The paper deals with the analysis of stochastic variables that can have significant influence on the transient impedance of the horizontal grounding electrode. Deterministic model of the grounding electrode is based on the antenna theory and corresponding Pocklington integro-differential equation in frequency domain. The Pocklington equation is solved using analytical method and transient impedance is subsequently obtained. Stochastic collocation analysis is applied by taking into account parameters of the lightning pulse (front time and time-to-half) and performing subsequent analysis of their impact on the transient impedance of the grounding electrode.