The 0-500 m storm-relative helicity (SRH) is the amount of streamwise horizontal spin present in the lowest 500 meters of the atmosphere, viewed from the storm's frame of reference. It is the shallowest layer routinely used to describe near-ground inflow feeding a supercell.
What the layer captures
SRH in general measures spin available for a rising updraft to tilt into vertical rotation. Restricting the integration to the lowest 500 meters focuses on the exact air feeding into the region where near-ground vertical vorticity is generated during tornadogenesis, rather than the deeper inflow that becomes the mid-level mesocyclone.
Because the layer is so shallow, small changes in low-level wind direction or magnitude produce large changes in the value. A veering surface wind of only a few knots can turn a modest number into a large one.
How forecasters use it
The 2019 study by Coffer, Parker, and Thompson evaluated 20,194 right-moving supercells and found that 0-500 m SRH was the strongest single-parameter discriminator they tested, with a true skill statistic of about 0.53. Their reported medians were roughly 224 m squared per second squared for significantly tornadic supercells and 64 for nontornadic supercells, a ratio of about three and a half to one.
That result led SPC to publish an updated Significant Tornado Parameter formulation that substitutes 0-500 m SRH for 0-1 km SRH, and the near-ground layer is now a standard field on SPC mesoanalysis and sounding output.
Important limits
The shallow depth is both the strength and the weakness. Objective analyses interpolate between rawinsonde and surface observations, and the resulting near-ground shear profile can shift meaningfully from one hourly update to the next. Point comparisons at storm scale are noisier than the deeper layers.
The value also assumes a plausible storm motion. If the storm is not tracking near the assumed right-mover motion, if it is anchored to a boundary, or if the boundary layer above the surface is decoupled from the inflow the storm is actually ingesting, the number can mislead in either direction.