Tilting of Horizontal Spin into Spin around a Vertical Axis
The storm-relative winds (vectors) on the animation below represent winds that blow relative to the storm (the arrows indicate direction, and the length of the vectors correspond to magnitude). To get storm-relative winds, we subtract out the forward motion of the storm (a vector) from the observed winds (also a vector). In effect, we treat the storm as if it were stationary and note the pattern of winds blowing relative to the storm.
On the flash animation below, the first three storm-relative winds above the ground generate spin about a horizontal streamline that traces the path of air flowing into the storm (in other words, storm-relative inflow). When this air spinning around this horizontal streamline encounters a convective updraft, it gets tilted into the vertical, setting the stage for rotating updrafts (supercells).
Think of the bottommost vector (just above the ground) as the footprint of the low-level jet stream. The stronger the low-level jet stream, the greater the horizontal spin around the streamline marking the storm-relative inflow. Once tilted into the vertical, the enhanced rotation of the convective updraft heightens the risk of tornadogenesis.
I'll add here that the storm-relative inflow tends to restrain the storm's gust front (the leading edge of rain-cooled air associated with storm downdrafts). Otherwise, the gust front would run out ahead of the thunderstorm and initiate new convection, thereby choking off the flow of warm, relatively moist air into the storm. Forecasters describe such a process as "undercutting" the storm's updraft.
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