In previous work [Kawamura et al., Plasma Sources Sci. Technol. 25, 054009 (2016)] and [Kawamura et al., J. Phys. D: Appl. Phys. 50, 145204 (2017)], 1D kinetic particle-in-cell (PIC) simulations of narrow gap (1 to 4 mm), high frequency (27 MHz) or dc-driven, He/2%H 2 O atmospheric pressure plasmas (APPs) showed an ionization instability resulting in standing striations (spatial oscillations) in the bulk plasma. We developed a steady-state striation theory which showed that the striations are due to non-local electron kinetics. In both the high frequency and dc-driven cases, the equilibrium electron density n 0 in the plasma bulk was stationary. In this work, we first conduct 1D PIC simulations of a 1 mm gap He/2%H 2 O APP, driven by a sinusoidal current at a low frequency of f ¼ 50 kHz such that x ¼ 2pf is well below the ionization frequency iz. In this case, n 0 varies with time, and we observe a time-varying instability which quasistatically depends on n 0 (t). At each phase of the rf cycle, the discharge resembles a dc discharge at the same n 0. At higher frequencies (200 kHz-1 MHz), x approaches iz , and quasistatic equilibrium at each phase breaks down. The discharge is also driven with a 200 kHz, 50% duty cycle square wave pulse with a short rise and fall time of 0.1 ls in an attempt to directly measure the striation growth rate s during the on-cycle before it saturated. However, the spike in current during the rise time leads to a spike in electron temperature T e and hence iz and s at the beginning of the rise which saturated during the beginning of the on-cycle. To predict the instability growth rate and saturation during and after the current spike, we extend our striation theory to include time-varying n 0 , T e , iz , as well as terms for the nonlinear saturation and noise floor of the striation amplitude. The timevarying global model predictions are compared to the PIC simulations, showing reasonable agreement.