The diabatic tracer scheme has been recently implemented in the MetOffice Unified Model (MetUM), it consist of a set of passive tracers of Potential Vorticity and theta for each model parametrization. A pseudo-operational MetUM forecasts with the diabatic tracers are run for the North Atlantic Waveguide and Downstream Experiment campaign period (NAWDEX, 15 September to 15 October 2016). Predictability barriers, a sudden drop in the forecasts skill, are found and associated to diabatic processes. With the use of diabatic tracers, these diabatic processes are investigated to understand the possible causes of model error.