Instability builds the storm; shear organizes it. This page covers the wind side of severe weather forecasting: bulk and effective shear, storm-relative helicity, hodographs, and the composite indices like STP and SCP that mash everything into one number. Read it alongside Instability & CAPE and you can follow any forecast discussion in the XWD Discord.
Wind shear is a change in wind speed or direction with height. In tropical meteorology, high wind shear (>20 kt) tilts a cyclone's circulation, ventilates its warm core, and inhibits development. In severe weather meteorology, wind shear (especially directional shear in the low levels) organizes supercells and creates tornado-favorable environments.
0–6km Bulk Shear is the vector wind difference between the surface and 6km up. >40 knots supports supercells, >60 knots supports long-lived violent supercells.
0-1 km shear is the change in wind speed and direction between the surface and 1 km up. Values above 20 kt are favorable for tornadoes; 30+ kt combined with strong SRH dramatically raises tornado potential. Low-level shear is the single most important parameter for tornadogenesis once a supercell exists.
0-6 km shear (deep-layer shear) is the change in wind between the surface and 6 km up. Values of 30-40 kt support organized multicells; 40+ kt supports supercells; 50+ kt supports long-lived, classic supercells. It's the storm-mode parameter — how organized convection will be once it fires.
Effective Shear is bulk shear calculated over the layer storms actually ingest (effective inflow layer to half the EL). More physically meaningful than fixed-layer shear, especially for elevated storms.
Storm Relative Helicity (SRH) measures the streamwise vorticity ingested by a storm — basically how much rotation it can draw from the environment. 0–1km SRH > 150 m²/s² strongly favors tornadoes; >300 favors strong tornadoes.
Streamwise vorticity is horizontal spin oriented parallel to the storm's inflow — the component of environmental rotation that an updraft can 'tilt and stretch' into vertical rotation (a mesocyclone). The higher the streamwise vorticity in the inflow layer, the more efficiently a supercell can generate rotation. 0–1km SRH is the primary proxy for low-level streamwise vorticity.
Veering = wind direction shifts clockwise with height (S → SW → W). Backing = counterclockwise shift (S → SE → E). In the U.S., veering winds with height indicate warm air advection and are strongly favorable for supercells because they create curved, elongated hodographs that generate streamwise vorticity — the rotation that feeds tornado-producing mesocyclones.
A hodograph plots wind vectors at various heights as a curve. Long, curved (clockwise) hodographs in the low levels indicate strong shear and high tornado potential.
The Bunkers method is a way to estimate the motion of a right-moving (cyclonic) supercell from the wind profile. SPC mesoanalysis plots Bunkers motion vectors directly on hodographs as the RM (right mover) and LM (left mover) marks. Storm motion matters because SRH and other tornado parameters are calculated relative to it.
A Low-Level Jet (LLJ) is a narrow ribbon of fast-moving air in the lower atmosphere, typically 1,000-5,000 ft above the surface. The nocturnal LLJ over the Great Plains strengthens after sunset as friction decouples from the surface, pumping warm, moist air northward at 40-70+ kt. LLJs boost low-level wind shear and moisture transport, which is why severe weather and tornado outbreaks often ramp up overnight in places like Oklahoma and Kansas.
SIGTOR is shorthand for the Significant Tornado Parameter (STP), which combines CAPE, low-level shear, SRH, and LCL into a single index designed to forecast EF2+ tornadoes. STP ≥ 1 means an environment supports significant tornadoes; STP 3-5+ is a major tornado outbreak environment. See also: STP.