Skies can be mostly cloudy (BKN) when the relative humidity is at least 70% at some level in the atmosphere. The following link will take you to the National Center for Atmospheric Research's Research Applications Program (RAP) numerical models page for the Eta model. Select the desired Forecast Time and then select 925-500 Temps/Humidity. You will receive a 4-panel plot of relative humidity forecasts at 500 mb (about 18,000'), 700 mb (about 10,000'), 850 mb (about 5,000'), and 925 mb (about 2,300'). If the relative humidity is at least 70% at any level, there's a good chance that skies will be cloudy. The lower the relative humidity, the more likely the skies will be clear. Statistical guidance ("model output statistics", or MOS) based on the NGM is also available for sky conditions.