Project Controls..the weatherman of the EPC world

As my role changes and evolves; diverting from focus on mechanical and technical aspects of the Turnaround and Operations environment to the challenges of Capital Projects I have recognized some crossover and disconnects of terminology. Case in point regarding Earned Value the expression of "budget" and "forecast" have two distinct meanings. Often budget and forecast used interchangeably, which can be challenging if there has been no discernible no distinction between the two.
Understanding the budget is derived from the planned estimate of the anticipated cost to complete the scope defined within each work package which is then rolled up to a WBS/CBS component. The planned estimate is an estimate; however, it is an essential component to provide a forecast. The budget plan becomes the performance measurement baseline.  The budget (Cost Baseline) is therefore a rigid plan of spending that is established prior to the start of work.  The forecast is a separate mechanism for predicting the final outcome once the work starts or it becomes clear that things aren’t going exactly according to plan.
Think of it in these simple terms: we forecast the weather, but we cannot plan it.   The weather, in this case, is the unexpected thing that happens to our projects once work gets under way.  Cost overruns, delays and rate changes are common examples of the weather we encounter.
If there is a delay that starts to require more hours than planned, the Project Manager can use forecasting tools within the Earned Value Management system to manage the risk of delay, ignore, mitigate or accept the problem and then show the effect of the risk management plan. In short, forecasting is used to protect the Estimate At Completion (EAC) figure and manage the project to keep this figure within thresholds.  What the manager cannot do is change anything that affects the budget without following a formal process.  If the budget was changed every time there was a problem and without a formal audit trail of the change, then the performance measurement baseline would be compromised along with the integrity of the earned value data.