$\sigma_n^2 = \lambda \sigma_{n-1}^2 + (1 - \lambda) \mu_{n-1}^2$

$\sigma_t = \sqrt{(0.94)(0.015)^2 + (1 - 0.94)\left(\frac{30.5 - 30}{30}\right)^2} = 0.01510519 = 1.5105\%$