Storm motion is the speed and direction at which a thunderstorm moves across the ground. It is the combination of advection by the mean flow and the storm's own propagation, and it sets the ground track a forecaster or chaser must plan around.
What controls it
Two ingredients drive storm motion. Advection is the passive transport by the environmental winds through the depth the storm occupies. Propagation is the storm's tendency to build new cells on a preferred flank, which shifts the effective centroid of the storm in that direction even when the mean wind does not change.
For ordinary multicell storms, the mean wind through a deep layer, often 0 to 6 km, is a first-order estimate. For rotating supercells, propagation is large and consistently deviant. Right-moving supercells in the Northern Hemisphere propagate to the right of the mean wind and move slower and more southerly than the mean flow. Left-moving supercells propagate to the left, faster and more northerly.
How forecasters read it
On a radar, storm motion is estimated by tracking centroids across successive scans. Operationally, it is compared with the forecast Bunkers vector or with an interpolated environmental estimate. A cell that consistently moves 15 to 30 degrees to the right of the mean wind is behaving as a right-moving supercell and is treated as such for warning purposes.
The Storm Prediction Center sounding display shows two forecast motion vectors, one for the right-moving supercell and one for the left-moving supercell. Chase-day planning starts from those vectors and then adjusts once a cell has been on radar long enough to establish an observed motion.
Important limits
A single environmental hodograph does not force a single motion. Two supercells in the same warm sector can move at meaningfully different speeds and headings depending on their own dynamics, their proximity to boundaries, and how strongly propagation is influenced by cell mergers and near-storm inflow.
Boundary interactions in particular can shift a storm off its predicted vector. A supercell riding along a warm front tends to slow down or curve leftward relative to Bunkers as the boundary anchors the propagation. Chase logistics and warning decisions that assume a fixed motion vector age poorly against these mesoscale interactions.