In many applications it is necessary to determine whether an observation from an incoming high-volume data stream matches expectations or is anomalous. A common method for performing this task is to use an Exponentially Weighted Moving Average (EWMA), which smooths out the minor variations of the data stream. While EWMA is efficient at processing high-rate streams, it can be very volatile to abrupt transient changes in the data, losing utility for appropriately detecting anomalies. In this paper we present a probabilistic approach to EWMA which dynamically adapts the weighting based on the observation probability. This results in robustness to data anomalies yet quick adaptability to distributional data shifts.