Multicells: how storms move by building new storms

The commonest storm there is

The Field Guide calls the multicell clusterA cluster of storms in which new cells keep forming on one flank while old ones die on the other. The cluster lasts far longer than any cell in it, which is the whole trick. the most common storm type, and gives the warm-season pattern: new cells form on the west or southwest edge, old cells decay on the east or northeast edge, each individual cell lasts 20 to 30 minutes, and the cluster as a whole lasts an hour or more.

Read those two timings together. The cluster outlives its own cells by a factor of three or more. Nothing in the cluster is old. The cluster is old.

The mechanism is lesson 2's cold poolThe dome of rain-cooled air a storm leaves sitting on the ground under and around itself. Denser than its surroundings, so it spreads out, and what it spreads under decides whether the storm lives or dies., used constructively instead of fatally. The gust frontThe leading edge of cool air rushing out of a storm, ahead of the rain. It is what makes the wind pick up sharply just before a storm arrives. from the mature cells keeps spreading, and where it runs into warm, moist inflow it lifts that air to its LFCLevel of Free ConvectionThe height above which a parcel is warmer than its surroundings and keeps rising on its own. Everything below it has to be paid for by some source of lift. and a new updraftThe column of rising air inside a storm. Everything a thunderstorm does, from hail to tornadoes, is downstream of how strong and how organised this is. goes up. The old cell dies exactly as before. It just leaves a successor.

Two motions, not one

This is the idea the lesson exists for. What a radar loop shows you is the sum of two different things:

1. Advection. Each cell is carried along by the mean wind through the depth it occupies. This is cell motion, and it is roughly the same for every cell in the cluster.
2. Propagation. New cells form preferentially on one flank and old ones die on the other. This is the cluster growing in one direction and shrinking in the other, and it is a motion of the system that no individual cell is performing.

System motion is the vector sum. If new cells keep forming on the upwind flank at the same rate the cells are being carried downwind, the two cancel and the system does not move at all while every cell inside it is moving at 30 kt. That is the flash-flood case, and it is why "the storms are moving fast" is not by itself a reason to relax.

backbuilding

New cells forming repeatedly on the upwind flank, so the system extends itself upwind as fast as its cells depart downwind. The system stalls or moves backwards relative to the cells.

training

Cells moving one after another along the same line, like carriages passing a level crossing. Each cell is brief; the rain over any point underneath is not.

Corfidi vectorsA pair of vectors estimating where a storm system will go, by adding the direction new cells keep forming in to the direction the existing ones are being carried. One end of the idea is a stalled flood-maker, the other is a fast-moving wind event.

Corfidi put the propagation argument into two vectors that can be read off a forecast soundingA vertical profile of temperature, moisture and wind through the atmosphere, from a weather balloon or a model. Everything in this section is read off one.. The propagation vector is taken as the opposite of the low-level jet, because that is where the moist inflow is arriving from and therefore where the new cells will go up.

Upwind (backbuilding) propagation
    V_prop = V_cloud_layer  -  V_LLJ

Downwind (forward-propagating) MCS motion
    V_mcs  = V_cloud_layer  +  ( V_cloud_layer - V_LLJ )

Learn the shape of the argument rather than the arithmetic. The upwind vector says a system fed by a strong low-level jet will try to build back into that jet, and can therefore sit still or move against the mean wind. The downwind vector says a system whose cold pool is doing the lifting adds propagation to advection instead of subtracting it, and can move considerably faster than the wind at any level. Those are the two ends of the same idea and they produce opposite hazards: flooding at one end, a bow echoA segment of a line of storms that has surged forward into a bow shape on radar. The apex of the bow is where the strongest straight-line winds are. crossing three counties in an hour at the other.

Boundary collisions

Where two outflow boundariesThe edge of a storm's cold pool, still travelling hours after the storm that made it has gone. New storms often form along one, which is why a dead cell is still worth watching. meet, air has nowhere to go but up, and both boundaries are already convergent lines in their own right. Collisions are among the most reliable initiation points there are, and on a summer afternoon here the collision is often between a sea-breeze front moving inland and an outflow boundary from an earlier storm moving the other way.

This is a genuinely useful nowcasting habit: watch the boundaries on the low-level reflectivityHow much of the radar pulse bounced back from whatever the beam was passing through on that tilt. It is the familiar green-to-red precipitation picture, and it describes what is up in the beam, not what is reaching the ground., not just the storms. The fine lines are where the next hour happens.

MCSMesoscale Convective SystemA collection of thunderstorms behaving as one system rather than as separate storms. Clusters, squall lines and bow echoes are all examples. and MCC

MCS

Mesoscale convective system. A general term for a collection of storms behaving as one system, of which a multicell cluster, a squall lineA long, narrow line of thunderstorms moving as one system. It uses the cold air it produces as its own source of lift, which is how it keeps going for hours. and a bow echo are all examples.

MCC

Mesoscale convective complex. A specific, satellite-defined subset of MCS with size, shape and duration criteria attached.

Maddox defined the MCC in 1980 from infrared satellite imagery: a cloud shield with an area of at least 100,000 km² colder than −32 °C, containing an interior area of at least 50,000 km² colder than −52 °C, meeting both size criteria for at least six hours, and with an eccentricity of at least 0.7 at maximum extent. The point of showing you those numbers is not to memorise them. It is that MCC is a defined term with a measurable test, and MCS is a description. Using them interchangeably is a small error that compounds.