Today has me considering how much weather can negatively affect schedule. This month we have had 5 snow days. Last fall our site in Miami was closed for a week due to a hurricane. Forces of nature are hard to predict and can definitely negatively affect schedule. The best we can do is try and account for those in planning phase. You might want your workers to have the ability to work remotely. You want multiple people trained to perform certain tasks/duties and you would want approvers to be able to delegate authority.