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Yesterday’s Weather is an Extreme Programming planning technique used to estimate the capacity of a Team based on their past performance. Yesterday’s Weather is a guide to help Teams neither under commit nor overcommit to a set of Product Backlog items during Sprint Planning. It is based upon the old story about developing weather forecasting software; if you want to predict today’s weather, all you need to do is repeat the forecast from the previous day and you have a 70% chance of being correct.
So does this work for software teams? In my experience, it does. If a Scrum Team is working in timeboxed Sprints of two weeks, they will likely deliver the same amount of working software in the next two weeks as they completed in the previous two weeks. This is due to two very important, simplifying assumptions associated with Scrum.
So what does a Scrum Team do when their Yesterday’s Weather is no longer accurate? I would revert to Commitment Based Planning and go with the Team’s instinct on what they think is possible for the next few Sprints. Once the velocity has stablized, then go back to Yesterday’s Weather (if you need to).