Snapshot diagrams, which visualize in-memory program state, are frequently used in programming education to demonstrate new concepts and help students develop a better understanding of program functionality. In this paper we introduce Snapdown, a textual language for drawing snapshot diagrams, designed for use by both students and instructors of programming courses. Snapdown is designed with an emphasis on learnability and simplicity: both to be picked up by students in a classroom setting in a matter of minutes, and to enable creation and maintenance of diagrams in instructional content with minimal overhead. We introduce several use cases of Snapdown and describe the design and features of its textual language. We also describe a deployment of Snapdown during two semesters of emergency remote teaching in CS 300 (anonymized for review), an introductory software engineering course taught by the authors.