Many wildfire behaviour modeling studies have focused on fires during extreme conditions, where the dominant processes are resolved and smaller-scale variations have less influence on fire behaviour. As such, wildfire behaviour models typically perform well for these cases. However, they can struggle in marginal conditions (e.g. low-intensity fire) as small-scale variations significantly influence fire physics at scales below grid resolution. In an effort to generalize wildfire behaviour models and improve their overall performance, we have developed a new set of equations for wet and dry fuel to capture the finer-scale sub-grid variations in temperature and moisture. We explore the behaviour of these equations in simple scenarios ranging from high- to low-intensity fire. Furthermore, we evaluate the performance against observations of surface fire. In all cases the proposed model performs well after peak temperature is reached; however, the rise of fuel temperature at the onset of combustion is faster than expected.