Urgent computing requires computations to commence in short order and complete within a stipulated deadline to support mitigation activities in preparation, response and recovery from an event that requires immediate attention. Missing an urgent deadline can lead to dire consequences where avoidable human and financial losses ensued. Timely allocation of resources to meet the deadline is crucial. Robustness is of great importance to ensure that small perturbations on the computing systems do not affect the makespan of the allocations so that the deadline can be met. This work focuses on developing a general mathematical makespan model for urgent computing to enable a robust allocation of ensemble forecasts while minimising the makespan. Three patterns of different resource allocation will be investigated to illustrate the model. The result will aid in satisfying the most crucial requirement, the time criterion, of urgent computing.