The 0-1 km storm-relative helicity (SRH) is the streamwise horizontal spin available to a storm through the lowest kilometer above ground, calculated in the storm's frame of reference. It is the traditional low-level layer used in supercell tornado forecasting.
Why the 0-1 km depth
The lowest kilometer roughly matches the depth of the boundary layer inflow that feeds the low-level mesocyclone of a supercell. Rotation generated in this layer can be tilted into vertical spin by the updraft and stretched near the surface where tornadoes actually form.
The layer is deep enough to average over noise in a single sounding level, and shallow enough to focus on the air a storm is directly inhaling rather than the broader environment aloft.
How forecasters use it
Low-level SRH is one of the shear inputs to the original Significant Tornado Parameter and to many operational supercell composites. NWS Louisville's hodograph guidance treats larger 0-1 km SRH values as one of the ingredients favoring more tornadic supercells, especially when they show up together with curved low-level hodographs and a low LCL.
Because the layer has been in operational use since the early 2000s, most forecasters carry a mental scale for it. Values that climb well above the local climatology on a given day tend to raise the tornado threat, particularly when supported by deeper-layer shear and adequate low-level moisture.
Where it misleads
The 2019 Coffer, Parker, and Thompson study showed that the 0-1 km layer is a less sensitive discriminator than the 0-500 m layer. A profile can score comfortably in 0-1 km SRH while the crucial lowest few hundred meters carry weak or crosswise spin, so this layer alone can overstate tornado potential.
As with any SRH, the value is only as good as the storm motion it is calculated against. A storm anchored to a boundary or moving off the assumed right-mover vector can experience very different low-level inflow than the value on the map implies.