MCS Characteristics, Structure, and Propagation
Green Line Separater

A study in Weather and Forecasting (Wes Junker et al.) looked at many mesoscale convective systems (MCSs) that affected the Mississippi and Missouri River Valleys from June to September 1993. They considered all MCSs that caused 2 inches or more of rain in 24 hrs (85 total events). They categorized the events from smallest in areal extent of heavy rain/lightest amounts to greatest in extent/heaviest amounts. The combination of these MCSs produced flash flooding and widespread serious river flooding from Missouri and Kansas north to the Dakotas, Minnesota, and Wisconsin. Below is a summary of environmental weather conditions associated with these events.

1) The majority of cases were nocturnal, occurring between 0000 and 1200 UTC. Heavy rainfall from some MCSs lasted past 1200 UTC.

2) High precipitable water (PW), K index, and mean relative humidity (RH) values were associated with heavy rain events. For the most extreme cases (largest areal extent of 2, 3, 4, and 5 inch rain amounts), PW averaged 1.65 inches; K averaged 35. For smaller events, PW and K averaged about 1.4 inches and 30, respectively. Highest values were positioned just upstream from the location of the heaviest rainfall, i.e., MCSs usually occurred on the northern periphery of high moisture values. The exception was mean RH which was more co-located with heavy rain. High RH values equate to less environmental dry air entraining into an MCS to cause evaporation. Thus, high mean RH increases the precipitation efficiency of the MCS, as does a large warm (>0 degrees C) cloud depth.

3) The majority of cases were associated with a low-level boundary as evident in either the low-level wind or moisture/temperature (i.e., equivalent potential temperature [theta-e]) gradient fields.

4) The large majority of events were associated with only weak-to-moderate vertical speed shear but with winds that veered with height. This is the case along or north of a low-level boundary where boundary layer winds with an easterly component veer to south or west aloft.

5) Many cases occurred in an area of significant warm air advection/isentropic lift. The stronger the warm advection, the higher the potential for heavier rainfall over a larger area (assuming all other factors are equal), i.e., stronger synoptic-scale/mesoscale forcing is present.

6) The heaviest rain usually fell just north of the strongest south or southwest 850 mb winds.

7) The heaviest rain usually fell north of the maximum 850 mb theta-e in a theta-e gradient zone but still in or near a theta-e ridge axis.

8) The heaviest rain often occurred within an area of significant 850 mb positive theta-e advection, but frequently just south of the maximum theta-e advection values.

9) The majority (about 60%) of events were centered near the 500 mb ridge axis (where warm advection can be strong).

10) A much higher percentage of cases were associated with low-level/850 mb warm advection than with 500 or 300 mb differential positive vorticity advection (DPVA).

11) Major heavy rain/MCS events occurred with 700 mb temps less than 12 EC (mostly 7-11 EC). An air mass usually is too capped for deep, organized convection at 700 mb temperatures above 12 EC. Some smaller events, however, did occur at higher temperatures.

12) MCS events usually were associated with an area of 250 mb divergence, but in the gradient south of the maximum divergence due to the sloped ascent of low-level unstable air parcels toward the upper-level divergence maximum. About 60% of all heavy rain events occurred within the right entrance region of an upper-level jet streak, although some cases were not associated with a jet streak or did not fit the typical divergence patterns of a jet streak.

13) Heaviest rain amounts often occurred north of an area of relatively strong 850 mb moisture transport.

14) The length, location, and orientation of the low-level moisture flux convergence, low-level jet, and instability determined propagation characteristics which ultimately affected rainfall amounts in any one area. For very heavy rain totals, these factors should be long in length, directed along/nearly parallel to the mean (850-300 mb) wind, and be located upstream (i.e., on the west or southwest side) of the initial MCS. This favors new cell growth on the upstream side (rear flank) of an MCS resulting in a slow moving or quasi-stationary MCS as forward cell movement is negated by a backward propagation vector.

15) Extreme rainfall events were associated with the strongest 850 mb winds and moisture flux that moved east/downwind slower than in lighter/less widespread events. Slower movement favors upstream cell development versus new cell formation on the forward flank of an MCS for a faster moving area of strongest winds and moisture flux, even if rainfall rates are nearly the same for both type systems.

16) One difference in larger/heavier rainfall events from smaller scale, lighter cases (but ones that can still produce local flash flooding) was the width of moisture flux convergence. The large the width (area) of convergence into a boundary, the greater the potential for merging and training convective cells and, therefore, higher rainfall amounts. There was less correlation between the magnitude of moisture flux convergence and the size/scope of events than there was with the width of high moisture flux convergence. The depth of moisture flux convergence also was important in producing heavy rainfall as deeper convergence factors heavier and more widespread rainfall totals.

Below, moisture convergence, maximum low-level winds, and instability are located on the upstream (west or southwest) quadrant of the MCS. Thus, this is the preferred area for new updrafts and cell development. After cells develop, they move downwind with the mean (850-300 mb) environmental flow but weaken as they encounter less favorable conditions downstream. Renewed development can continue upstream with cell training over the same area. Thus, individual cells may move forward, but propagation is backward, resulting in a system movement that is slow-forward, quasi-stationary, or backward. Hail and some wind damage may occur with this type of MCS, especially in initial stages when some dry air aloft may be present. However, upon maturity, this type of system can be a prolific rainfall producer and a major flash flood threat.

Below, moisture convergence, low-level inflow, instability, and strong updrafts are maintained on the leading edge of the MCS. This favored environment moves downwind with time, as does the MCS as new cell growth occurs along the leading/forward quadrant of the system. Fast environmental flow regimes with moderate-to-strong wind shear and a progressive shortwave trough aloft favor a fast-forward propagating convective system. This can result in squall lines and bow echoes that produce significant wind damage along their leading edge, along with possible hail and transient tornadoes within bowing segments. A system-relative front-to-rear flow can cause trailing precipitation behind the leading thunderstorm line. Depending on the speed and direction of movement of the leading squall line and amount of trailing precipitation, this type of MCS can produce heavy rainfall amounts and a possible flood threat. However, high rainfall rates usually are short-lived with less flash flood potential than with quasi-stationary/backward propagating MCSs.

Below, schematics of stationary/backward and forward MCS propagation are shown. Moist, unstable inflow provided by the low-level jet is maintained on the west or southwest side of the stationary/backward MCS, while the inflow is maintained along the leading edge for forward propagating MCSs (including bow echoes).


Green Line Separater

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