Homogeneous-charge compression ignition (HCCI) engine technology is capable of reducing NOx and soot emissions while achieving higher thermal efficiency than spark ignition engines. In order to obtain a smoother pressure rise, some degree of temperature stratification is introduced in the otherwise chemically (nearly) homogeneous fuel-air mixture. As some authors recently showed by means of Direct Numerical Simulation [E. R. Hawkes, R. Sankaran, P. P. Pébay, J. H. Chen, Combustion and Flame 145(1-2):145-159], temperature inhomogeneities lead to the coexistence of two ignition regimes - spontaneous ignition fronts, and deflagration fronts - Thus posing a modeling challenge to the multizone approach under high stratiFIcation conditions. In this paper we focus on the ignition of a pocket of lean H2/air mixture ( φ = 0:1, p = 41 atm and T = 1070 K). The igniting mixture parcel is lumped into a single reactor and a Probability Density Function (PDF) based model with sole treatment of micromixing and chemistry is applied. Our approach improved the predictions of the multizone model and it is found capable of quantitatively reproducing the time history of the global heat release rate, mean temperature and pressure rise from published DNS data. Most importantly, the dependence of the heat release rate on the extent of the initial temperature stratification in the charge is also well captured.