The rear-inflow jet, or RIJ, is a stream of relatively strong, mid-level air that enters a mesoscale convective system from its rear. When it descends to the surface behind or near the leading edge, it is a leading producer of damaging straight-line winds in bow echoes and squall lines.
How the jet develops
A mature convective line generates a deep, cold pool at the surface and warm, buoyant air aloft on its downshear side. That vertical arrangement produces a horizontal buoyancy contrast between the cold pool and the environment above, and the resulting pressure field draws mid-level environmental air rearward into and through the system.
As the rear-inflow jet accelerates toward the leading edge, it can descend along the back of the cold pool. In systems with substantial trailing stratiform precipitation, evaporation and sublimation along the descending flow add negative buoyancy that helps drive the jet toward the surface.
Connection to bow echoes and derechos
Enhanced rear-inflow is a recognized ingredient of bow-echo structure. The jet is often strongest along and behind the axis of a bowing line segment, and its descent at or near the updraft-downdraft interface is where the most damaging surface winds tend to concentrate. Persistent derecho-producing systems are commonly organized around sustained bow echoes with mesoscale vortices and a well-defined rear-inflow jet.
In moderate to strong ambient shear, the jet stays elevated until near the bow apex before descending sharply, producing narrow but intense corridors of damage. In weaker shear, the jet descends earlier and spreads along the leading edge, producing broader but generally less extreme surface winds.
How forecasters read it
Radar signatures of a strong RIJ include a rear-inflow notch on the trailing side of the line, a mid-level velocity maximum feeding the notch, and a bowing or accelerating leading edge. Warning meteorologists watch for descending reflectivity cores and for the transition from an elevated to a surface-reaching jet, which often coincides with a step-up in damaging wind reports.
Line-end vortices on the north and south ends of a bow segment can enhance the rear inflow by channeling flow between them. Contracting bookend vortices and a strengthening RIJ together are among the operational cues that a wind-driven line is entering a period of increased threat.
Where the picture is incomplete
A rear-inflow jet that stays several kilometers above the ground can look impressive on a mid-level velocity slice yet produce little at the surface. Forecasters check multiple radar tilts to see whether the strongest flow is actually reaching the boundary layer.
The RIJ also does not act alone. Downward momentum transport from the ambient environment, the strength of the cold pool, and the moisture profile in the lowest kilometer all shape the surface wind that ultimately arrives. Reading the jet in isolation from those ingredients overstates or understates the threat, depending on the case.