Data from the OSCOUR® network (Organization for Coordinated Emergency Monitoring) are used by Santé publique France as part of the Heat Wave and Health Alert System (SACS) to estimate the health impact of heat waves. The objective of this study was to evaluate the ability of this surveillance system to reliably and robustly estimate the health impact of a weather alert in the Occitanie region. Emergency department visit summaries (RPU) from the 66 emergency facilities in the Occitanie region were collected daily from June 2 to September 8, 2017, to assess the completeness of diagnoses (completion rate), responsiveness (from Day 1 to Day 8 following the visit), and the stability of the surveillance system, with a focus on heat-related conditions. Quality criteria for data transmission and completeness were also proposed to rank departments based on their ability to provide comprehensive and timely information. The results show that the use of syndromic indicators the day after a heat wave alert would lead, at the regional level, to a 35–40% underestimation of the health impact of this event (number of visits and emergency room admissions for heat-related conditions) and that the data consolidation time is approximately 2 to 3 days. Nevertheless, disparities exist between departments, and for five of them, an estimate of the health impact associated with a heat wave can be considered reliable as early as the day after it begins. Monitoring the proportion of visits for heat-related conditions, however, is a reliable and robust indicator as early as the day after an event for the entire region. This study provides useful insights for interpreting real-time syndromic surveillance data and offers avenues for improving data quality as part of ongoing work in collaboration with the Federation of Emergency Observatories (FedORU).