In this paper, we present a formal definition in temporal logic for an authorization model for an adaptable workflow within cloud environment. Cloud computing is a strong driving technology reshaping cyber space transactions. Security is characterized as the prime challenge for cloud computing. Workflows are core business activities which were previously running in trusted environments. Running workflows in cloud environment is a growing practice which brings agility to institutions as well as presenting a new challenge. New challenges were brought in the picture due to the factors of the distributed environment. In this paper a formal model for the access control problem of workflows within cloud environment is presented. A temporal event-based periodic role-based authorization model is given for workflows on cloud where synchronization of workflows with authorization flow and separation of duties are supported. The model combines the necessary mechanisms needed to answer the authorization challenges. The model will help to answer some of the pressing needs of security within the new developing cloud environment besides the classical needs of workflows. The model presented creates a set of mechanisms in response to those challenges, and then combines all the mechanisms in a single model. Yet another challenge this work lays ground for is presenting a formal model where it is practical to explore the security-reliability nexus. That gives support to the cloud system design process.