Updraft helicity is a model-derived value that measures how strongly a simulated storm's updraft is rotating. It multiplies upward motion by vertical vorticity, the spin around an upright axis, and adds that up through a layer, most often 2 to 5 km (about 6,600 to 16,400 feet) above ground, giving units of square meters per square second.
Why models track it
Convection-allowing models produce individual storms, but a forecaster cannot study every simulated storm at every time step. Updraft helicity was designed to flag rotation in simulated storms. Researchers at the Storm Prediction Center (SPC) and the National Severe Storms Laboratory (NSSL) explain that it targets model mesocyclones because they may indicate supercells.
Model output often includes the hourly maximum, the largest value at each grid point during the hour. Mapped over time, those maxima form swaths that trace where the model's rotating storms traveled.
How forecasters read it
NSSL describes updraft helicity (often shortened to UH) as a proxy for right-moving, rotating storms. Ensembles turn it into a probability, such as the share of members exceeding a set value.
Thresholds depend on the model and the layer. Products from NOAA's Warn-on-Forecast research program use 60 square meters per square second for the 2 to 5 km layer and 20 for a 0 to 2 km layer, while a 2015 NWS notice set 25 for probability products in a different ensemble, so a number from one system does not transfer to another.
What it is not
It is not storm-relative helicity. Storm-relative helicity describes the environment, the potential for updraft rotation in the low-level wind profile, while updraft helicity is calculated from the model's own storm.
It is also not a tornado forecast. Long tracks can come from elevated or high-based simulated storms with low tornado chances, or from line-end vortices in lines and clusters, and even tornadic supercells usually produce tornadoes for only part of their lives.
Completing the picture
Read updraft helicity alongside the environment the model storm is moving through, including low-level moisture, cloud base heights and low-level shear. Strongly negative values can also appear, and they mark rotation in the opposite direction, typical of left-moving storms.