Quasi-Stationary Convection

1
Quasi-Stationary Convection
6-hour Rainfall Totals for the 28 July 1997 Fort
Collins, CO Flood
2
Quasi-Stationary Convection

• Basic Concept
• Conceptual Models
• Climatology of Extreme Rainfall Events
• Historical Events
• Big Thompson, CO flood 31 July 1976
• Johnstown, PA flood 19 July 1977
• Fort Collins, CO flood 28 July 1997
• Forecasting

3
Basic Concept

• A Offsetting Multi-Scale Process
• The motion of a given storm system is the sum of
  two vectors
• Cell Velocity
• Motion of individual cells that compose the
  system
• Roughly equivalent to the deep layer mean
  synoptic wind
• Propagation Velocity
• Motion due to the formation of new cells on the
  storm periphery
• Related to the motion of mesoscale boundaries
  and their
• interaction with the storm inflow (e.g., cold
  pool propagation)
• May occur on any side
• On the leading edge ? system accelerates
• On a side ? systems move to the right or
  left
• On the rear flank ? systems decelerates

Right Moving Supercell
Mean Wind
Mean Cell
Propagation
Storm
Squall Line
Mean Wind
Propagation
Mean Cell
Storm
Quasi-Stationary Storm
Mean Wind
Mean Cell
Propagation
Zero Storm motion
4
Conceptual Models

• Synoptic Setting
• Maddox et al. (1979) examined the synoptic
  environments
• for 151 quasi-stationary convective events
  that produce
• significant flash floods
• Identified two regular situations with many
  common
• characteristics
• Frontal Forcing
• Nearly stationary synoptic-scale surface front
• Meso-High Forcing
• Nearly stationary, mesoscale, surface-outflow
  (cold pool)
• boundary generated by previous convective
  activity
• Characteristics Common to Both

Frontal Forcing
Meso-High Forcing
5
Conceptual Models

• Mesoscale Organization
• Schumacher and Johnson (2005) examined radar
• data for 116 MCS that produced extreme
  rainfall
• Identified two regular organization types
• Training Line Adjoining Stratiform
• 65 of cases
• Convective cells develop behind a front or
  boundary
• Cell motion parallel to the boundary, as new
  cells
• form and move over (or train over) the same
  location
• Stratiform precipitation forms behind the line
  but
• also moves parallel to the boundary
• Backbuilding Quasi-Stationary
• 27 of cases
• Convective cells repeatedly form upstream from
  their

From Schumacher and Johnson (2005)
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Climatology of Extreme Rainfall Events

• Definition and Regional Statistics
• Extreme Rainfall Event ? When a surface rain
  gauge reports a 24-hour total rainfall
• greater 125 mm ( 5 inches)
• Five year period (1999-2003)
• 382 events
• Most common in July
• Only occur in the summer
• across the northern U.S.
• Year-round in southern U.S.
• 74 of warm season events
• were associated with MCS
• (or meso-high) forcing
• 95 of cold season events
• were associated with strong
• synoptic forcing

From Schumacher and Johnson (2006)
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Historical Events

• Big Thompson Canyon Flood 31 July 1976
• Quasi-stationary MCS produced 8-10 inches of
  rain in 1.5 hrs
• Occurred 830 -1000 pm LST
• Killed 145 people (six bodies never recovered)
  and produced gt40 million in damages
• Strong low-level easterly (upslope) flow of
  warm, moist air
• Weak westerly winds aloft (above 700 mb)
• Details http//pubs.usgs.gov/fs/2006/3095/pdf/FS
  06-3095_508.pdf

Mouth of Big Thompson Canyon
1 August 1976
Today
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Historical Events

• Johnstown Flood 19 July 1977
• Associated with a slow-moving MCC that
  originated four days prior over South Dakota
• Produced 12 inches of rain in 10 hrs across
  central Pennsylvania
• Occurred between 700 pm and 500 am LST
• Extreme precipitation caused multiple dams to
  overtop and break
• Killed 76 people and produced gt200 million in
  damages
• Details Bosart, L. F., and F. Sanders, 1981
  The Johnstown flood of July 1977 A long-lived
• convective system, Journal of
  Atmospheric Science, 38, 1616-1642

Primary dam that broke
Johnstown
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Historical Events

• Fort Collins Flood 28 July 1997
• Quasi-stationary MCS produced gt10 inches of rain
  in 6 hrs
• Occurred 500 -1100 pm LST
• Killed 5 people and produced gt200 million in
  damages
• Details http//olympic.atmos.colostate.edu/pdf/P
  etersen-Flood97.pdf

From Peterson et al. (1999)
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Historical Events

• Fort Collins Flood Synoptic Conditions
• A 500-mb ridge axis over northeast CO
• Strong low-level easterly (upslope) flow
• Deep layer of moist air
• Modest CAPE at 850 J/kg
• Weak southwesterly winds aloft

Denver Sounding
Edge of warm moist air
Cold cloud tops
Surface Winds
500mb Vorticity Maxima
From Peterson et al. (1999)
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Historical Events

• Fort Collins Flood Mesoscale Organization
• A quasi-stationary back-building MCS
• New cells repeatedly developed
• over southern Fort Collins as
• their mature and decaying
• predecessors moved slowly north
• across the city

From Peterson et al. (1999)
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Forecasting

• Guidelines
• Prior to the development of convection, look
  for
• Strong low-level flow normal to a front, outflow
  boundary, or orographic feature
• Warm and moist low-level air
• Modest CAPE (500-1000 J/kg)
• Large CIN (gt200 J/kg)
• Weak mid- and upper-level flow parallel to a
  pre-existing front or boundary
• Once convection has developed, look for and
  monitor
• Surface wind observations roughly equal and
  opposite middle and upper-level winds
• Cell motion parallel to fronts or outflow
  boundaries
• Slow moving, back-building systems on radar
• Radar-derived estimates of total precipitation

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Quasi-Stationary Convection

• Summary
• Basic Concept
• Cell Velocity
• Cell Propagation
• Conceptual Models
• Frontal Forcing
• Meso-high Forcing
• Training Line
• Back-building
• Climatology of Extreme Rainfall Events
• Historical Events
• Big Thompson, CO flood 31 July 1976
• Johnstown, PA flood 19 July 1977
• Fort Collins, CO flood 28 July 1997

14
References
Bosart, L. F., and F. Sanders, 1981 The
Johnstown flood of July 1977 A long-lived
convective system, J. Atmos. Sci., 38,
1616-1642. Chappell, C., 1986 Quasi-stationary
convective events.  Mesoscale Meteorology and
Forecasting, P. S. Ray, Ed. Amer. Meteor. Soc.,
289-310. Maddox, R. A., C. F. Chappell and L. R.
Hoxit, 1979 Synoptic and mesoscale aspects of
flash flood events.  Bull. Amer. Meteor. Soc.,
60, 115-123. Maddox, R. A., L. R. Hoxit, C. F.
Chappell, and F. Caracena, 1978 Comparison of
the meteorological aspects of the Big Thompson
and Rapid City flash floods. Mon. Wea. Rev., 106,
375389. Petersen, W. A., L. D. Carey, S. A.
Rutledge, J. C. Knievel, R. H. Johnson, N. J.
Doesken, T. B. McKee, T. Vonder Haar, and J. F.
Weaver, 1999 Mesoscale and radar observations of
the Fort Collins flash flood of 28 July 1997.
Bull. Amer. Meteor. Soc., 80, 191216
Schumacher, R. S. and R. H. Johnson, 2005 
Organization and environmental properties of
extreme-rain-producing mesoscale convective
systems.  Mon. Wea. Rev., 133, 961-976.