Inclement weather effects have a direct impact on the efficiency of a Solar Power Tower plant and have the potential to damage the receiver by flash heating. An optimised aiming strategy for the heliostat field mitigates the risk of receiver damage and maximises plant efficiency. A stochastic integer programming approach is applied to optimise the aiming strategy of the heliostat field, with uncertainty in the cloud location, size and density. The optimisation technique is demonstrated with a test case and results are presented for near real-time simulation of the optimal aiming strategy.