This document provides the detailed processing model of a the Weighted Average Synthesis Processor (WASP), that delivers monthly syntheses of Sentinel-2 surface reflectance products over land (so called Level 3A products). WASP uses as input Sentinel-2 Level 2A products obtained with MAJA processor. The high quality of the cloud and shadow masks of these Level2A products enable to trust the cloud mask. As a result, the WASP synthesis method can use a weighted average of cloud free and shadow free pixels gathered during a synthesis period of 1.5 month, while most Level 3A processors are based on Best Available Pixel methods, which for each pixel select only one date which is the most likely to be cloud free. Thanks to the weighted average method, the Level 3A products generated with WASP have a very low amount of artefacts compared to classical methods. The methods used in WASP were defined within CESBIO in 2008 for the Venµs satellite, and then implemented within the Sentinel-2 for Agriculture project (Sen2Agri). The processor developped by the Sen2Agri consortium has later been integrated within Theia ground segment by CNES. The development of synthesis methods at CESBIO were funded by CNES TOSCA and THEIA projects. The operational processor was funded by ESA in the framework of the Sentinel-2 for Agriculture project and developed by a consortium led by the Université Catholique de Louvain, in collaboration with CESBIO laboratory, and CS-France and CS-Romania companies.