Bunkers storm motion is a hodograph-based technique for estimating the ground motion of right-moving and left-moving supercells from the environmental wind profile. It is the operational default on most sounding tools for anticipating supercell tracks.
How the method works
The technique starts with the non-pressure-weighted mean wind through the surface to 6 km layer. That mean wind represents pure advection and is where an unpropagating cell would travel. It also serves as the reference point on the hodograph.
The method then draws a line through the mean wind oriented perpendicular to the 0 to 6 km shear vector, and marks off a fixed distance in each direction along that line. That distance, roughly 7.5 meters per second or about 15 knots, comes from an observational climatology of supercell motions. The point on the right side of the shear vector is the forecast right-moving supercell motion, and the point on the left is the left-moving supercell motion.
The 2000 formulation is dynamic in the sense that a curved hodograph automatically produces asymmetric motions, because the shear vector rotates through the profile. Later refinements by Bunkers and colleagues used an effective-layer mean wind and effective shear rather than fixed 0 to 6 km values, which improved performance for elevated and shallow supercells.
How forecasters use it
Bunkers vectors are the default for storm-relative parameters on the Storm Prediction Center sounding display. Storm-relative helicity, effective inflow depth, and effective bulk shear are all computed with reference to the right-moving vector on a Northern Hemisphere hodograph, unless the user selects a different motion.
In chase planning, the Bunkers right vector is a first cut at where a supercell will be one, two, and three hours from initiation. Once a cell has been on radar long enough to establish an observed motion, the observed vector replaces the environmental estimate for real-time positioning. The Bunkers vector remains useful even then as a check on whether the storm is behaving like a supercell should for that environment.
Important limits
The predicted motion often differs from the observed one. Field studies from the University of North Dakota and the NWS Rapid City forecast office document that right-moving supercells in the northern Plains average several knots slower than the Bunkers forecast, and that individual storms can deviate by 10 knots or more. The direction can also shift, particularly for storms interacting with boundaries.
The method also assumes an isolated supercell in a homogeneous environment. Storm mergers, near-storm outflow, and boundary anchoring can all pull a supercell off the Bunkers vector. The most deviant tornadic supercells, sometimes producing hard leftward turns near warm fronts or outflow boundaries, are a known failure mode that the standard vector cannot anticipate.
The effective-layer refinement
Bunkers and colleagues updated the technique in 2014 to base the mean wind and shear on the effective inflow layer rather than the fixed surface to 6 km layer. The effective-layer version handles elevated supercells and shallow cool-season supercells better than the original, because the base of the calculation matches the air the storm is actually ingesting.
In practice, the fixed-layer and effective-layer vectors agree closely for classic surface-based Plains supercells and diverge for elevated or short storms. Forecasters usually check both, treat the effective-layer vector as the default for supercell diagnostics, and fall back to the fixed-layer vector when the effective inflow layer is undefined or shallow.