The numerical calculations of the transition phase of a streamer to an arc in nitrogen is reported. The proposed model includes an extensive set of collisional processes leading to gas heating. Radial variations of the species concentration and temperature are included in the calculation. Starting with the streamer parameters as an initial condition, the calculations are carried out until ‘‘quasisteady’’ state arc conditions are reached. The time evolution of the species concentration, the discharge voltage, and the current are discussed. The steady‐state arc temperatures, radii, currents, and voltages are compared with available experimental data.