Storm-relative velocity (SRV) is the radial velocity field displayed after an estimated storm motion vector has been subtracted at every gate, leaving the winds a viewer would see if they were traveling along with the storm.
How the field is built
Base radial velocity is the sum of two motions. The storm as a whole is moving across the ground, so every echo inside it inherits that translation. Rotation, convergence, and other internal circulations are added on top. When a supercell is moving northeast at 40 knots, the whole velocity image is biased in that direction, and a real 40-knot rotation on the back side of the mesocyclone can be masked because the inbound rotation cancels part of the outbound translation.
SRV subtracts the estimated storm motion vector from every gate before display. What is left is the wind relative to the storm itself. The mesocyclone is no longer competing with a strong background wind, and its inbound and outbound halves sit at nearly equal magnitudes around the axis of rotation.
How forecasters read it
The primary read is mid-level and low-level rotation. A well-defined couplet of inbound and outbound values that persist through several volume scans, and that tighten and strengthen over time, is the classic supercell mesocyclone signature. Low-level rotation strong enough to warrant a tornado warning is typically diagnosed on SRV at the lowest available tilt, where the beam is closest to the surface.
SRV also cleans up inflow and outflow structure. An inflow jet feeding into a rotating updraft is more obvious once the storm motion is removed, because the beam is no longer showing the sum of storm motion and inflow. Rear-flank downdraft surges, hook occlusions, and cyclic mesocyclone behavior are all easier to see and to time on SRV than on base velocity.
What it does not prove
SRV requires a storm motion vector. That vector may come from tracking the parent cell across recent volume scans, from a manually entered value, or from an algorithm. Any of those inputs can be wrong, especially when a cell is deviating, splitting, or newly formed. A wrong storm motion can smear a real rotation, create the appearance of rotation where there is none, or push the mesocyclone image away from its true location.
SRV also does not overcome the fundamental beam-projection limit. Rotation with an axis close to the beam direction still shows up cleanly; rotation with an axis perpendicular to the beam is harder to see. And a strong SRV couplet aloft does not confirm a tornado at the surface any more than a strong base velocity couplet does. Read SRV alongside dual-polarization data, spotter reports, and the base velocity image, and treat the couplet as a rotation diagnosis, not a hazard verdict.