the Cressman interpolation method [@cressman59] with the cutoff radius set to $2\varepsilon$ (consistent with [@cai05]). Vorticity lines were then computed near the low-level mesocyclone $\sim500$ m above ground level (AGL) every $5$ minutes once a distinct low-level mesocyclone had formed (as discerned from visual inspection of the $75$-m vorticity field). As in [@cai05], vorticity lines were fit to $300$ m $\leq \varepsilon \leq 9600$ m. Tornadogenesis was considered to occur once the maximum axisymmetric tangential wind velocity, $V_T$, around the intensifying surface vortex associated with the low-level mesocyclone exceeded $20$ m s$^{-1}$. The $V_T$ was retrieved using the vortex detection and characterization technique [@potvin13].

As in [@cai05], the vorticity lines steepen prior to tornadogenesis (see Figures \[fig:vorticitylines-ok-nssl\] and \[fig:timeseries-ok-nssl\]), consistent with the concentration of vorticity from larger to smaller scales.

![Vorticity lines computed at $t=134$ min (black, least steep), prior to the development of a discernible surface vortex; at $t=154$ min (red, in-between), by which time a relatively weak surface vortex is present; and at $t=169$ min (blue, steepest), near the time of tornadogenesis. Points used to create the least-squares fit are denoted by dots, while points not used in the vorticity line computation ($\varepsilon<300$ m) are denoted by asterisks.[]{data-label="fig:vorticitylines-ok-nssl"}](vorticitylines-ok-nssl.pdf){width="60.00000%"}

