The communication of system uncertainties may be key for overcoming challenges related to overtrust in automated driving. Existing approaches are limited to conveying uncertainties using visual displays in the instrument cluster. This requires operators to regularly monitor the display in order to perceive changes which impedes the execution of non-driving related tasks and thereby degrades the user experience. This study evaluates variables for the communication of uncertainties using peripheral awareness displays, considering changes in brightness, hue, position, size, pulse frequency, and movement speed. All variables were assessed in terms of how well participants can distinguish different instances, how logical they are, and how interrupting to a secondary task. With the exception of changes in position, all variables were ranked highly in terms of logic while changes in pulse frequency were perceived as most interrupting. The results inform the development of unobtrusive interfaces for uncertainty communication.