The 0-3 km storm-relative helicity (SRH) is the streamwise horizontal spin available to a storm through the lowest 3 kilometers above ground, calculated in the storm's frame of reference. It is the original SRH layer used to describe the full inflow depth that becomes a mid-level mesocyclone.
The layer's history
The 3 kilometer depth traces back to the earliest operational SRH work, when the inflow layer of an idealized supercell was assumed to be a fixed depth roughly matching the boundary layer plus a portion of the free troposphere above. Davies-Jones's streamwise vorticity framework provided the physical rationale, and the 3 kilometer choice matched the depth over which most environmental spin could reasonably be tilted by an updraft.
It remains the default deep-layer SRH on many operational displays because it is easy to compute, comparable across regions, and reasonably diagnostic of whether a storm can organize as a supercell at all.
How forecasters use it
The SPC bulk shear reference and NWS Louisville's operational guidance both treat 0-3 km SRH as a supercell organization parameter. Large values indicate that the storm has plenty of ambient spin to build a persistent mid-level mesocyclone.
Forecasters typically pair it with 0-6 km bulk shear for storm-mode assessment. A profile with strong 0-3 km SRH and adequate deep-layer shear favors sustained rotating updrafts, whereas a strong 0-3 km number with weak deep shear tends to produce transient rotation.
What it does not tell you
A supercell can carry a stout 0-3 km SRH while offering little low-level rotation, and a storm can go tornadic on a day with modest 0-3 km SRH but strong near-ground spin. The 2019 Coffer, Parker, and Thompson analysis found the shallower 0-500 m and effective layers to be materially better discriminators of tornado potential than the 0-3 km value alone.
This layer is best read as one line in the shear stack. Compare it with the 0-1 km and near-ground values to see whether the low-level portion of the profile is doing enough of the work to matter for tornadoes.