sometimes occur simultaneously
● Terrain can anchor new cell development and foster quasi-stationary convective
systems given favorable background flow and nearly saturated lower levels
Summary
● MCS motion is a sum of cell advection and cell propagation
● Propagation occurs most readily on those portions of the system gust front
where relative inflow is maximized; this is often --- but not always --in the direction of and at a rate proportional to the LLJ
● Momentum transfer dictates that, over time, MCS cold pools elongate in the
direction of the mean cloud-layer shear; parts of the associated gust front
become nearly stationary, while other parts remain progressive
● Assuming the presence of sufficient convergence, moisture, and buoyancy to
support storm development along an elongating gust front:
Upshear-propagating / regenerative MCSs are favored on the stationary parts;
these systems are most likely to produce flash floods
Downshear-propagating MCSs are favored on the progressive parts; these
systems are most often associated with damaging winds, but can produce
flash flooding when the environment supports efficient processing of large
quantities of moist, buoyant air
● Upshear and downshear-propagating MCSs sometimes occur simultaneously
● Terrain can anchor new cell development and foster quasi-stationary convective
systems given favorable background flow and nearly saturated lower levels
● MCVs and various environmental heterogeneities can affect MCS motion
Acknowledgements
Greg Carbin
Ariel Cohen
Mike Coniglio
Sarah Corfidi
David Cleaver
Jeff Evans
Jared Guyer
John Hart
Bob Johns
Steven Weiss
Mike Woolridge