Evidence decay in PGMs is achieved using virtual evidence nodes that create and send lambda messages that when combined with the other evidence force specified beliefs onto the decaying evidence nodes. The virtual evidence nodes compute a step along a path from the decaying node's current belief to a target belief to determine the specified belief. Belief propagation is executed to process the pi and lambda messages to update the current beliefs for all nodes. Observation evidence is removed from the model. For each decaying node, belief propagation is executed absent the evidence of that node to generate an updated target belief. Following an observation, the node's belief will decay in a smooth, continuous manner.