The NWS produces expert commentary (aka "forecaster's discussion") that can explain the deviations from forecasted weather. For example, there is a stalled weak cold front over the US right now. Many areas are getting variants of "Though not currently included in our forecast, very minor rain chances could return on Wednesday with models indicating some eastward movement of the ridge and an increase in PWAT values" based on the underlying uncertainty behind where exactly this frontal system will stall out.