The formulation of cloud electrification is introduced for the first time into the three-dimensional Regional Atmospheric Modeling System (RAMS) mesoscale forecast model. As a first step, this paper deals with the electrical development in an isolated thunderstorm. A subsequent paper will deal with the electrical development in a cloud field. As an example, for this demonstration we chose wintertime conditions off the northern coast of Israel. Simulations of the charge separation in thunderclouds and the evolution of the electric field were conducted until the conditions for lightning were met. The numerical formulation of the electrical processes includes only the parameterization of the noninductive graupel-ice charge separation mechanism. Two different parameterizations of the charge separation by the noninductive charging mechanism were compared. The first was based on the experimental studies of Saunders et al. (1991) and the other on the experimental results of Takahashi (Takahashi (1978) and Takahashi and Miyawaki (2002)). The numerical results show that Takahashi's scheme produces a tripolar charge distribution in the cloud, prior to the first lightning flash. The Saunders' parameterization produces an inverted dipole before the lightning time. The charging rate was similar in both schemes.