Background <p>Emergency departments (ED) are struggling with an increased influx of patients. One of the methods to help departments prepare for surges of admittance is time series forecasting (TSF). The aim of this study was to create an overview of current literature to help guide future research. Firstly, we aimed to identify external variables used. Secondly, we tried to identify TSF methods used and their performance.</p> Methods <p>We included model development or validation studies that were forecasting patient arrivals to the ED and used external variables. We included studies on any forecast horizon and any forecasting methodology. Literature search was done through PubMed, Scopus, Web of Science, CINAHL and Embase databases. We extracted data on methods and variables used. The study is reported according to TRIPOD-SRMA guidelines. The risk of bias was assessed using PROBAST and authors’ own dimensions.</p> Results <p>We included 30 studies. Our analysis has identified 10 different groups of variables used in models. Weather and calendar variables were commonly used. We found 3 different families of TSF methods. However, none of the studies followed reporting guidelines and model code was seldom published.</p> Conclusions <p>Our results identify the need for better reported results of model development and validation to better understand the role of external variables used in created models, as well as for more uniform reporting of results between different research groups and external validation of created models. Based on our findings, we also suggest a future research agenda for this field.</p> Clinical trial number <p>Not applicable.</p>