Repositioning is moving to preserve a useful and acceptable observing position as a storm evolves, changes motion, or renders the current spot unsafe or unproductive. It runs continuously from initial contact with a cell until the chaser drops the storm or ends the day.
What triggers a reposition
Storm motion is the most common trigger. Supercells routinely deviate from the mean flow, with right-moving cells turning further to the right of the environmental wind. The Bunkers method of forecasting storm motion is widely used to anticipate that deviation and to plan the next observing point.
Other triggers include a cycling mesocyclone that has shifted the tornado threat area, a rear-flank downdraft surge, hail cores overtaking the current road, deteriorating visibility, or a change in the road network that makes the current spot a dead end.
How chasers execute it
A clean reposition starts before the current spot has failed. The navigator identifies the next intersection, the intended new stopping point, and the escape options from that new point. The driver then executes without lingering in the transition.
The move itself is planned to stay on the visibility side of the storm, out of the forward-flank core and out of the rear-flank hail path. NWS spotter guidance to observe from four-way intersections and keep a buffer between vehicle and storm applies to the new position just as much as the old one.
Where repositioning goes wrong
The two dominant failure modes are moving too late and moving too far. Moving too late puts the vehicle in weather it was trying to avoid. Moving too far surrenders the storm to a competitor cell or to distance, and often results in trailing the storm through worse terrain than the original position offered.
A useful discipline is to name the reason for each move out loud, the new target position, and the new escape route. If any of those cannot be named clearly, the reposition is probably not ready yet.